Dutch concept of ALMA control building.
Dutch concept of ALMA control building.
The next ASAC meeting is on Monday 10th April (telecon) at 21:00 UT.
Minutes for meeting by Peter Shaver.
ALMA Science Advisory Committee
Leiden, 10-11 March 2000
Participants: Baars, Bachiller, Benz, Blake, Booth, Bronfman, Brown, Cox, Crutcher, Emerson, Evans (chair), Fukui, Guilloteau, Gurwell, Kawabe, Kurz, Menten, Nakai, Richer, Scoville, Shaver, van Dishoeck, Walmsley, Welch, Wild, Wilson, Wootten, Yamamoto, Yun
1. Welcome and Introduction
N. Evans opened the meeting with introductory comments and presented the ASAC Charter which requires that a report be prepared before every ACC meeting. This was a major objective of the present meeting. Sub-groups were to be formed on the second day of the meeting to make recommendations in specific areas for ratification by the whole ASAC.
2. Interim Report on Definition of Phase 2
R. Brown summarized plans and progress towards Phase 2. All scientific wishes from both sides have now been incorporated into the baseline project, and this defines the scientific and technical scope of the project. The "value" of the project has yet to be completely defined. An estimate will be given to the ACC in April, and final figures by the next ACC meeting in Oct/Nov. The impact on the final project definition of the cost estimates, and of the likely Japanese participation, were discussed.
3. Report from ALMA Liaison Group
S. Guilloteau reported on the three meetings of the ALG that took place on 3 Decmber, 16 February, and 9 March. ASAC advice is needed for the ACC. Priorities for Japanese involvement in ALMA were suggested by the ALG in two categories: the baseline project, and an enhanced project. S. Yamamoto commented that the priorities listed by the ALG are identical to the Japanese priorities, with objectives of (1) greater sensitivity, (2) increased spectroscopic capability, and (3) improved imaging quality. It is important to Japan that it makes a major contribution to the baseline project. S. Yamamoto said that a proposal will be prepared this year for full Japanese participation starting in FY 2001. Notes are included in the ASAC report to the ACC.
4. Discussion
The above reports were discussed by the ASAC in a restricted session.
5. Receivers
W. Wild reviewed recent developments in the receiver area, outlining the current specs and design plans. He pointed out that, with over 1000 receiver components per antenna, as many as three failures per day can be expected in total, so reliability is crucial. There was discussion about whether the marginal improvement in sensitivity using SSB rather than DSB really justified the significant increase in complexity and cost. J. Lamb is preparing a report on this. The ALMA schedule requires working mixers a year from now, and the receiver construction scenario must also be decided within a year. W. Wild listed a number of issues for ASAC input: priorities for receiver bands, total power stability, WVR specs, polarization requirements, calibration accuracy, SSB vs. DSB, and priorities in the case of cost reductions.
6. Report on Polarization
R. Crutcher presented the report on polarization, starting with an overview of the science case. He said that the science drivers imply optimization for continuum linear polarization mapping, perhaps for a prime polarization observing frequency (eg. 345 GHz?). The relative advantages of linearly vs. circularly polarized feeds were discussed. Calibration problems and possibilities were discussed, including the possibility of a linearly polarized CW signal from an aperture in the secondary. Recommendations included optimization for polarization measurements in one band, stable polarization properties of both primary and secondary, a CW source in the secondary, OTF capability also for polarization, and implementation and testing of polarization capabilities in the test interferometer. The Report is included in the ASAC report to the ACC.
7. Report on System Review
J. Baars summarized the System review meeting recently held in Garching, and presented the recommendations of the Review Committee. A DRAFT of each of the the System Review and of the Test Interferometer CDR reports is available.
8. Discussion
The above reports were discussed by the ASAC in a restricted session. There was a strong feeling that SSB was not justified, in view of cost, complexity, and reliability. There was general agreement that high-quality polarization capability is a major objective, although doubts were expressed about the usefulness of a secondary CW signal as it does not calibrate the primary or secondary. Dual polarization should generally be provided, at least at the lower frequencies, although this is not so clear at the higher frequencies.
9. Q-Band Receivers
J. Carlstrom presented his report via telecon, and took part in the discussion that followed. He gave a cost of US$ 3.2m, much lower than estimated by the project, although this was later debated. The sensitivity would be more than that of the upgraded VLA, with a loarger FOV giving faster mosaicing. ALMA's contribution would be on 1-100 arcsec scales, where it would be essentially unique. A wide variety of potential science was outlined. S-Z and CMB studies would be major drivers, and the potential of ALMA in this field would bring strong support from CMB researchers. J. Richer showed results from simulations at Cambridge, which strongly support Carlstrom's conclusions. K. Menten pointed out that this is the frequency range where free-free emission shows up best relative to synchrotron and dust emission. The engineering problems of incorporating the Q-band receivers were discussed. Design possibilities have to be further explored. It was generally felt that this band should be incorporated if the cost is reasonable and it does not disturb other bands. The Report is included in the ASAC report to the ACC.
10. Report on Configurations and Long Baselines
K. Menten gave a scientific case for the longest (10-20 km) baselines, showing the high resolution performance at high brightness sensitivity and giving examples of stellar envelopes and planet formation.
