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titlebf Report of the ALMA Scientific Advisory Committee:
September 2001 Meeting
par
titleOctober 15, 2001
par
title
par
vspace2.5cm
par
affilALMA Scientific Advisory Committee
authorR. Bachiller
(Spain), A. Benz (Switzerland), G. Blake (USA, Vice-chair), R. Booth (Sweden), L. Bronfman (Chile), P. Cox (France), R. Crutcher
(USA), N. Evans (USA), Y. Fukui (Japan, Vice-chair), M. Gurwell
(USA), T. Hasegawa (Japan), H. Matsuo (Japan), N. Nakai (Japan),
J. Richer (UK), S. Sakamoto (Japan), P. Schilke (Germany), N. Scoville (USA)
, K. Tatematsu (Japan), M. Tsuboi (Japan)
, E. van
Dishoeck (Netherlands, Chair), M. Walmsley (Italy), J. Welch
(USA), C. Wilson (Canada), S. Yamamoto (Japan), M. Yun (USA)
par
affil
par
vspace1.5cm
par
affilEx-officio members
authorS. Guilloteau (ESO), R. Kawabe (Japan)
, J. Mangum (NRAO),
P. Shaver (ESO), A. Wootten (NRAO)
par
affil
par
vspace1.5cm
par
affilOther participants
authorJ. Baars (ESO), R. Brown (NRAO), Y. Chikada (Japan),
B. Glendenning (NRAO), P. Gray (NRAO), E. Hardy (NRAO),
D. Hofstadt (ESO), R. Kurz (ESO), R. Lucas (France),
D. Mardones (Chile), S. Myers (NRAO, part-time),
K. Morita (Japan), L.-AA. Nyman (Sweden), J. Payne (NRAO),
S. Radford (NRAO),
M. Rafal (NRAO), E. Vera (Chile, part-time)
par
affil
vspace1.5cm
Not present at Chile meeting, but provided input to ASAC report
par
newpage
par
sectionExecutive Summary
par
This report covers the developments in the ALMA project between March
and September 2001, a period of great activity on many fronts. The
ASAC applauds the enormous efforts made by the Project and the
technical working groups on many science-related issues. The
discussion at the ASAC meeting centered on the science cases for the
enhancements in the 3-way project, and a prioritization of these
enhancements was made (see S refprior). As part of this process,
the ASAC heard presentations of the extensive set of imaging
simulations carried out for the Atacama Compact Array (ACA), which
confirmed its importance for recovering smooth extended emission and
for increasing the dynamic range of the maps.
par
Scientific operations were a major point of discussion, and the ASAC
has listed many specific recommendations in a separate report in
Appendix C, including the need for a powerful ALMA observing
simulator, for a single Science Operations Center where the pipeline
produces and stores the official archive, and for Regional Support
Centers (RSC) responsible for support of the observer.
par
Concerning software, the ASAC was impressed by the work of the
Software group on the pipeline and offline requirements. It notes that
critical milestones on the use of AIPS++ as the offline package are
coming up in mid-2002, which may lead to a review of the software
effort. The ASAC suggests that the Software working group defines a
core program to test both the pipeline and offline analysis software
and obtain early user feedback.
par
Good progress was reported on the development of prototype receivers
and other receiver issues, but the Project faces an important upcoming
decision on the LO system. The ASAC urges the Receiver group to put
more effort into studying the total power stability and to present
detailed scenarios for mass production of the receivers at the next
meeting. The ASAC was pleased to see the joined effort on the 2nd
Generation (2G) correlator and looks forward to a more specific plan
next year. The ASAC has provided guidelines for the 2G correlator
specifications.
par
Regarding calibration, the ASAC accepts the recommendation that a flux
accuracy of 1% in the millimeter bands and 3% at submillimeter
wavelengths be the design goals for ALMA. It also urges the Project to
establish swiftly a calibration group with
a well-defined leader and a dedicated person for
polarization issues.
par
Following a detailed comparison of site testing on Chajnantor and
Pampa La Bola, the ASAC recommends that Chajnantor is chosen as the
center of the ALMA array. The ASAC encourages continued tests and
comparisons of the different phase correction methods and asks for a
study of cloud cover on Chajnantor using satellite data and through
installation of an infrared camera.
par
newpage
par
sectionIntroduction labelintro
par
This document reports on the fourth face-to-face meeting of the ASAC,
held in Santiago, Chile, on September 11-12, 2001. The meeting was
overshadowed by the breaking news of the terrorist attacks on the USA
on September 11. The ASAC managed to stay focussed and concentrated
under these difficult circumstances and carried out an in-depth
discussion of all items on the agenda. The program on the first day
centered on the charge by the E-ACC to provide science cases for the
enhancements, as well as a prioritized list of the enhancements. The
science cases are contained in a separate document to the E-ACC
entitled ``Scientific Justification for the ALMA Enhancements'',
whereas the prioritization is summarized in S refprior. The main
topics on the second day were the science operations (see S
refops) and software (see S refsoftware) developments, together
with updates on other technical developments. The report of our
discussions and the resulting issues are given below, with the overall
recommendations summarized in S refsummary.
par
Prior to the meeting in Santiago, most of the ASAC members visited San Pedro de
Atacama and the Chajnantor site on September 9-10. At San Pedro, the
ASAC enjoyed the warm hospitality of Casa Don Tomas and discussed a
presentation by D. Hofstadt on site operational issues.
The ASAC also heard an account by
T. Readhead (Caltech) on his experiences with his CBI instrument at
the Chajnantor site.
On the evening of September 9, the ASAC met with E. Goles,
president of CONICYT, and with S. Berna, the mayor of San Pedro, at a dinner
hosted by ESO. The
visit to the site on September 10 was literally breathtaking, and gave
the ASAC members a much clearer picture of the situation and lay-out of
the terrain, as well as a better understanding of the challenges
that the project has to face. A summary of the discussion of the site
issues is contained in S refsite. On the evening of September 11,
the ASAC met with representatives of the astronomy council of CONICYT at
a dinner hosted by NRAO.
par
On September 13, the ASAC participated in an ALMA science day at the
School of Engineerings of the Universidad de Chile, to present the
ALMA project to Chilean astronomers and engineers. The program of
this day is listed as Appendix D. The ASAC enjoyed the
hospitality of the Universidad de Chile throughout its visit.
par
sectionProject Status and Management
par
The ASAC meeting in Santiago started with a number of presentations on
the status of the project. R. Brown gave an overview of progress
since the last face-to-face meeting. He described the evolution from a
2-way to a 3-way project. It is hoped that construction can
begin in 2002, and that completion can be achieved in 2010. Issues
include the implications of delayed Japanese involvement in
construction, permission for use of the site, project schedule and
resources, and the establishment of a centralized management
structure. Issues presented for specific consideration by the ASAC are
the project scope, which affects its cost and schedule, and the
possibility of incentives for technical performance during
construction and for scientific staffing in Chile for array testing
and early operations. The ASAC's response on the project scope is
contained in S refprior and in the document ``Scientific
Justification for the ALMA Enhancements''. The ASAC postponed an
in-depth discussion on the incentives to a later meeting.
par
T. Hasegawa described the funding situation in Japan. The MEXT budget
request to the Ministry of Finance includes 929 million Yen (about
$7.7 million) for ALMA R & D, but the main construction budget was
not included. Very positive comments about the project have been made,
and it is still hoped that the government may express its commitment
to the project. In a letter to the E-ACC in August, N. Kaifu stated
that a major step is a budget that includes a 12-m prototype, and that
the decision by MEXT practically means the start of the project, but
approval of the overall construction budget could be delayed by one or
two years. NAOJ spent 200 million Yen per year so far on the project
on other R & D efforts, and plans to continue this. Planning for the
3-way project scope and task division continues as before.
par
R. Kurz gave current cost estimates for the 3-way project. The total
resources in the -10% model including Phase 1 amount to $ 870
million (2000). Counting just the amounts from September 2001 onwards,
the resources are $ 816 million, and the cost estimate including the
high priority enhancements (categories 1-3, see S refprior)
is $ 816 million. Thus, the project is feasible with the projected
resources, as concluded in June 2001. The ASAC proceeded with the
prioritization process under this assumption.
par
M. Rafal gave a brief summary of the history and charge of the
AMAC. It presently includes representatives from the US and Europe and
will be augmented to include representatives from Japan. At its first
meeting in June 2001, the AMAC expressed its positive impression about
the project, recommended the establishment as soon as possible of an
international project office with a director with well defined
authority, stressed the need for a rapid definition of a legal
organization in Chile, accepted the IPT structure, pointed out that
the prototype antenna difficulties have project-wide implications, and
said that the procurement model for the antennas needs additional
study to meet the overall project constraints. The ASAC concurs with
these recommendations.
par
sectionPrioritization of the Enhancements labelprior
par
As described in detail in the document ``Scientific Justification for
the ALMA Enhancements'', there are strong science cases for each of
the enhancements considered in the 3-way project compared with the
baseline ALMA project of 64
12m antennas equipped with 4
receiver bands. There will be only one ALMA world-wide, and it has to
serve a large and diverse scientific community with interests ranging
from distant galaxies to comets in our own solar system. Many of the
enhancements, in particular the receiver bands, have been advocated by
the ASAC from the start of the project. Certain enhancements are also
key features for the Japanese community. It is therefore with
considerable reluctance that the ASAC went ahead with the
prioritization process requested by the E-ACC.
par
At the face-to-face meeting, the ASAC heard and discussed
presentations of the science cases of all enhancements. The ASAC
subsequently ranked the enhancements based primarily on scientific
merit, with issues such as technical readiness and implementation
schedule considered of secondary importance. Nationalistic, political
or budgetary factors were not taken into account in the ranking.
par
The following scientific ranking is unanimously agreed upon by the ASAC. Within
each group of two, the rankings are equal. Categories 1-3 are close in
absolute ranking.
par
beginenumerate
par
item it Top priority: Band 10 and the ACA
par
item it Very high priority: Band 1 and the Second Generation Correlator
par
item it High priority: Band 4 and Band 8
par
item it Medium priority: Band 2 and Band 5
par
endenumerate
par
With the current estimate of the project costs, it is assumed that all
enhancements in categories 1-3 can be fitted in the budget for the
3-way project under the ``-10% option'', with only the
implementation of Bands 2 and 5 deferred to the operational phase. The
ASAC requests that it is consulted on further prioritization should
budget pressures make it necessary to defer implementation of any of
the higher priority enhancements to a later date.
par
sectionAtacama Compact Array: Design and Simulations labelaca
par
The addition of a compact array of smaller antennas (the `Atacama
Compact Array' - ACA) has been considered as an enhancement to the
baseline 2-way project for some time. Following the February 2001
ASAC meeting in Florence, a request was made to evaluate the
robustness of the results of the initial simulations with respect to
calibration errors (amplitude and phase), and to optimize the
deconvolution method and the ACA configuration.
par
Using different deconvolution methods, simulations of the ACA
including calibration errors were performed by three independent
groups (J. Pety, F. Gueth & S. Guilloteau at IRAM; K.-I. Morita at
Nobeyama; M.A. Holdaway at NRAO). The ACA was taken to be an array
containing 12 dishes with a diameter of 7-meter. The simulations were
made on a set of images with different properties, and were performed
for ALMA-only, ALMA + the single-dish data from four 12-meter
antennas, and ALMA + single dish + ACA. A detailed account of the
these simulations is given in ALMA memo 393 (see also tt
http://iram.fr/
alma). The ASAC appreciates the massive amount of work
carried out by the simulation groups.
par
The first important result is that the conclusions of the three
independent studies agree and that they do not depend on the adopted
deconvolution method. A second significant result is that the data
processing including the ACA remains simple and that no significant
additional computing power is required. However, the ACA will add some
complexity in operations, construction and maintenance, since it
represents another array with a different type of antenna and more
receivers, even though efforts are made to duplicate as many elements as
possible from the main array.
par
The three studies demonstrate that the ACA is essential to the ALMA
project for the following reasons:
par
vspace0.2cm
par
beginitemize
par
item The ACA brings robustness in the imaging, making the results more
immune to pointing and primary beam errors.
par
item The ACA recovers information on scales between 8 and 15 meters,
which are intermediate between those sampled by the 12-meter
antennas in interferometric mode and those sampled by the
total-power measurements of a single 12-meter antenna. The image
fidelity (which is the inverse of the relative error) will
therefore be improved by 30 to 100% and reach the values
expected in typical observing conditions, i.e., 30 to 60.
par
item The addition of the ACA brings an insurance on the quality
of the result. The resulting images will have a high reliability
thereby opening ALMA to the astronomical community at large, including
non-expert users.
par
enditemize
par
Subsequent scientific analysis of the simulation results has shown
that the inclusion of the larger scales is very important in the
interpretation of the images, but that the detailed advantages of the
ACA depend on the image/source structure and content. Without the
ACA, key aspects of the source structure can be missed or, in some
cases, inaccurate conclusions can be reached when interpreting the
data in terms of physical conditions. Further considerations on the
ACA together with scientific examples are provided in the ``Scientific
Justification for the ALMA Enhancements''.
par
it Recommendation: The ASAC recognizes the importance of
the ACA for improving the image fidelity. It asks the groups to
continue investigating the effects of thermal noise, primary beam
errors, misalignment of optics, and atmospheric correlated noise on
the images, and to model in more detail the effects of such errors on
the single-dish data.
par
sectionReceivers labelreceivers
par
The ASAC was pleased to hear of the excellent technical progress being
made on issues related to the ALMA first light receivers, in particular the
prototype receiver efforts for bands 6, 7 and 9. This
rapid progress will enable at least some of the first light bands to
be installed on the prototype antennas (and ASTE, see S refaste),
enabling detailed assessments of the performance of actual ALMA
electronics during the testing phase.
par
Encouraging research is also well underway on a totally photonic LO
approach in Japan, Tucson, the UK, and Germany. The Project faces an
important go/no go decision in July 2002 on whether to adopt the
totally photonic approach, which may well both simplify the LO system
and reduce its cost. The ASAC encourages the Project and JRDG to develop
immediately a detailed testing plan with definitive milestones. The
coordination of the partner efforts should also be explicitly
delineated to ensure maximum progress.
par
A number of other recurring issues were also considered by the ASAC,
and are summarized at greater length below. The ASAC reiterates
its request for a report concerning the mass production plans from all
ALMA partners. This report should include the personnel mix, the
training of technicians, and the role of the Integration Centers during
and after construction. Task division recommendations for the 3-way project
are due within the next month, and so a report should be possible by
the next ASAC face-to-face meeting in early 2002.
par
bf Frequency Bands. The ASAC reiterates that the goal for ALMA
should be complete coverage of the atmospheric windows across the
millimeter and submillimeter spectrum. A discussion of the
prioritized rankings of various baseline and enhanced capabilities of
ALMA can be found in the document on the ``Scientific Justification
for the ALMA Enhancements''. The ASAC stresses that the additional
receiver bands that should ultimately be added to the array will
enable unique science to be performed that would otherwise not be
possible.
par
bf Total Power Stability. There was no additional discussion of the
specification of 
/
10
, but the ASAC again
notes that it would enable total power on-the-fly maps to be
generated without the need for a nutating subreflector. This
stringent requirement is driven by the superb ALMA site and the
excellent sensitivity of the array in interferometric mode. The
specification is aggressive, but it has been achieved on an existing
millimeter-wave array, and the ASAC urges the JRDG to carefully
consider the means of achieving this important capability, and the Project
to make resources available to study this.
par
bf Polarization. As noted in previous reports, polarization
measurements form a pivotal scientific capability for ALMA. To date, the
JRDG has attempted to mitigate problems associated with polarization
purity etc. At this point, the most important steps that can be taken
are to undertake detailed measurements of receiver/antenna optical
properties that affect polarization performance during the prototype
testing period. The most significant issue is that of temporal
variations and variations with ALT/AZ, for which system stability is
absolutely pivotal. Should prototyping results call into question the
overall receiver and telescope performance in polarization
measurements, the ASAC recommends that
the polarization properties of the 345 GHz receiver are optimized for
both continuum and molecular line work by optimizing the optics
outside of the cryostat.
par
bf Calibration Accuracy. The ASAC has re-assessed the overall ALMA
calibration specifications, and agrees with recent recommendations of
1% accuracy at millimeter-wavelengths increasing to 3% above 300
GHz, where the calibration should essentially be limited by the
atmosphere (see S refcal). Further work on the two temperature
load scheme in the secondary and the partially transmitting vane
assemblies should be pursued and is best undertaken by the calibration
team, as it involves so many aspects of the system.
par
bf Receiver Modes. As technology matures, careful assessments of
the cost/performance tradeoff for SSB versus DSB operation of the
receivers, IF sub-system, and correlator, must be undertaken. The
ASAC has made a recommendation on the current state of affairs, and
this is included as an appendix to this report (see Appendix refdsb).
Briefly, SSB receivers are vital to ALMA's future, but at present DSB
receivers offer better performance/price ratios for many of the bands,
especially at submillimeter frequencies. The JRDG is the best forum
for further examinations of this issue, and the ASAC would like to be
keep informed of their deliberation as construction proceeds.
par
it Recommendations: The ASAC confirms the importance of the
different receiver bands, and urges the Project to make available
sufficient resources and manpower to carry out the many tasks needed
to design and build them. The ASAC provides recommendations as to the
DSB/SSB nature of the initial receiver complement, and asks the JRDG
to carefully consider the means to achieve the required total power
stability. It notes the upcoming decision on the LO system and
encourages the Project and the JRDG to develop a testing plan with
milestones. The ASAC again requests a presentation at our next meeting
of a detailed plan for the mass production, integration and testing of
the ALMA production receivers.
par
sectionCorrelator(s) labelcorrelator
par
subsectionBaseline Correlator
par
The correlator of the ALMA baseline project (the ``Baseline Correlator'')
is being built to accommodate 64 antennas, and continues to be on
schedule. The specifications of this correlator are given in chapter
10 of the ALMA Project Book. The prototype correlator chip has been
simulated recently, and a Critical Design Review should follow in
early 2002. A first portion of this correlator, which will serve as
a single baseline correlator for the ALMA test interferometer, should be
working in the laboratory by the end of 2002, and will be delivered to
the VLA site in May 2003. The ASAC has no
further comments on this development, which
appears to be proceeding well.
par
subsectionSecond Generation Correlatorlabel2gen
par
bf Background.
As noted in previous ASAC reports, the ALMA project includes
developments on a 2nd Generation (2G) Correlator aimed at providing a
greater number of channels, higher sensitivity, and higher flexibility
than the Baseline Correlator.
The scientific merit of the 2G Correlator is discussed in
the document ``Scientific Justification for the ALMA Enhancements''.
It should combine high spectral resolution with very broad frequency
coverage, high sensitivity without any spectroscopy capacity losses,
and a highly flexible use of the bandwidth and the antennas. The 2G
Correlator would be comparable to the Echelle spectrometers that are
a ``must-have'' instrument for all first-class optical and
infrared telescopes around the world. A major goal of this correlator
is 3-bit (or even 4-bit) correlation format, which improves sensitivity
by about 9 % without spectroscopic capacity losses. This is equivalent
of adding 9 % of collecting area to ALMA (about 6 dishes).
par
Two different technical schemes, one in Europe and another in Japan,
have been developed so far for designing the 2G Correlator. The
European project was a Digital Hybrid XF (DHXF) Correlator (see ALMA
Project Book, Chapter 10), whereas the Japanese project was a FX
Correlator (see ALMA Memo 350). In its previous reports, the ASAC
strongly encouraged a tight collaboration between the European,
Japanese and North-American teams to optimize the design of this 2G
Correlator. The ASAC also provided a set of guidelines for the
specifications and goals of the 2G Correlator.
Following such recommendations, a joint multinational
working group was formed which meets and/or keeps teleconferences on a
regular basis.
par
Y. Chikada reported on the recent face-to-face meeting of the 2G
Correlator WG which was held in Nobeyama in August 5-7 2001. The ASAC
was pleased to hear that the working group is clearly oriented toward
a high level of cooperation by working jointly toward a ``Unified
Design". Nine areas of common interest, which
are independent of the final architecture to be adopted, were
identified at this meeting, and joint reports on these areas will be
produced by the group at the end of 2001 for consideration by the ASAC
at its next meeting. An initial timeline for the 2G Correlator
development was presented.
The working group also
discussed the guidelines for the specifications and asked further
clarification to the ASAC (see below).
par
it Recommendations: The ASAC endorses and encourages the current
effort to jointly develop the 2G Correlator for ALMA. As a consequence, the
ASAC reiterates its previous recommendation that the Correlator
working group continues during 2002 to establish the best possible
``Unified Design" architecture. The working group should prepare a
detailed 3-way work plan for the correlator development and
production. The detailed design, precise cost estimate, and production
plan should be made available no later than September 2002. Frequent
progress reports on the working group activities should continue to be
provided to the ASAC.
par
The ASAC expressed some worries about the areas which are not of
common interest, and about the procedures to be used for adopting a
``Unified Design". It seems urgent to establish a concrete strategy
for evaluating the performances of the different design options. The
ASAC also calls attention to the EVLA/WIDAR concept, which is closely
related to the initial European design and whose performance
specifications are in some aspects similar to the requirements for the
ALMA 2G Correlator. The ASAC encourages the working group to consider
also the WIDAR architecture in their design considerations.
par
The ASAC discussed and adopted updated guidelines for the
specifications of the 2G correlator, which are included in Appendix A.
These guidelines are for orientation only and define the
minimum features which should be met by the ``Unified Design''. The
ASAC recommends that the ALMA Project Scientists establish the actual
specifications and goals by taking these guidelines as a starting
point. They should ensure that they are consistent with the
requirements from other areas of the project, in particular those
related to the LO, calibration, and software. For instance, the data
rates implied by the specifications need to be studied by the Science
and Software Requirements (SSR) working group.
par
sectionScience Operations labelops
par
Following the E-ACC and E-AEC meetings in June 2001, the ASAC was
asked to consider the ALMA operations from an astronomer's point of
view. As a result, an ASAC operations study group was formed chaired
by N. Evans, C. Wilson and Y. Fukui. The group was asked to produce
a report which could serve as input to the discussion at the
face-to-face meeting in Santiago. The study group did not address
technical operations issues, such as the siting of the OSF, the work
schedules, etc. Instead it focused on the operational issues that
might affect the scientific productivity and vitality of ALMA, looking
at the questions from the point of view of the future ALMA observer.
par
One major goal of the study was to start addressing the operational issues
raised by the Software group in their requirements document. To
achieve this goal, a close communication between the head of the
Software group and the operations study group was maintained. A
second major goal was to define the roles of the regional centers. It
was agreed that these centers should provide support, broadly
conceived, rather than merely being data repositories. In recognition
of this conclusion, the ASAC suggests that the names be changed to
Regional Support Centers (RSCs).
par
The full report, explaining the recommendations and describing the
remaining issues, is given in Appendix C. Here we summarize the basic
assumptions and approach, and repeat the recommendations and issues for
further study that are discussed in the report.
par
The basic assumptions for science operations are that (i)
Non-experts should be able to use ALMA as easily as possible. To ``use"
is to propose, obtain, reduce, analyze, evaluate, and publish observations.
(ii) Information on what has already been done (or approved to be done) should
be readily accessible.
(iii)
The dynamic scheduler must match the projects to the existing
observing conditions to make the maximum use of the best observing
conditions.
(iv)
Reliable and consistent calibration of all data is essential to
achieving the full scientific capability of ALMA.
(v)
The system should provide the maxiumum flexibility to observers that is
consistent with smooth operations.
(vi)
Data should become public in a timely fashion.
par
vspace0.3cm
par
it Recommendations:
par
beginenumerate
par
item Complete information on the source parameters
(coordinates, velocity, frequency, resolution, rms noise)
in approved and completed projects
(both proprietary and public) should be available in the archive.
par
item
Routine calibration should be primarily a responsibility
of the ALMA system.
par
item ALMA should develop a powerful simulator that is capable of a complete
end-to-end observing simulation of a project composed of a number
of scheduling blocks.
par
item ALMA should adopt the concept of stringency. This concept may be
defined as
where
is the total observing time available
and
is the total time during which a given project can be done.
par
item The dynamic scheduler should include science ranking,
stringency, and execution status as three of its key parameters.
par
item A dynamic scheduler for the ACA needs to be included in the
software planning.
par
item The ASAC should have a role in defining the operations
of the TAC to ensure that scientific considerations are included.
par
item
Opportunities for the observer to interact with ALMA operations
through `eavesdropping' and `breakpoints' in the observing script
should be encouraged where possible.
par
item
There should be a single Science Operations Center (SOC),
operated by the
ALMA observatory, where the pipeline produces and stores the official
archive. The natural location for the SOC is in Chile.
par
item
Regional Support Centers (RSC) should be responsible for support of
the observer, from proposal preparation through data reduction and
analysis. They may also provide data portal and software development.