M. Yun reviewed the work done by the Configuration Working Group. The design constraints include the image quality (UV coverage, image library, short spacings), the site (topology, pipeline, etc), and practicalities (cost of pads, transport, frequency of moves, calibration). The site constraints are severe; shadowing, ground slope. The strawman array concept consists of:
a) compact array (150m)
b) intermediate arrays (150m - 3km)
c) 10+ km array The topological constraints can be met; 12 km is better than 10 km from this point of view. Most of the discussion involves the intermediate array, and there are a few different concepts: spiral, double ring, and zoom. Final decisions must be reached this year. L. Bronfman commented that an expansion of the science reserves (including outrigger stations) could be considered in the context of the renewal of the agreement for use of the land. Issues for the ASAC: provision of an astronomical image library, matching resolution vs operational ease, resolution vs clean beam, ultracompact array.
11. Report on Antennas
R. Kurz summarized the status of the prototype antenna contracts. They were signed on 21 and 22 February. The PDRs are scheduled for June, and CDRs for September (US) and October (Europe), and acceptance in October 2001 (US) and November 2001 (Europe). Tests and evaluation are scheduled for the period January 2002 - March 2003. The contractors must update their cost estimates for the production antennas late this year, and give a binding bid after delivery of the prototypes.
12. Report on Total Power, Nutating Secondaries
J. Welch summarized his report on total power observing with ALMA. He commented on the need to map large structure efficiently - too large for nutators with their limited throw. There are three broad possibilities: nutating secondaries, focal plane chopping (hard to achieve good balancing of beams), and on-the-fly scanning (requiring gain stability of 10**-4 in 1 sec). This only concerns the continuum, as the sky is not the limiting factor for line observations. The OTF mapping can be done, using all 64 antennas, assuming the 10**-4 gain stability. He recommends that OTF be made possible on all 64 antennas, that this be tested on the prototype, and that at least one nutator be provided for comparison. The nutator should be capable of a few Hz at 3 arcmin throw. S. Guilloteau commented that this could also be done with nutators on just four antennas, although it was pointed out that using 64 antennas averages out pointing systematics better and gives better stability. The Report is included in the ASAC report to the ACC.
13. Report on Water Vapour Radiometry
J. Richer reviewed the status of WVR developments for ALMA. A 3' offset gives a 1m offset at 3 km altitude. Probably any offset <10' would be acceptable. Anomalous refraction will limit mosaicing. The baseline 183 GHz system involves an uncooled Schottky; a cooled Schottky could be considered, but probably not SIS. J. Richer gave a list of open issues - whether the 183 GHz system should be pursued, at what cost, use on the prototypes, requirements for the system. N. Scoville said that cooled Schottky should be used as the baseline. K. Menten asked about the use of HFETs, in view of their reliability; J. Richer replied that there is still a problem of stability. W. Wild and J. Welch expressed concerns as to the full adequacy of the atmospheric modelling. J. Welch commented that, in conditions of poor seeing, one could just observe at low frequencies. The Report is included in the ASAC report to the ACC.
C. Wilson reported on the new IR WVR system being developed by D. Naylor at Lethbridge. It operates near the peak of the Planck curve for atmospheric temperatures. The wavelengths only include transitions from water vapour. The detector is simple and high-speed. The theoretical atmospheric models are supported by the FTS measurements. Comparison with the 183 GHz system gives quite good results (although they were pointed in slightly different directions). There was a question as to the effect of clouds, which needs study. The plan is to operate the system on Mona Kea for 3 months. It was commented that 1 sec timescales must be monitored. The Report is included in the ASAC report to the ACC.
14. Discussion
R. Brown went over the costing of the Q-band receivers, in view of J. Carlstrom's comments. He reiterated that he projected cost is $12m without the dewars, or $20m with.
There was further discussion on the above agenda items. R. Booth asked whether Chajnantor should be kept as the site of the compact array, and there was general agreement that it should. The configuration group must come up with a recommendation for the intermediate array configuration. K. Menten made the point that ALMA must be able to make excellent images at resolutions comparable to that typical of optical/IR telescopes, 0.1-1 arcsec, as that is what many in the wider community will require.
For the prototypes, it was emphasized that polarization and fast scanning tests must be made on the test interferometer, although the 183 GHz tests are better done on Mona Kea. A. Wootten said that the prototypes will be used over the longer term to test equipment for the production antennas.
Concerning the WVR, it was recommended to build a second generation 183 GHz system, for a final prototype, before production. The baseline should include a cooled Schottky. The WVR should also be used to estimate extinction and emission. Anomalous refraction was de-emphasized; in cases of poor seeing one could concentrate on the low frequencies.
15. Preparation of Report
Sub-groups of the ASAC met separately to propose recommendations for the individual items listed below. The recommendations were then presented to the whole ASAC for discussion. The final recommendations will be combined in a report to be presented to the ACC. That report is now available in html. It will also be available in PDF format.
Responsibilities for writing the various sections of the report were assigned as follows:
a) Introduction - N. Evans
b) ALG issues - P. Cox
c) Receivers - Blake
d) System - R. Bachiller
e) Configurations - K. Menten
f) Antennas - M. Walmsley
g) WVR - J. Richer
h) Future projects, issues - N. Evans (UC array, configurations, Lo system, s/w, spectrum manag., site (Bronfman), outreach)
i) Summary - N. Evans
Appendices will be comprised of the reports: polarization (Crutcher), total power (Welch), WVR (Hills & Richer), IRWVR (Naylor, Wilson), Q-band receivers (Carlstrom)
The timescale is: 17 March (contributions due); 21 March (to committee for comments); 26 March (comments back to N. Evans); 29 March (complete report available).
17. Election of new Vice-Chairperson
J. Welch was elected as the new vice-chairperson of the ASAC. K. Menten replaces N. Evans as chairperson now, and J. Welch will become chairperson following the Oct/Nov ACC meeting.
18. Next Meeting
The next ASAC meeting is on Monday 10th April (telecon) at 21:00 UT.