They should be operated with an international and collaborative
spirit.
par
item
Each RSC should have a core functionality provided by the ALMA observatory.
The partners may choose to add other functionality (computer resources,
financial support for travel, students, publications, ...) from their own
resources outside the ALMA project.
par
item
The ALMA archive should be open to the worldwide community and be fully
compatible with the Virtual Observatory (VO) and the Grid paradigm.
par
item
The proprietary period for regular projects should be 1 year as is
commonly used in the currently working instruments, with some
exceptions for legacy projects and for long-term projects.
par
endenumerate
par
The ASAC recommends the following topics for further study and
discussion at future ASAC meetings: (i) The definition of stringency:
are separate parameters for water vapor content, phase stability, and
wind conditions (i.e., pointing) needed? (ii) Details of dynamic
scheduling: how should the three key priorities for the dynamic
scheduler be balanced? (iii) ALMA TAC(s): There should be further
study of how the ALMA TAC(s) should work, including a review of how
existing TACs for other facilities operate. (iv) Flexibility: how
much flexibility to adjust approved programs should be allowed in the
Phase II stage and once observing has started? How should breakpoints
and/or eavesdropping be implemented to avoid overly complicating
operations? (v) The core functionality of the RSCs should be further
considered and defined, including the number of RSCs that are needed.
(vi) Reduced images: should images produced by observers, as well as
those produced by standard scripts, be placed in the official ALMA
archive?
par
sectionSoftware labelsoftware
par
The ALMA pipeline and offline software requirements for science
operation were a major topic of discussion at the ASAC meeting.
The lists of the
requirements and the tasks needed to meet them, as presented by the
Software working group (SSR), are impressive (and
daunting) in scope. The ASAC is pleased to be informed of these
plans, which appear to be consistent with the Science Operation
requirements (see S refops) in that they consider easy access
to ALMA and analysis of its data even by non-radio astronomers to be
essential. The ASAC applauds the excellent worldwide collaboration
initiated by the SSR team.
par
The data analysis software will consist of
par
- a data simulator, initially envisioned as a tool to assist in
proposal preparation but which could be expanded into a powerful
data analysis module (see S refops)
par
- a data pipeline of automated processes that will include
calibration, real-time quick-looks (dirty images), and an initial
reduced (e.g. calibrated and cleaned) science quality
image set, and
par
- an offline analysis package allowing further data manipulation,
additional reduction, and analysis. It will be easily operated through
a Graphical User Interface (GUI) for non-specialists and through a
Command Line Interface (CLI) for in-depth analyses by more experienced
researchers.
par
Both the pipeline and offline packages will be based upon the same
software modules. These modules may be adopted from existing packages,
such as AIPS++, AIPS, GILDAS, and MIRIAD, to minimize duplications.
Additional capabilities will be added as needed to support improved
imaging or calibration algorithms, and it will be possible to rerun
the pipeline process within the offline package as enhanced processes
become available.
par
At the current time, it is assumed that the offline analysis
package will be largely based on AIPS++. To explore this possibility,
a test has been initiated to analyze Plateau-de-Bure data with
AIPS++. The test will be finished by the end of April 2002, and then the
applicability of AIPS++ for ALMA data will be studied. An ``audit"
will conclude the project. The ASAC appreciates the efforts of the
Plateau-de-Bure team for this AIPS++ test, and looks forward to seeing
how many of the offline requirements are already implemented, and
how much is to be newly developed.
par
The ASAC has received draft documents of the requirements for the
pipeline and offline analysis. The ASAC is particularly concerned
with calibration requirements, and a section in the draft pipeline
requirements report has been implemented to address these issues. Some
feedback to the SSR group has been given and further iteration will occur.
par
The discussion also took note of the scientific potential of
simulations, not only in the preparation phase of observations where it
is essential, but especially in the analysis phase. The simulator
would provide an important tool for understanding ALMA data. For this
purpose, the ASAC recommends to develop the simulator beyond the
current state (for ACA evaluation) to level 3 specifications, allowing
the complex modeling of arbitrary images using typical weather
statistics, etc.
par
The ASAC is impressed by the wide scope of the presented requirements.
It also realizes the enormous number of items in the data analysis
packages. It therefore has the following comments and recommendations.
par
vspace0.3cm
par
it Recommendations:
par
beginenumerate
par
item In view of the expected size of the work, the ASAC would like to
better understand the allocated and lacking resources of the
software team, and the current management plan of the entire data
analysis project in its tripartite form.
A detailed timeline including milestones covering the ALMA
construction phase until 2010 is highly desirable.
par
item The ASAC notes that the upcoming milestones, including the review
of pipeline package (end of 2001) and the ``audit" of AIPS++ for ALMA (mid
2002), are critical to the software part of ALMA.
The ASAC therefore suggests to use the conclusion of the above audit to
review the entire software for ALMA in the course of 2002.
par
item The ASAC suggests studying the software requirements for the
addition of the ACA. The ASAC has initiated simulation
studies of the imaging improvements by the ACA, which may be
a source of considerable commentary on where current algorithms
fail in the production of mosaics with single-dish, ALMA, and ACA
data.
par
item The ASAC suggests that the Software working group defines a core program
for both the pipeline and offline analysis. The core should include
much less than the priority 1 items in the current
SSR documents and allow simple analyses of
one mode of simulated data. The goal is to define a narrow path of
reduction software from which to expand the packages. Such a core program
would be a significant milestone and would allow a first feedback
from the users' side.
par
item User feedback should be generally encouraged and well established
at an early phase (see point 4). The goal of this feedback is user
friendliness of the ALMA software.
par
endenumerate
par
sectionCalibration labelcal
par
The most significant recent development in the calibration area has been the
preliminary design review (PDR) in Cambridge in June 2001, where
interested parties presented relevant ideas on amplitude, phase
and passband calibration. It is crucial that the ALMA project builds upon
this important first step toward developing a coherent strategy for
calibration: many of ALMA's ambitious science goals depend upon
accurate calibration of all aspects of the system, and the review
report clearly identifies many weaknesses in the organization of this
part of the project. The missing items in the design review
(polarization, single-dish) should be addressed promptly, and people
and resources allocated to form the proposed Calibration group. In
very general terms, the plans for phase calibration, using a
combination of water vapour radiometry and fast switching, are
reasonably well in hand: the phase 1 project to build prototype
radiometers 183 GHz is underway, and good progress on atmospheric
modeling is helping to understand how successful these schemes should
be. On the other hand, amplitude calibration appears to be receiving
less attention within the project, and efforts to remedy this position
are needed, in particular to bring the semi-transparent vane and
secondary-mirror based hot load methods to the status of a critical
design review.
par
The ASAC discussed the proposal of the Calibration PDR that a flux
accuracy of 1% in the millimeter bands, and 3% at submillimeter
wavelengths, be the design goals for ALMA. These are somewhat
softened from the ASAC's original goal of 1% at all frequencies, but
it was felt that these were still stringent and that the bulk of ALMA
science would be affected only modestly by this proposal. Accordingly,
the ASAC adopts these new goals, but nonetheless notes that there is
science which will be lost. The ASAC continues to recommend that
amplitude calibration receive a high priority within the receiver and
calibration groups.
par
The ASAC also noted the new results on phase correction already
obtained on Chajnantor by the site testing group. Two relevant pieces
of work by Canadian groups were also presented. The first of these
involves interferometric phase correction experiments at the SMA,
using a clone of the Cambridge-designed 183 GHz radiometer, and the
existing 183 GHz system borrowed from the CSO. These should give
results within a few months and have the added advantage of having a
radiometric beam perfectly aligned with the astronomical beam. The
second Canadian experiment is to use the IRMA 20-micron systems for
phase correction at Chajnantor: this is more speculative as the
physics of the line emission process has not been studied in detail;
nonetheless these tests are very worthwhile and should have results
within 12-18 months or thereabouts. In the meantime, there has been
progress in the phase I project to design and test second-generation
183 GHz radiometers for ALMA (work being done at MRAO and Chalmers),
but it is still 18-24 months before this project will have lab
results.
par
One new area addressed at the ASAC by H. Matsuo is the
problem associated with water droplets in the atmosphere, which may
limit the radiometric phase correction techniques under some conditions.
It is possible to measure this accurately and correct for it if one
can use simultaneous total-power observations at, say, 220 and
650 GHz. These ideas are being written up into a memo and will be
discussed at the next ASAC meeting.
par
vspace0.2cm
par
it Recommendations:
par
beginenumerate
par
item The ALMA Project should act upon the findings of the Calibration design
review, allocating manpower and resources swiftly, and establishing
a group with a well-defined leader.
par
item The ASAC accepts the recommendation of the Calibration PDR that a
flux accuracy of 1% in the millimeter bands, and 3% at
submillimeter wavelengths, be the design goals for ALMA.
par
endenumerate
par
sectionPolarization labelpol
par
Polarization has been extensively discussed at the previous two ASAC
meetings, and details can be found in the reports from the Berkeley
September 2000 and Florence February 2001 meetings. The ASAC was
pleased to hear that current plans for testing the prototype 12-m
antennas at the VLA site now include evaluating the polarization
properties of these antennas and a prototype receiver
(see S refreceivers), and installation of a stable photonic
system for calibration. A concern expressed in previous ASAC reports
has been that the polarized beams of the antennas must be very
accurately known so that polarized beam artifacts may be removed from
polarization maps. Since measurements of the polarized beam to
sufficient precision will be very time consuming, it is essential that
the polarized beam be stable so that it can be measured
infrequently. This property of the antennas will now be assessed for the
prototype antennas, with the results available for input into the
antenna selection process. Further, the decision to go ahead with a
bandpass and polarization photonic calibration source will mean that
it will be possible to calibrate the polarization properties of the
receivers and electronics systems easily and frequently. Both of these
developments are positive for ensuring that ALMA will be able to meet
the science polarization goals discussed in previous ASAC reports. The
remaining area of concern is how both the ALMA interferometer and
single-dish polarization data will be calibrated. The ASAC urges that
this issue be addressed expeditiously by the Calibration team.
par
it Recommendation: The ASAC recommends that a dedicated person, probably
within the Calibration group, is identified to concentrate on polarization
issues.
par
sectionSite Issues labelsite
par
bf Configuration. The ASAC notes that the concept of a self-similar
array design for the inner 3 km array (by J. Conway) was agreed at
the Grenoble design review meeting in February 2001. A detailed
design, based on the site topological information, will be available
before the end of the year to be iterated on by the engineering and
soil analysis team. It was also agreed that the ACA site will be
close to the center of the array, near the Chajnantor-south
position. A 60m diameter area will be left clear awaiting the final
ACA configuration. The ASAC endorses these recommendations.
par
bf Site testing. Reports on site-testing were given by S. Radford,
L.-AA. Nyman and S. Sakamoto.
Comparison of the 220 GHz opacity and
radio seeing monitoring data on Llano de Chajnantor by the
US/European team and on Pampa la Bola by the Japanese team
show that Llano de Chajnantor is the better of the two sites in terms
of opacity and seeing. With the recent successful studies by
the Configuration working group that demonstrate the feasibility of
fitting the second largest configuration into the Llano de Chajnantor
area and with the recent progress on the `direct link' option of
access from the OSF to the site, S. Sakamoto reported a
consensus within the joint site testing team of having the central
location of the array at Llano de Chajnantor.
par
vspace0.2cm
par
it Recommendation:
The ASAC recommends that Chajnantor is adopted as the ALMA array center.
par
bf 183 GHz radiometers and phase correction.
Attempts to understand the phase correction ability of the 183 GHz
radiometers are still on-going. A comparison of the water vapour
measurements with the radiometers and the 11 GHz interferometers frequently
shows good correlations but there are anomalous effects. It is thought that
these are due to the difference in frequency of the two experiments and the
difference in the optics of the two systems (also because of frequency).
Some 10% of the time-varying ionospheric delays affect the 11 GHz data but
not the 183 GHz total power measurement. There is also a problem when the
inversion layer is low enough to be in the near field of the interferometers.
par
vspace0.2cm
par
it Recommendations: The ASAC recommends that:
par
beginenumerate
par
item Further attempts are made to understand the anomalies between the
radiometers and the interferometers.
par
item The proposed trials of the 183 GHz radiometers on the SMA dishes
be carried out (see S refcal).
par
item The people responsible for the radiometers in Chile contact the
Calibration group to better coordinate efforts on ALMA phase correction.
par
item The IRMA infrared detectors (see S refcal) be deployed on
Chajnantor as soon as possible and that a joint comparison of data
with those from the radiometers is performed.
par
endenumerate
par
bf Cloud cover.
T. Readhead presented reports
of cloud cover over Chajnantor when there is none over
Pampa la Bola. Clouds may be investigated in two ways:
(i) Using satellite data over the site. The analysis of A. Erasmus
with a pixel centered on Cerro Chascon, and one each approximately over the
two sites, may be compared. A contract with Erasmus should be considered (and
costed) since his data may also be useful in the ALMA scheduling process.
(ii) Using 10 micron infrared cameras.
Such cameras exist (e.g. by H. Matsuo) and could be deployed on the site in
about 1 year.
par
it Recommendation:
The ASAC encourages comparison of water vapour data on
Chajnantor with cloud data from satellites and infrared cameras
par
bf Site development. Presentations by D. Hofstadt at San Pedro and
R. Brown in Santiago informed the ASAC of the current ideas on the
placement of the OSF much closer to the ALMA site than San Pedro, at
an altitude around 3000 m. The plan is that a new road will be cut
from the OSF onto the site (the `direct link' option). This will allow
the safe and quick transport of the antennas and personnel to and from
the site. Whether part of the residences could be in San Pedro is
still under discussion. T. Readhead stressed at his presentation in San Pedro
the importance of the use of oxygen in enhancing the work efficiency and
preventing errors.
par
it Recommendation:
The ASAC suggests that the project should
study the site experience of T. Readhead and profit by it.
par
sectionALMA Antennas labelantennas
par
The ASAC heard discussion of the three prototype 12-m antennas and
the plans for evaluation of the prototypes. The US prototype being
built by Vertex/RSI is farthest along and is scheduled to be accepted
at the test site in Socorro in April 2002. A pie section on a back-up
section is complete and has been thermally cycled between -20 and 45
C, with the shape being measured by accurate photogrammetry. There was
no surface deviation detected greater than the experimental limit of
5
m over this temperature change, a very satisfying result.
par
There have been delays in the development of the European antenna
from the EIE Consortium of at least nine months relative to the Vertex
prototype as a result of both design uncertainties and funding. A
recent back-up structure design improvement promises to allow a
surface accuracy of 20
m. A second panel manufacture design based
on a metal hex-core structure with an electroformed nickel surface
plate is under consideration along with the machined aluminum panel
option. The new option is funded by the European Space Agency as a
continuation of a space telescope (XMM) development, which may use it
for space craft antennas as well. In addition, a new consortium is
being put together to complete this prototype. The hope is for
delivery of this unit to Socorro by the end of 2002.
par
In Japan, the project has developed a prototype design based on
the tests of the new 10-m ASTE antenna (see S refaste) with
extrapolations to the 12-m scale. Although the specifications are
aggressive, it is felt that they can be achieved. The back-up
structure will consist of CFRP tubes with Invar joints, and the panels
will be of machined aluminum. A `Request for Quotation' on
development of the design will be submitted to Japanese industry soon,
and it is hoped that the prototype can be delivered to the test team
at Socorro by April of 2003. The project has completed very detailed
tests of ASTE, built by Mitsubishi. The
tracking is very smooth with RMS pointing errors less than 120
arcsec, even in winds up to 8 m/sec. The pointing accuracy based
on an optical guide telescope is excellent,
. Holographic
setting of the surface at Nobeyama has achieved a preliminary accuracy
of 55
limited by the atmospheric conditions of the site.
Based on these results, a good prototype for the 12-m is expected.
par
A detailed schedule of both single dish and interferometric tests is
planned for the Socorro site. The bulk of these will be single
antenna tests, so that the arrival of the prototype antennas at
different times should not cause serious delay in the overall
evaluation. The three prototypes will be compared with one another
and with the specifications. Pointing will be evaluated with a cooled
optical camera installed on a 10 cm optical telescope. The surfaces
will be set by holography, whereas the antenna gains, patterns,
polarization, and calibration will be tested with final versions of
the receivers, and the nodding secondaries will be tested.
Interferometric tests will follow. The antenna electronic systems,
including monitor and control components, will be studied.
The evaluation team will consist of 7-8 members, and their work will
result in an evaluation report. The ASAC notes that these tests and
the overall schedule of evaluation completed by the end of 2003 seem
satisfactory, provided that this schedule can be met.
par
sectionOther Chajnantor Projects
par
subsectionASTE labelaste
par
The Atacama Submillimeter Telescope Experiment (ASTE) is a project to
install and operate a 10-meter submillimeter antenna at Pampa la Bola
(4800 m in elevation) . The project is driven by NAOJ in collaboration
with Universidad de Chile, and also with Japanese astronomers in
universities, e.g., the University of Tokyo, Nagoya University, and
Osaka Prefecture University. As the precursor to the ALMA project and
the test bench for ALMA, ASTE will provide an important
occasion to construct and test an exposed, high precision
submillimeter antenna under the actual conditions in Chile. Another
important asset of ASTE is that it will allow the ALMA receiver system
to be tested.
par
The ASTE 10-m dish was designed to have a high surface accuracy
(
25
m rms; goal is 17
m). Low
thermal metal (Invar) and CFRP are employed for the backup
structure. Adjustable machined aluminum panels are equipped on the
surface. The Japanese 12-m ALMA prototype antenna will be based
on the design of, and experiences with, ASTE.
par
At its Cassegrain focus, an ALMA type receiver with three inserts for
cartridges will be installed. Recently, the dewar and Sumitomo
3-stages GM refrigerator were delivered to Mitaka, and the cryo-system
is under testing. The Band 8 prototype cartridge has been designed
and will be completed within six months. A Band 4 prototype cartridge
will be constructed in the Osaka prefecture university group. The
receiver can accomodate one guest ALMA prototype cartridge from Europe
or the US, and communications on that issue have already started in
the Joint Receiver Design Group (JRDG) of ALMA .
par
The ASTE 10-m antenna was constructed in February 2000 at Nobeyema in
Japan, and tested for a year. The antenna was disassembled in July
2001 for shipping to Chile, where it will arrive later this year.
Construction at the Pampa La Bola site start in January 2002, to be
finished by March 2002 and followed by installation of the
instruments. About one year is needed evaluations and tests. An ALMA
prototype receiver can be installed in about one year.
par
it Recommendation: The ASAC applauds the opportunity that ASTE
offers to test ALMA prototype receiver cartridges and encourages the
ALMA Project and the JRDG to communicate with the ASTE project on this
issue.
par
subsectionAPEX
par
The textbfAtacama textbfPathfinder textbfEXperiment is a
modified copy of the VERTEX ALMA US prototype antenna, which will be
put on Chajnantor by a consortium of Max-Planck-Institut für
Radioastronomie, Germany; Universität Bochum, Germany; ESO and
Onsala Space Observatory, Sweden. The modifications consist of adding
two Nasmyth cabins to accomodate additional receivers, and a
corresponding change in the secondary/tertiary optics. The surface
goal is also modified with respect to the ALMA prototype to be
18
m instead of 20
m. The antenna is anticipated to be
located in the general area of Chajnantor North, at about 5050 m, in
order not to interfere with construction activities at the ALMA array
center at Chajnantor South. Current plans foresee erection and
testing of the antenna until March 2003, so that after a period for
holography the operation is expected to commence in mid-2003. MPIfR
is responsible for the construction; the operation will be jointly
between the partners. The observing time is to be shared between
MPIfR/Bochum (45%), ESO and OSO (22.5% each) and Chile (10%).
par
The intitial instrumentation will consist of a large (
300
elements) bolometer array operating at 870
m, a smaller array
(100 elements) operating at 350
m, the CHAMP+ 16 pixel heterodyne
array receiver, and single pixel receivers covering all other
atmospheric bands between 1.3 mm and 300
m. Receivers in the
Terahertz region are also foreseen on an experimental basis.
par
newpage
par
sectionSummary labelsummary
par
The major ASAC recommendations are summarized below. These are in the
order discussed in the text and not in any priority order. More
detailed recommendations can be found in the section referenced by the
major recommendations.
par
beginenumerate
par
item The ASAC has carried out the priorization of the enhancements,
as described in S refprior. The ASAC requests that it is consulted
on further prioritization should budget pressures make it necessary to
defer implementation of any of the high priority enhancements to the
operational phase.
par
item The ASAC asks the ACA simulation groups to continue
investigating the effects of various sources of noise on the results,
paying special attention to more realistic modeling of the single-dish
data (see S refaca).
par
item The ASAC has the following Receiver
recommendations (see S refreceivers):
par
- The Project and JRDG should develop a detailed testing plan with definite
milestones for the upcoming decision on the LO system in mid-2002.
par
- The JRDG should analyze the means to achieve the required total
power stability.
par
- The JRDG and Project should present at our next meeting a detailed
plan for the mass production, integration and testing of the ALMA
production receivers.
par
- The Project should make available sufficient resources and manpower
for the many tasks required to design and build the ALMA receivers.
par
item The ASAC endorses and encourages the current effort to jointly
develop the 2G Correlator for ALMA, and has the following comments
(see S ref2gen):
par
- The Correlator working group should continue to establish the best
possible ``Unified Design" architecture and prepare a detailed 3-way
work plan for the correlator development and production by September
2002. In areas which are not of common interest, procedures should be
developed for adopting a ``Unified Design''.
par
- The ASAC encourages the working group to also consider the EVLA-WIDAR
architecture in their design considerations.
par
- The ASAC provides updated guidelines for the specifications of the
2G correlator (see Appendix A), which should be translated into actual
specifications and goals by the ALMA Project Scientists.
par
item The ASAC has considered Science Operations in detail and has the
following major recommendations (see S refops and Appendix C):
par
- ALMA should develop a powerful simulator that is capable of a complete
end-to-end observing simulation of a project composed of a number
of scheduling blocks.
par
- The ASAC should have a role in defining the operations
of the TAC to ensure that scientific considerations are included.
par
- There should be a single Science Operations Center (SOC),
operated by the ALMA Observatory, where the pipeline produces and
stores the official archive. The natural location for the SOC is in Chile.
par
- Regional Support Centers (RSC) should be responsible for support of
the observer, from proposal preparation through data reduction and
analysis. They may also provide data portal and software development.
They should be operated with an international and collaborative
spirit.
par
- Each RSC should have a core functionality provided by the ALMA observatory.
The partners may choose to add other functionality from their own
resources outside the ALMA project.
par
item
The ASAC has the following recommendations on Software issues
(see S refsoftware):
par
- A better understanding of the resources of the Software
team and the current management plan of the entire data analysis
project in its tripartite form is needed.
par
- The conclusions of the upcoming critical milestones, including the
review of pipeline package (end of 2001) and the ``audit" of AIPS++
for ALMA (mid 2002), should be used to review the entire software
effort for
ALMA in 2002.
par
- The Software working group should define a core program
for both the pipeline and offline analysis. Such a core program
would be a significant milestone and would allow a first user feedback.
par
item
The Project should act upon the findings of the Calibration
preliminary design review, allocating manpower and resources
swiftly, and establishing a group with a well-defined leader and a
dedicated person for polarization issues (see S refcal and
refreceivers).
par
item
The ASAC accepts the recommendation of the Calibration review that a
flux accuracy of 1% in the millimeter bands, and 3% at
submillimeter wavelengths, be the design goals for ALMA (see S
refcal and refreceivers).
par
item
The ASAC has the following recommendations on the site
(see S refsite):
par
- Chajnantor should be adopted as the center of the ALMA array.
par
- Further attempts should be made to understand
the anomalies between the 183 GHz radiometers and the 11 GHz
interferometers, and the proposed trials of the 183 GHz
radiometers on the SMA dishes should be carried out
(see S refcal and S refsite).
par
- Comparison of water vapour data on Chajnantor with cloud data from
satellites and infrared cameras is encouraged.
par
item
The ASAC applauds the opportunity that ASTE offers to test ALMA
prototype receiver cartridges and encourages the JRDG and the Project
to communicate with the ASTE project on this issue (see S refaste).
par
endenumerate
par
newpage
centerlinebf APPENDICES
appendix
sectionASAC Guidelines for the Second Generation Correlator labelcorguide
par
The following specifications and goals should be taken into account
by the European, Japanese and North-American teams working in the design
of a 2nd Generation (2G) Correlator for ALMA.
par
In general terms, the ASAC stresses that the Enhanced Correlator
developments should be guided by the goals of achieving:
par
beginitemize
par
item high number of channels in wide band modes
item high configuration flexibility
item high sensitivity
item high spectral resolution, and
item power consumption as low as possible.
par
enditemize
par
The ASAC strongly encourages a tight collaboration of the different
teams to optimize the design and to establish the best possible
architecture and manufacturing method within the budget limits.
par
subsectionSpecifications and goals
par
In the following, a ``baseband" denotes an individual input band of 2
GHz width which is analyzed by a single A/D converter. A ``sub-band"
denotes a continuous frequency chunk to be analyzed spectroscopically
(i.e. a sub-band is a sub-element of a ``baseband"). A ``sub-array"
denotes a sub-set of 12-m antennas which can operate as a logically
independent interferometer (i.e., a sub-array can work at a frequency
different from the rest of the ALMA antennas, and can receive specific
control commands: start, stop, integration times, etc).
par
beginitemize
par
item In addition to the 64 ALMA antennas of 12-m, the 2G Correlator must
accommodate the ALMA Compact Array (ACA). The ACA specifications are
not yet established. Current ACA simulations assume 12 antennas of 7-m
diameter. For calibration purposes, the ACA will be correlated jointly
with about 4 antennas of 12-m.
par
item A total number of 8000 channels is the minimum required. This
seems sufficient for most astronomical observations. Observations
using multiple sub-bands and polarizations would accordingly have less
channels available per spectral product (per sub-band and/or
polarization).
par
A more ambitious goal would be to obtain 4000 to 8000 channels
per spectral product (sub-band and/or polarization). This goal should
be fixed by considerations of technical feasibility and cost.
Nevertheless, if the total number of channels were significantly
larger than 8000, there should be ways of selecting or compressing
them for further processing.
par
The Baseline Correlator provides 4096 channels in most modes.
When used at the maximum bandwidth, full polarization, 256 channels
cover 8 GHz, corresponding to a a resolution of 31.25 MHz. With one
polarization, 1024 channels give a resolution of 7.8125 MHz.
par
item Three-bit digitizing format and three-bit (or even four-bit)
correlation format are recommended to obtain high sensitivity by
diminishing quantization losses.
par
In its widest bandwidth, the Baseline Correlator provides a two-bit
digitizing format. In narrower modes, three and four bit correlation
are available (though three bit quantization at the digitizers and FIR
filter limit usefulness of the latter).
par
item A highest spectral resolution of 5 kHz is required. This
corresponds to 0.05 km/s at 30 GHz, which is necessary, e.g., for the
observation of lines in cold dark molecular clouds. The bandwidth
obtained at this highest resolution will be determined by the maximum
number of channels provided by the correlator (see item 2).
par
The Baseline Correlator can provide a resolution of 1.9 kHz
single baseband single polarization ; it is 15.3 kHz for full
polarization single sub-band. As in example D4 of Table 1 of Memo 194, a
resolution of 1 kHz is possible.
par
item A reasonable goal for the 2G Correlator is to provide 16 sub-bands
(in total, not per polarization). The equivalent number of sub-bands
in the Baseline Correlator is 8.
par
item ALMA will have the ability to be split in different
logically-independent sub-arrays, and to observe at a maximum of 4
different frequencies. Thus the 2G Correlator should be able to
accommodate a minimum of 4 independent sub-arrays and the ACA (see
item 1). The Baseline Correlator has the capability of accomodating
16 sub-arrays.
par
item For continuum observations of the Sun and flare stars, the
required minimum integration times are 10 milliseconds (specification)
and 1 millisecond (goal). To allow mapping large areas reasonably
quickly (on-the-fly mosaics) in spectral lines, the required minimum
integration times are the same: 10 milliseconds (specification) and 1
millisecond (goal). As specified in ALMA memo 192, observations of
fast pulsars would require integration times as short as 10
microseconds, but a non-imaging mode of the interferometer would be
sufficient, and the data could perhaps be collected by sampling the
phased array output with a modest off-line system, as currently done
at the VLA.
par
enditemize
par
newpage
par
sectionIssues Associated With DSB vs SSB Receivers
for the Initial ALMA Complement labeldsb
par
A number of ALMA memos (numbers 168, 170, 301, and 304) and reports to the
ASAC have recently considered the potential sensitivities of double sideband
(DSB) versus a number of single sideband (SSB, which here includes variants
such as sideband separating, or 2SB, approaches) receiver designs. While the
conclusions differ to some extent, the numbers and general trends driving the
potential decisions are similar and are worth summarizing:
par
-The potential sensitivity gains with various SSB options are greatest for
observations of transitions in a single IF sideband or for observations of
lines in separate sidebands where the needed correlator capacity is less
than that available.
par
-The potential sensitivity gains with SSB receivers increase as the receiver
noise contribution to the total system temperature decreases. That is,
SSB receivers provide improved performance as the atmosphere begins to
dominate the overall noise.
par
-For continuum or wideband spectral line observations that "fill up" a
correlator bandwidth matched to that available from the DSB receiver IF,
SSB receivers with the same IF bandwidth are sometimes less sensitive
because the DSB bandwidth is effectively sqrt(2) larger.
par
With the current estimates of achievable receiver noise temperatures, the
estimated improvements with SSB receivers range from 1.4-1.2 (low frequencies
to high) for observations in a single sideband (Memo 304, Figure 2). As
receiver noise temperatures drop, the improvement attainable with SSB
receivers gets larger. Under the same conditions, continuum observations
with DSB receivers are more sensitive, particularly at high frequencies.
It is worth stressing, however, that with better receivers SSB approaches
will be equal to or superior to DSB receivers for all observing modes,
and that the potential improvement corresponds to a very large number of
additional antennas. Recent work at submillimeter frequencies has
demonstrated that the SIS mixers themselves can operate near the quantum
limit, and that in the future it will be possible to build receivers that
are much more sensitive than those likely to be initially installed on ALMA
as our understanding of materials at THz frequencies improves.
par
At that point, SSB receivers will clearly be superior, especially if their
IF bandwidths can be made sufficiently large to occupy most of an atmospheric
window and fill a very large correlator with a single sideband (and with
dual polarization receivers). In the meantime, the overall gains (or losses)
in sensitivity with SSB versus DSB receivers are a complex function of the
assumed receiver temperatures, the atmospheric conditions under which
observations are performed, the correlator capabilities, and the temporal
mix of observing modes used by the array. It is largely differences in
these parameters that drive the differences in the various ALMA memos and
reports. SSB receivers also are more complex to design, build, and maintain,
and so if the gains are small or negligible then DSB receivers provide
better value from a total project perspective in terms of cost and risk,
especially early in the project lifetime.
par
Given the likely pace of design and development after ALMA construction,
it seems unavoidable that both DSB and SSB receivers will be implemented
on the array at some point. It is therefore important for the project not
to preclude either option at this time, at least in terms of making decisions
now that make it extremely expensive to implement new receiver layouts in
the future. Some specific recommendations, by no means exhaustive, might
include:
par
-Dual polarization, DSB receivers provide the best alternatives for bands
8-10 at present, and should be the baseline design. The correlator(s) must
therefore provide for phase switching demodulation of the upper and lower
receiver sidebands.
par
-Design and development of SSB receivers is critical for ALMA and should
continue. Decisions on when it is appropriate to implement SSB designs,
especially for the lower frequency bands which are likely to have SSB
implementations ready first, are best made by the Receiver and System
IPTs. The ASAC requests regular updates on the progress in this area,
especially as regards the first light receiver bands.
par
-The IF distribution and correlator downconverter systems should not
preclude the introduction of SSB (read 2SB) receivers, or at least should
not make the conversion to SSB/2SB approaches prohibitively expensive.
par
-The cryostat design, cryogenic systems, and interfaces should be compatible
with a gradual migration from DSB to SSB receiver cartridges.
par
newpage
sectionALMA Operations Plan: Recommendations and Issues
newedcommandee[1]mbox
newedcommandeten[1]mbox
newedcommandmmmillimeter
newedcommandsubmmsubmillimeter
newedcommandfirfar-infrared
newedcommandmirmid-infrared
newedcommandnirnear-infrared
newedcommandirinfrared
newedcommandlsunmboxL
newedcommandCIImbox[Cscriptsize II]
par
vspace0.5cm
par
centerlineit ASAC Operations Study Group:
par
vspace0.5cm
par
centerlineNeal J. Evans II, Roy Booth, Leo Bronfman, Yasuo Fukui,
Mark Gurwell,
centerlineJohn Richer, Seiichi Sakamoto, Peter Shaver, Christine
Wilson, Malcolm Walmsley
par
vspace0.5cm
par
bf Abstract.
This document is intended to provide recommendations regarding ALMA operations
from a scientific point of view. In some areas, we recommend further
study and discussion between the various entities concerned with operations
planning. We divided our
considerations into two main aspects: before and during the observations;
after the observations. In the first area (S C1-C6),
a sub-group led by Christine Wilson considered the issues.
Yasuo Fukui led the effort in the second area, which
covers issues of operational and support centers, archives, and
proprietary periods (S C7-C11). At the end of most sections, our
recommendations or topics for further discussion appear in italics.
We conclude with a summary restatement of the recommendations (S C12)
and topics for further discussion or study (S C13).
par
subsectionProposing to Use ALMA
Preparing the Phase I proposal will lead to the first encounter with the
ALMA operational system that most users will have. We believe that it is
particularly important that this encounter be as welcoming as possible so
that astronomers unused to radio interferometry are encouraged to observe
with ALMA. It is also essential that the refereeing process be clear and
informed by knowledge of what has been done and what is possible.
While the details of time allocation remain to be worked out, we focus on
conditions that we believe should be met by whatever method is eventually
adopted.
In particular, we consider the following areas.
par
beginenumerate
item Access to information about completed and currently scheduled
projects.
item Tools for preparation of both Phase I and Phase II proposals, such
as time estimators, ALMA simulators, etc.
item A process for technical review, including a quantitative measure of
the stringency of the requirements.
endenumerate
par
The simulation tools have relevance to Phase I, Phase II, and operations during observing,
and so are discussed in detail in the next section.
par
subsubsectionPhase I Proposal
par
The first step in planning any observing proposal is to assess what
has already been done. For data past the proprietary period, the
ALMA archive should provide this information. However, there will be
many observations that are not yet in the archive, especially in the
early years of ALMA. We believe that a more limited archive of information
about completed projects and a still more limited archive of
information about approved, but uncompleted, projects should be
available. For completed projects still in the proprietary period,
a prospective user should be able to learn the names of the proposers,
the coordinates covered, the source names, the frequencies, and the
rms achieved in the pipeline reduction. For approved proposals, all the
same information should be available, but the rms noise or integration time
approved by the review panel should be supplied. The frequency information
should include rest frequency and either velocity or redshift.
This information should be in
an easily searchable data base that could be in the regular ALMA archive
or in a separate data base.
par
The tools for proposal preparation should be easy to use and yet
powerful. On the simplest level, we agree with the SSR report
that the whole proposal process should be electronic. It should be
possible to upload proposals to the proposal data base, but also
to download one's own proposals, modify them, and upload them
again, up to the deadline time. The proposal should include enough
information for a scientific and technical review, as well as enough
information to allow automated checking of the final observing scripts
against the parameters of the approved proposal. This will mean that a detailed
source list is required, including information on coordinates, frequency
and field of view. There will have to be exceptions or special procedures
for time-variable sources or targets of opportunity. Clearly, time variable
sources and, with justification, other sources can be observed again
in the same way. There may be special cases, in which making source coordinates
or line frequencies public would be unfair.
In addition, in the case of very long source lists, particularly those
that are easily characterized by other means, alternative solutions
may be acceptable. The observer should be
able to apply for and justify an exception to the detailed source list
rule for Phase I proposals.
par
medskip
it Recommendation: Complete information on the source parameters
(coordinates, velocity, frequency, resolution, rms noise)
in approved and completed projects
(both proprietary and public) should be available in the archive.
par
subsubsectionPhase II Proposal
par
The main goal in the Phase II proposal is to create appropriate Scheduling
Blocks that realize the written scope of the successful proposal.
One issue is how to get advice from an expert if it is needed, and
where that expert should be located. From the user's point of view,
it would be useful to have expert advice located in a similar time
zone (see S 9); it might also
be important to be able to get expert advice in the native language
(even if Phase I and II Proposals are all in English). However,
as long as the advice is readily available when it is needed,
it could be possible to have all the experts located in Chile, if
that was the decision of the project. The amount of human
interaction in Phase II will probably be higher in the early
stages of ALMA and settle down to some lower level as the project
matures. However, there will likely always be some need for expert
advice in Phase II preparation from beginning observers or
from observers wishing to develop complex programs.
par
subsubsectionCalibrations
par
The key question here is what is the responsibility of the project
and what is the responsibility of the observer. Since we want ALMA
to be accessible to non-experts, some basic calibration responsibilities
need to be accepted by the project. For example, perhaps the Observing
Tool can be designed to be clever enough to make sure that the minimum
necessary calibrations are done to achieve some basic calibration
accuracy. Calibration strategies can be recommended or even required
by the system based
on the required calibration accuracy specified by the user.
Each time-contiguous piece of a program must always have a preamble and
a postamble to make sure a complete set of calibration data are available.
par
It is also important that all observers have access to flux
calibration information
from all programs or that the flux calibration is done by the system in a
regular way.
Further consideration of this issue will be appropriate once the calibration
strategy for ALMA is better defined.
par
medskip
it Recommendation: Routine calibration should be primarily a responsibility
of the ALMA system.
par
subsectionThe ALMA Simulator
par
It is important to have powerful tools to aid the novice in mm interferometry.
One should be able to specify things like resolution, field of view,
and rms in various frequency bands and receive a recommended set of
configurations, correlator setups, and integration times. The effects of
phase noise and decorrelation under different atmospheric conditions
should be included in the simulator. It would also
be important to receive a file with the beam map that is likely to result
from the proposed observations. The ideal system would fully simulate
ALMA observations of a model source, which could be a Gaussian, or any
other model distribution of intensities supplied by the user. This tool
would then be extremely useful for data analysis at a later time.
par
The ALMA simulator will also be invaluable in preparing Phase II proposals. For example,
a tool to assist in setting up mosaics and a tool to show the chosen
correlator setup overlaid on a simulated spectrum of the source
would be very useful. Many tools may be useful for both Phase I and
Phase II preparation. Non-standard observing scripts should
certainly be allowed for the expert user, although these should
be verified as much as possible. At a minimum, the verification process
should check that basic requirements such as pre/postambles,
calibration, and pipeline processing for the archive are included.
par
Given the basic policy that ALMA should be friendly to non-expert
users, the scheduling blocks generated by the default setting of
the software should be
good enough that most users, including experts, would be willing to
adopt the default setting to generate their scheduling blocks.
Recommended settings
may be particularly useful for observations of Galactic objects
in some of the ``standard'' lines and continuum, for which the
users may just need to specify the pointing center and the
required rms noise level. The issue of whether the project can
guarantee a requested rms noise level or only the time calculated by
the simulator is an open question not addressed here.
par
For complex programs, it would be useful to be able to simulate
running a set of interdependent scheduling blocks. This simulation could
perhaps function as the validation stage or could be more
sophisticated, i.e., a real observing simulator. This test could
turn up errors in the specification of the interdependency
between blocks, for example. It could be useful to
have the simulator include a weather model so that the observer
could see a longer program being broken up into several smaller
pieces, for example as weather conditions shift from day to day.
This type of simulator would check that the preamble and postamble
always work properly. An advanced simulator like this could
go a long way towards checking non-standard observing scripts.
par
It is quite clear from the recommendations above that a powerful
ALMA simulator is an important aspect of making ALMA maximally useful.
It is also clear that electronic data bases must be flexible, yet
secure. Much of this technology is available, and we need only adopt it,
but the ALMA simulator will be more challenging. Some of the current work
on the study of the imaging characteristics should provide a framework
for the simulator.
par
medskip
it Recomendation: ALMA should develop a powerful simulator that is capable
of a complete end-to-end observing simulation of a project composed of a number
of scheduling blocks.
par
subsectionProposal Review Procedures
par
At least in the early years
of ALMA, it will be important to have a technical review before the
scientific review. Use of the ALMA simulator can make much of this technical
review
automatic, as long as the simulator is updated regularly to take account
of ALMA development and experience with actual observations.
par
subsubsectionStringency
par
Part of this
technical review should be a quantitative measure of what has come to be
called the ``stringency". The stringency can be defined as
,
where
is the total observing time available and
is the
total time during which this project can be done. In practice,
will
be calculated based on the
required water vapor, seeing, pointing, uv coverage, sensitivity, etc.
The stringency is then the inverse fraction of the time that the observations
can be done, according to statistics that are built up over time. This
concept is described in the SSR document. This information should be
available to the scientific review panels to aid them in designing
a program that has reasonable coverage of the ``observing condition
phase space". Filling this space is a well-known problem for observatories
operating at submm frequencies. The stringency should be calculable from the
parameters given in the Phase I proposal and the ALMA simulator. In the
early operations phase, human judgment may be necessary to apply
appropriate corrections, but the goal should be an evolving simulator
that does this as automatically as possible. In order to check the
Phase II observing scripts versus the approved parts of the proposal,
it is essential that the review committee be able to add their
recommendations into the electronic data base of the proposal. In this
way, it should be possible for the committee to approve only parts of
proposals, to assign different priorities to different parts of
proposals, etc.
par
medskip
it Recommendation: ALMA should adopt the concept of stringency. This concept may be defined as
where
is the total observing time available and
is the total time
during which a given project can be done.
par
medskip
it Discussion: Consider further the definition of stringency:
do we need separate parameters for water vapor content, phase stability,
and wind conditions (re pointing)?
par
subsubsectionTAC Operations
par
While the structure of the time-sharing agreement is of course
the responsibility of the E-ACC, the nature of the Time Allocation
Committee(s) may affect the scientific productivity of ALMA.
Consequently, the ASAC should also have some input into the
functioning of the TAC. Scientifically, there are arguments both
for single and for multiple TACs, and of course the partners have
other considerations. For the following, we use the acronym TAC to
refer generically to one or more committees.
We offer here some preliminary considerations,
and we suggest that a study of the structure and functioning of existing
TACs for multi-partner observatories could be of value.
par
From a user point of view, there should be at least two proposal
deadlines per year (one per year is very restrictive, especially from the point
of view of student thesis work). Individual proposals need to be reviewed
both scientifically and technically, and need to be checked for
overlap with scheduled proposals and other proposals submitted in
the same period. In addition, the proposals need to populate the
available observing conditions well and may need to satisfy constraints
on the fraction of time awarded to each partner. We suggest
that, with a good simulator, the scientific and technical feasibility
could be handled by a single reviewer. However, checking for overlap
and in particular comparing proposals against available observing
conditions is more complex and probably should be an
observatory task.
par
There are many possible models for how proposal review might operate.
The reviewers may supply
reviews by mail to a TAC, or they may actually meet and make recommendations
to the TAC, or they may themselves constitute the TAC.
One set of questions concerns the reviewers.
Should there be a few
reviewers who grade all proposals on scientific and technical merit?
Should there be a few groups of reviewers, with each group reading
all the proposals in a single science category? Should there be a
large number of reviewers, perhaps 1-3 for each proposal with each
reviewer reading 1-3 proposals? Should the reviewers be exclusively
ALMA staff, or exclusively not ALMA staff, or some combination?
A second set of questions concerns the
TAC. Should the TAC be the same people who are also the reviewers?
Or should the TAC be a different set
of people? These questions probably do not need to be decided now,
but the division of tasks between the reviewers and observatory
staff will have implications for staffing levels.
par
The Science Software and Requirements document suggests that
``Reviewers should take into account the percentage of
observing conditions in each category and accept proposals accordingly.''
This is a very large task that will probably be best done by the
TAC with help from the observatory. In general, we may need
to populate a three-way space (RA, observing conditions, partner
share). This task will probably require some clever software
to display how things are progressing throughout the semester
as well as experienced people to monitor it and potentially
tweak the inputs as the semester goes on. One way to deal with this is to reject only truly infeasible proposals
in the lowest frequency bands (
GHz, or perhaps even
GHz,
depending on the weather statistics at the site). By keeping low priority
proposals that can use poor weather, we maximize the likelihood of ALMA
always having a source to observe. In fact, we might reject
only truly infeasible projects at any frequency and just give a very low
rating to poor proposals. One could then rely on the
dynamic scheduler itself to ensure a reasonable coverage of observing
condition space, as long as enough proposals were available to it.
par
The TAC will likely need to be able to assign different priorities to
different parts of a program. At a minimum, one could envisage an
accepted program that happened to include a previously observed
source, and so the entire proposal was approved except for a single
source. At a higher level, a program might be approved with different
rankings for several different sources or for the same source at
different wavelengths.
par
Another question to be considered is whether approved projects
are carried over from a previous semester and, if so, should
they get a higher priority for completion?
This decision might be a complicated function of how close to completion the
program is, how high a scientific ranking it had originally, and what
its stringency requirements are. Clearly highly ranked, high stringency,
nearly completed programs should be completed before new ones in a similar
class are started. It seems reasonable that programs that are highly
ranked and nearly completed be carried over and completed the next semester
with a high priority, regardless of their stringency.
par
medskip
it Recommendation: The ASAC should have a role in defining the operations
of the TAC to ensure that scientific considerations are included.
par
medskip
it Discussion: There should be further study of how the ALMA TAC should
work, including a review of how existing TACs operate.
par
subsectionFlexibility in Phase II and During Observing
par
One key issue for observers is how to maintain sufficient flexibility
to update scheduling blocks, for example, to take advantage of new information
gained from other telescopes or to incorporate results from
ALMA on the first few sources from a large sample. This desire for
observer flexibility will probably be constrained by the need to ensure that
the scheduling blocks match the approved proposal and that the set
of scheduling blocks is sufficiently stable with time that the
dynamic scheduler can operate efficiently. It should be
straightforward to make sure that the scheduling blocks match
the approved proposal if sufficient information is specified
it for each source in the Phase I proposal. A reasonable
compromise between infinite ability to change and a single fixed
submission might be the following: scheduling
blocks should be changeable until the program is started, and again
after any breakpoint is reached.
par
An alternative approach would be to allow only very limited
flexibility in updating scheduling blocks after the
proposal has been accepted.
The reasoning behind this approach is as follows.
Assume that the outputs from reviewing by the TAC
not only include scientific rating and technical
feasibility but also reflect some attempt to fit the program to
the available observing parameter space. The successful proposal
may thus be split by the TAC into several parts of different
ratings depending on the observing frequency, LST range of the
source, and required resolution, as well as the scientific merit and
technical feasibility of the entire proposal.
Allowing too much flexibility to the users after time allocation
is complete may reset all these careful considerations and lead to
problems with a time-varying ensemble of scheduling blocks.
Break points could still be used to allow the observer to evaluate
the status of the observations and perhaps update the scheduling
blocks. However, these updates would be limited to slight
modification of the pointing (i.e. because of possible offsets of
the spatial distribution of millimeter/submillimeter sources
to their counterparts in other wavelengths) or frequency (e.g.,
slightly wrong) or correction of obvious careless
mistakes.
par
One key parameter that observers might not be allowed to change
would be the resolution of their observations, since this
could affect the configuration schedule. In general, the continuous
reconfiguration currently envisaged makes this a complex subject worthy
of further consideration.
par
medskip
it Discussion: How much flexibility to adjust approved programs should be allowed
in the Phase II stage and once observing has started?
par
subsectionSetting Priorities in Dynamic Scheduling
par
The Science Requirements and Use Cases document gives a lengthy list of
factors to be considered in dynamic scheduling. To first order, these
factors can be divided into two categories: conditions that must be
satisfied for the scheduling block to be even considered for scheduling;
and conditions that are used to set relative priorities between eligible
scheduling blocks. Factors in the first category include things such
as LST range (is the source currently visible?) and atmospheric opacity
(can the required frequency be observed?). To some degree these types of
factors are mostly ``go/no go'' choices and are fairly straightforward, so
we will concentrate here on the second set of factors, those used to set
relative priorities.
par
In setting relative priorities, the most obvious determinant is it scientific
ranking. However, on instruments like ALMA for which certain frequencies
are only observable a small fraction of the time, it stringency should also
be an important consideration. For example, projects with lower scientific
ranking that require very good weather conditions may need to be given
preference above projects with higher science ranking that can use a
wide range of weather (for example, projects at 3 or 7 mm).
par
A final important consideration is the it execution status of a project.
This factor is designed to give some priority to completing projects that
require only a small amount of time to be finished. This is a particularly
important consideration for the project that is currently being executed.
For example, suppose the weather suddenly changes from 3 mm weather to
350 micron weather. How quickly the scheduler should decide to stop the
current project and move to one that will take advantage of the better
weather should be a function of the status of the current project. For
example, suppose the current project would be completed in just 10 minutes
(perhaps 5 minutes of source observations and 5 minutes of postamble
observations). It would probably make sense to complete the current project,
rather than stopping (and still needing to spend 5 minutes on the postamble),
and then spending perhaps 15 minutes at a later time (now including
preamble observations as well). On the other hand, if the current project
still needed 30 minutes to finish, it would probably be most efficient to
wind the current session up quickly and move along. It is clear from
this example that the typical time needed to be spent on preamble and
postamble observations associated with each session on a project will
influence the exact timing of these decisions. Also, completion in
this context may usefully be defined as completion of the project as a
whole, completion of a single source in a larger project, or completion
of a single source in the current configuration of a multi-configuration
project.
par
We suggest the following three factors are the important ones to
be considered in designing
the dynamic scheduler.
par
beginenumerate
item scientific ranking
item stringency
item execution status
endenumerate
par
These should not be considered absolute, but should be assigned some
weights. These weights might be different over different timescales.
For example, execution status might be weighted most highly if the
current program required only 5 minutes to finish, while stringency
and scientific ranking are clearly more important factors on
timescales of days or weeks, respectively.
par
Another consideration is whether to include a delay in the situation
when the weather changes. For example, if the weather is slowly degrading,
should we continue to observe the current project for some short period
of time in marginal weather before switching to a project that is
better suited to the current weather? Similarly, if the weather seems
to be improving, how long do we wait to make sure it is going to continue
improving before switching to a new program? Again, both these decisions
will be affected by the overhead involved in switching programs too
often, i.e., the time spent in preamble and postamble observations.
par
Finally, unlike what is described on page 6-8 of the ESO Operations
Proposal, it seems likely that eventually the dynamic scheduler will
have to be it almost completely automatic (i.e. it not
prioritized in real time by a support astronomer). Real-time
prioritization by a support astronomer of hundreds of projects is hard
enough to do at existing telescopes like OVRO and the JCMT, which have
programs using blocks of several hours and only a range of 3-4 in
frequency; it seems likely to be impossible for ALMA. However, one can
imagine the human scheduler making a choice, based on recent
experience, between a restricted set of projects presented by the
automatic scheduler. This experience should gradually be incorporated
into the automatic scheduler as much as possible. In the early days
of ALMA, when we are still learning about the weather conditions and
the algorithms, more significant human interaction with the dynamic
scheduler will probably be required.
par
medskip
it Recommendation: The dynamic scheduler should include science ranking, stringency, and execution
status as three of its key parameters.
par
medskip
it Discussion: How should the three key priorities for the dynamic scheduler
be balanced?
par
subsubsectionDynamic Scheduling of the ACA
par
The ACA will also require dynamical scheduling, and so this needs
to be considered in the studies as well. Some of the parameters of the
ACA will likely be somewhat different from the main array. For example,
the ACA will likely observe fewer projects but for longer periods of
time. Depending on how long is required for each project, this could
make it harder for the dynamic scheduler, for example, if the demand for
very good weather became very high.
par
If the ACA and the main array are run by two independent dynamical schedulers,
there will likely need to be some passing of information between the
two. For example, a program that has completed its observations with
the main array might get a higher priority to complete its observations
with the ACA and vice versa.
par
medskip
it Recommendation: A dynamic scheduler for the ACA needs to be included in the software
planning.
par
subsectionSupport while Observing
par
ALMA observing will be ``Service Observing'' for a variety of reasons
(primarily to make efficient use of ALMA in varying weather conditions).
One of the penalties that one pays for service
observing is that the astronomer cannot react in the same fashion
to unexpected astronomical results
(we do not know ahead of the observations what we are going to find).
One question that ALMA operations will
pose is whether the inevitable loss of flexibility that service observing
involves can be minimized.
Maintenance of flexibility needs to be done in a manner that does not
cause the efficiency of ALMA operations in general to be reduced.
par
To a large extent, ALMA should base its policy on experience in
existing institutions (Plateau de Bure, BIMA, OVRO, NMA). It is true
that ALMA will be
``different'' but most of these differences will only emerge after actual
experience has been gained. The SSR report indeed makes use of that
experience and seems a good zero-order attempt to outline a reasonable
approach that ALMA might adopt. The following are merely some reflections
on the ways in which ALMA might interact with observers during actual
observations.
par
One can envisage two ways in which observers can be involved in real time
in ALMA observations.
par
beginenumerate
item Look at the data using standard (pipeline) reduction procedures and
communicate to the ALMA operations center if things look wrong. We
will call this ``eavesdropping'' and some options for implementing
it are discussed in more detail below.
item Pre-program break points in the observations at which one would stop taking data for
a certain minimum period (say 24 hours) thus allowing a more balanced look at the
data quality and results.
endenumerate
par
The first of these is clearly beneficial in that experience suggests
that it only the observer is in practice sufficiently motivated to
note unexpected features in the data. It involves some organization
because, with dynamic scheduling, one will never be quite sure when a
certain set of observations will be carried out. However, it should
not require data handling by the operations center over and above that
needed by the staff checking data quality. It requires a standard
pipeline package that can handle in real time programs with a
reasonable data rate. This package should allow the observer to decide
whether his (her) scientific goals are being reached and to
communicate rapidly with the operations center in the event that
something is going wrong.
par
The question of break points is trickier and in our opinion must be
handled in a manner that allows the (human) scheduler in the ALMA
control center the final decision. Observers cannot be allowed to
break off every 5 minutes to look at the data! More importantly, ALMA
operations needs one person in charge who decides on a day-to-day
basis what happens. On the other hand, one can envisage cases where
for example, a follow-up observation should only be made in the case
that a certain source is detected. The observer could program the
observations in such a way that there is a break-point when the
critical RMS for detection has been reached. He(She) must communicate
the result to the operations center within a certain time subsequent
to the break-point. Breakpoints will likely be it required for
long programs as well as in the early years of ALMA, to protect
against wasting large amounts of observing time.
par
subsubsectionDifferent possible levels of Eavesdropping
par
beginenumerate
par
item At a minimum, eavesdropping means being notified that your
project is now being observed, and monitoring the images as they come
out of the pipeline in real-time, perhaps by looking at a website. This
option is the minimum required and is currently mentioned in the Software
Requirements and Use Cases document.
par
item A second level of complexity would be to allow the observer to phone the
operator to say that something is going wrong and the observation should
be stopped until the observer can figure out what it is. This could
function within the current scheme by the operator being able to insert
an instantaneous breakpoint into the program, and the program will
not be scheduled again until the observer clears the breakpoint. How the
operator inserts this breakpoint would need to be worked out; presumably
the observer could then clear it in whatever way is used to clear
pre-planned breakpoints. This option might be a good one to have, but
the SSR people would need to figure out how it could be done.
par
item A third level of complexity would be to allow the observer to make
real-time decisions that are something OTHER than pausing the
observations until a later date. An easy example would be when the
observer is measuring a long list of objects, and one of them comes in
brighter than expected. The observer might want to stop observing that
source (i.e. the observations are deemed complete for that source at that
frequency) and move on to the next source in his/her list. Alternatively,
something might not be detected that was expected to be, and the observer
might want to integrate longer at the expense of doing fewer targets.
Some of this might be handled by the use of preset breakpoints if the
system is clever enough; for example, in the first case, you could
specify
OR
OR
,
where any one condition causes the
observations to cease. It is harder to see how you would specify the
second case, where one needs to integrate longer (
?),
but it might be possible. The observer would also
need to be clever about how he/she uses the software. If these decisions
could be handled by clever use of breakpoints, it may not require
any software and people beyond what would be required for the
interrupt option described above. We need to examine how clever we
can make the breakpoint software while still maintaining a robust system. A
sophisticated simulator would provide a vital check of complex
interdependencies between scheduling blocks.
par
endenumerate
par
medskip
it Recommendation: Eavesdropping and breakpoints should be included
as an option in the operations plan.
par
medskip
it Discussion: How should breakpoints and/or eavesdropping be implemented
to avoid overly complicating operations?
par
subsectionOverview: Operations and Support Centers
par
Operationally, one may distinguish between the following entities:
par
beginenumerate
item
A Science Operations Center (SOC), responsible for post processing,
quality control, and delivery of the data products to the astronomer
and archive. It is an bf operational center, and as such is not
necessarily involved in software development (S 8).
par
item Regional Support Centers (RSCs), located in the partner
continents (i.e., Europe, Japan, North America, and possibly South America).
These RSCs should provide support to users during all phases of the
observing process, from proposal preparation to data reduction and analysis.
In the VO/Grid era, astronomers should have easy access to
the archive wherever they are (that is just a matter of bandwidth),
but they may also require assistance from the experts in their RSC,
and in some cases they may travel to the RSC for hands-on assistance.
par
endenumerate
par
We note that the RSCs are the entities formerly known as Regional Data
Centers (RDCs); the suggested name change reflects our thinking on
the primary function of these centers (see S 9 for further discussion).
par
subsectionScience Operations Center (SOC)
par
There should be one location where the data are processed through
the standard pipeline and a uniform quality is assured. The SOC
should house the master archive. It is essential that
there be only one such center, to assure the homogeneity and quality
of the final data products. The location of the SOC is not
a scientific issue, but it should be a function of the ALMA observatory
rather than any of the partners. It should be accessible to everyone.
Chile seems a logical choice, as it is ``neutral ground" for the project (in
which case Santiago would be preferred, as it is easier to recruit
staff there), but this is not essential.
par
medskip
it Recommendation: There should be a single SOC, operated by the
ALMA observatory, where the pipeline produces and stores the official
archive.
par
subsection Regional Support Centers (RSCs)
par
As mentioned above (S 7) and implicit in the change of name, we
believe that the primary function of the regional centers should be
support of observing. This support runs the gamut from proposal preparation
to data access, reduction, analysis, and perhaps publication.
The physical location of the
bf data is less relevant in a world of high speed electronic
communication. The required number of such RSCs is unclear
from a scientific view point: suggestions range from a it single
RSC to four (one each in Japan, Europe, South America, and North
America). If only one such center were created, the ``Regional"
appellation would obviously be inappropriate. As with the issue of the
TAC (S 3.2), the E-ACC will doubtless consider various aspects of this
issue; we offer
here some considerations based on the goals of scientific productivity
and maximum impact of ALMA data on astronomy in general.
par
Both before and after observations are obtained with ALMA, the astronomer
will need continued interaction with support centers. This interaction will
be of varying degrees, depending on the experience of the astronomer
and the type of project undertaken. This interaction is primarily
related to preparing the best observing plan,
obtaining the data, whether pipeline reduced images or raw
visibilities, along with any ancillary data (``archiving''), and
use of or assistance with data reduction and/or analysis
(``data analysis support'').
par
The facilities the astronomer will utilize in this stage include one
or more of the Regional Support Centers (RSCs),
along with the astronomer's personal
workstations or home institute's other computing resources. A
working-model of the RSC is given in the ESO operations proposal. We
wish to provide advice in more detail on the possible types of
interactions that could arise and should be supported.
par
The major roles that the RSCs it should have are as follows:
par
beginenumerate
par
item Support in preparation of proposals, both Phase I and II.
The novice observer may need assistance even in Phase I to access
the archive to find out what has been done, to obtain and understand
technical information, and to avoid proposing impossible projects.
The support in Phase II will probably be more important, as the
generation of non-standard observing scripts may require consultation
with experts.
par
item Analysis Support - The RSC will provide help remotely to the
users. The help should span the range of simple advice related to the
default pipeline data reduction algorithms, as well as more
sophisticated requests, such as using advanced or specific algorithms
for reduction of the data. An issue here is whether the centers
should supply computing resources for really big reductions over the
net. These interactions should be basically fulfilled remotely, but
those who hope to get deeper into the processing may want to come and
stay at the RSC for a while. The RSC should then be able to support
them on a face-to-face basis.
par
endenumerate
par
In addition, the RSCs it may be responsible for the following roles:
par
beginenumerate
par
item The Data Portal - Another function of the RSCs is
to facilitate the transfer of data from the Array to the User. The
current plan (see the ESO operations proposal) is that each RSC
receives all the observed data from the Science Operations Center
(SOC) and creates a ``mirror'' archive, while the SOC keeps the master
archive. These archives include cleaned images as well as the raw and
calibration visibility data and other array data (weather, etc.), e.g.,
as requested in the Phase 2 Proposal Process (P2PP) obtained under the
proposal of the astronomer. A second option to be considered is for
the RSC to supply a gateway to the archive without keeping the
actual data (for example, compiling an archive of only the header data
for use in search and location of specific data). In this case only
the SOC would hold a true archive, and the load in hardware at the RSC
could be considerably less. In either case, the access to the RSC for
data retreival is basically to be made remotely by the astronomer.
par
item Software Development - One goal of the RSCs will be to work on
improved algorithms for data reduction, analysis (through Aips++ or
other packages), and archive mining, as well as development of tools for
interaction with existing or future archives, such as the National
Virtual Observatory in the US. This includes not only an
interface for retrieval of data from these archives, but a facility
for transfer of basic pipeline reduced data into the larger,
multi-band (NVO-type) archive.
The software should
also be portable to platforms at the workstations of home
institutions. The data rate we can handle between the RSC and the home
institute should increase quite a lot by 2008-2010. However, in the
early stages of ALMA it may require more reduction and analysis ``over
the net'', particularly for researchers at smaller institutions.
par
endenumerate
par
The relative roles of the individual RSCs has yet to be formally
defined. ALMA will need to balance the need to provide efficient (and
perhaps ``local", meaning within the continent) services to astronomers
against the cost of supporting several RSCs. The former is possibily
best handled by having the RSCs remain quite similar (the ``Clone''
model) with nearly identical resources, capabilities, etc. In the
face of limited resources, it may make for more cost-efficiency for
each of the RSCs to have areas of specialization, with some necessary
``core'' capability at each RSC (the ``Distributed'' model).
It is natural for
the support astronomers of each center to have their own areas of
expertise and for users to seek out the ``world-expert" in some area
of reduction, analysis, or computing, no matter where he or she resides.
par
Each has pitfalls. For example, the Clone model suggests a high
degree of redundancy that may not be necessary, as well as the need
for some sort of control to maintain the uniformity of the centers.
The Distributed model may force an astronomer to interact with an RSC
many time zones away, which may be inconvenient, and in some cases may
require the astronomer to travel to the RSC of a different partner for
face-to-face assistance. A more significant danger of the distributed
model is that the capabilities and compatibilities of the RSCs will
have a tendency to diverge, and a strong control will be needed to
ensure that they don't wander too far from each other.
par
At this time, we recommend a compromise of sorts, suggesting
that the centers should have a core of functionality that is common
to all. This core should be part of the ALMA operations to ensure
commonality. Partners should be able to add to this core functionality
with their own funding to meet different needs. For example, needs for
support of computer resources,
graduate students, travel, and publications differ greatly
between the partners, and the RSCs may differ in the extent to which
they provide this kind of support.
par
Finally, we note that the it scientific need for more than one RSC
is not as yet well-substantiated. We can conceive of integrating the
RSCs into a single Support Center, located within the scope of any of
the partners or even at, or adjacent to, the SOC. From the standpoint
of the human (political and social) elements, the need for more than
one RSC is justifiable, as the long-term RSC staff from each of the
three partners would presumably prefer to live closer to ``home''.
The RSCs also represent the most visible structural elements of ALMA
for the general public of each of the partners.
par
medskip
it Recommendation:
Regional Support Centers (RSC) should be responsible for support of the
observer, from proposal preparation through data reduction and analysis.
They may also provide data portal and software development.
They should be operated with an international and
collaborative spirit.
par
medskip
it Recommendation:
Each RSC should have a core functionality provided by the ALMA observatory.
The partners may choose to add other functionality (computer resources,
financial support for travel, students, publications, ...) from their own
resources outside the ALMA project.
par
medskip
it Discussion:
The core functionality of the RSC should be further considered and defined.
par
medskip
it Discussion:
How many RSCs do we really need?
par
subsectionArchive Issues
par
The role of the ALMA archive should be twofold. One is for the
pre-observing users to learn what has already been done and what is
planned to be done in a given observing session. The
other is the real archive of all the ALMA data open to the worldwide
community anytime.
par
The real ALMA archive may be further divided into two parts. The first
includes the visibilities, the standard reduction scripts, and the images
produced by those scripts.
The second includes the images produced by the observers, which may
contain substantially enhanced images or other relevant data products.
The responsibility for the second part should belong to the individual
observers. It needs further consideration if the second part should
remain within the official ALMA framework or rather should be
organized outside it.
par
The ALMA archive should be fully compatible with the Virtual
Observatory (VO) and the Grid paradigm on which the VO is based. ALMA
will be the first major observatory coming on-line post-VO. In the VO
context, the ALMA archive data should be available independent of
location and there should be no distinction between the ``master" and
``satellite" archives. Through the Grid, the astronomer's desk-top
computing power can be enhanced relative to what he/she has available
locally. If ALMA is going to provide processed data in a
user-friendly way to a non-expert community, then it should take
advantage of the VO environment and the underlying GRID technologies.
par
There are two major and distinct development areas:
par
beginenumerate
item development of the post processing, quality control and data analysis
software;
par
item development of the archive for the post-VO and Grid era.
par
endenumerate
par
medskip
it Recommendation:
The ALMA archive should be open to the worldwide community and be fully
compatible with the Virtual Observatory (VO) and the Grid paradigm.
par
medskip
it Discussion:
Should images produced by observers, as well as those produced by
standard scripts, be placed in the official ALMA archive?
par
subsectionProprietary period
par
We believe that a fairly short proprietary period will help ALMA to
have an early impact, along with the production of quality pipeline
images (as opposed to quality pipeline visibility data), allowing
astronomers of all flavors to utilize ALMA with minimal discomfort and
maximum scientific weight. A proprietary period of regular projects
should be 1 year as is commonly used in the currently working
instruments, with some exceptions.
par
A key question here is when the clock starts to count. The simplest
solution is to use the time when all the observations in a project are
completed on ALMA. This method works for most of the projects of
short observing times. We need to consider the effect of proprietary
periods on long-term projects extending over a long time frame
(years), including Key or Legacy projects. The term legacy might be
taken to mean large blocks of time in exchange for no or very short
proprietary period.
par
We do need to allow some flexibility in the proprietary period. We suggest
that it be possible for the observers to propose periods different from
the standard 1 year. Proposing a shorter period could be considered
a plus by the TAC. On the other hand, longer periods may be justifiable
for some projects, where large data sets are needed and the proposers
cannot produce scientific papers until all the data are in hand.
Some student theses are examples of such programs.
We considered a longer proprietary period for student theses, but
decided instead to recommend that it be possible to apply
for longer periods on a case-by-case basis.
par
It should be avoided as much as possible that the
community cannot see the data for years from such long-term projects,
since they are often valuable and of strong impact on science.
We therefore consider setting an upper limit for the proprietary
period like 2 years from the first day of observation for any
long-term projects.
par
medskip
it Recommendation:
The proprietary period for regular projects should be 1 year as is commonly
used in the currently working instruments, with some exceptions for legacy
projects and for long-term projects.
par
newpage
subsectionRecommendations
par
beginenumerate
par
item Complete information on the source parameters
(coordinates, velocity, frequency, resolution, rms noise)
in approved and completed projects
(both proprietary and public) should be available in the archive.
par
item
Routine calibration should be primarily a responsibility
of the ALMA system.
par
item ALMA should develop a powerful simulator that is capable of a complete
end-to-end observing simulation of a project composed of a number
of scheduling blocks.
par
item ALMA should adopt the concept of stringency. This concept may be defined as
where
is the total observing time available and
is the total time
during which a given project can be done.
par
item The ASAC should have a role in defining the operations
of the TAC to ensure that scientific considerations are included.
par
item The dynamic scheduler should include science ranking, stringency, and execution
status as three of its key parameters.
par
item A dynamic scheduler for the ACA needs to be included in the software
planning.
par
item
Eavesdropping and breakpoints should be included
as an option in the operations plan.
par
item
There should be a single SOC, operated by the
ALMA observatory, where the pipeline produces and stores the official
archive.
par
item
Regional Support Centers (RSC) should be responsible for support of the
observer, from proposal preparation through data reduction and analysis.
They may also provide data portal and software development.
They should be operated with an international and
collaborative spirit.
par
item
Each RSC should have a core functionality provided by the ALMA observatory.
The partners may choose to add other functionality (computer resources,
financial support for travel, students, publications, ...) from their own
resources outside the ALMA project.
par
item
The ALMA archive should be open to the worldwide community and be fully
compatible with the Virtual Observatory (VO) and the Grid paradigm.
par
item
The proprietary period for regular projects should be 1 year as is commonly
used in the currently working instruments, with some exceptions for legacy
projects and for long-term projects.
par
endenumerate
par
newpage
par
subsectionTopics for further study
par
beginenumerate
par
item Consider further the definition of stringency:
do we need separate parameters for water vapor content, phase stability,
and wind conditions (re pointing)?
par
item There should be further study of how the ALMA TAC should work, including a review
of how existing TACs operate.
par
item How much flexibility to adjust approved programs should be allowed
in the Phase II stage and once observing has started?
par
item How should the three key priorities for the dynamic scheduler be
balanced?
par
item
How should breakpoints and/or eavesdropping be implemented
to avoid overly complicating operations?
par
item
The core functionality of the RSC should be further considered and defined.
par
item
How many RSCs do we really need?
par
item
Should images produced by observers, as well as those produced by
standard scripts, be placed in the official ALMA archive?
par
endenumerate
par
newpage
sectionALMA Science Day Program
par
vspace0.5cm
par
centerlinebf Thursday September 13, 2001
par
vspace0.3cm
par
centerlineit School of Engineerings, Universidad de Chile, Santiago, Chile
par
vspace1cm
par
begintable[h]
begincenter
begintabularrll
noalignmedskip
noalignmedskip
9:00 - 9:10 & Opening & L. Bronfman
noalignbigskip
9:10 - 9:40 & The ALMA Project: General Aspects & S. Guilloteau
9:40 - 10:10 & The ALMA Project: Technical Aspects & A. Wootten
10:10 - 10:40 & The ALMA Site &T. Hasegawa
par
noalignbigskip
par
10:40 - 11:00 & Coffee Break
noalignbigskip
par
11:00 - 11:25 & Observational Cosmology & P. Shaver
11:25 - 11:50 & Distant Galaxies & P. Cox
11:50 - 12:15 & Nearby Galaxies & C. Wilson
12:15 - 12:40 & The ISM and Regions of Star Formation & Y. Fukui
noalignbigskip
12:40 - 14:45 & Lunch hosted by School of Engineerings
noalignbigskip
14:45 - 15:10 & Cosmochemistry & E.F. van Dishoeck
15:10 - 15:35 & Interstellar atomic carbon & S. Yamamoto
15:35 - 16:00 & Star and Planet Formation & N.J. Evans
16:00 - 16:15 & Concluding Remarks & G.A. Blake
par
noalignbigskip
16:15 - 16:45 & Visit to School of Engineerings
par
noalignbigskip
par
17:00 - 18:00 & Public Conference :
& `Observaciones de la formacion
de estrellas y galaxias con ALMA' & R. Bachiller
par
noalignmedskip
par
18:00 - 18:30 & Questions from public and media
par
endtabular
endcenter
endtable
par
enddocument
Next: About this document ...
Al Wootten
2002-04-05