Report of Calibration
Strategy Review Committee
ALMA Calibration Strategy
Preliminary Design Review
21-22 June 2001
ALMA Division Heads/IPT
Jaap Baars (email@example.com),
Bob Brown (firstname.lastname@example.org), Darrel Emerson (email@example.com), Stephane
Guilloteau (firstname.lastname@example.org), Tetsuo
Kohno Kotaro (email@example.com), Morita Koh-Ichiro (firstname.lastname@example.org),
Simon Radford (email@example.com),
Gie Han Tan (firstname.lastname@example.org), Wolfgang Wild (email@example.com), Al Wootten
Mel Wright (firstname.lastname@example.org),
Anne Dutrey (Anne.Dutrey@obs.ujf-grenoble.fr), Bob Sault (Bob.Sault@atnf.csiro.au),
Peter Schilke (email@example.com)
The Committee believes
that although a start has been made, there is a substantial amount of work
remaining before we can be sure that ALMA may be
calibrated to the
1) Getting an end-to-end description of the calibration system (whether
this be stability requirements on the hardware or calibration observations
or calibration technique) needs to be pursued vigorously. Moreno and Guilloteau's
work on the times required to calibrate the instrument is also obviously
an important adjunct to this.
2) It is attention to detail in much of this work that is required in the
long run, and it is too easy for approaches which involve sophisticated
modelling to fail to note or solve the real problems. It is important to
prototype as much as possible of the calibration system, and preferably
in an environment which is as similar to Atacama as possible.
3) The lack of coherent direction in the present design and development
stage of ALMA clearly reflects on work in the calibration area. A coordinated
Calibration Group is needed to direct and assess efforts. At the present
meeting, representatives of the system, receiver and science groups were
in attendance; this seems a sensible core for this group.
A Calibration Group has been named. Membership
includes: Front End Group: Carter and Vaccari; Software: Lucas; System:
Mangum; Science: Cotton, ASAC representative Lee Mundy; Polarization: Cotton,
Brogan, Holdaway and Myers; Atmosphere and WVR: Richer, Butler, Pardo;
and Project Scientists Wootten and Guilloteau.
4) Although the SSR group actively pursues examination of the observing
modes for ALMA they should do so in close collaboration with the Calibration
Lucas is the liaison between SSR and Calibration
5) The ASAC has recommended a goal: "The ALMA calibration specification
of 1% for absolute intensity is adequate scientifically perhaps even a
bit aggressive." This may be achievable in the millimeter region but will
be difficult in the submillimeter. The committee recommends that the specification
for acceptance be achievable, for instance 1% * (nu/100 GHz). At 3mm standing
waves may compromise achieving this; work should be directed at understanding
this aspect of calibration.
Welch described a system under study at BIMA incorporating a standard gain
horn used to observe a standard object. The calibration is then transferred
to the antenna by comparison with the antenna receiver, thence to the array.
Study should continue at BIMA and migrate to the ALMA project book for
incorporation into the array.
In conjunction with points 1 and 2 above, it is important for the project
to devote resources to implementing the semi-transparent vane calibration
system, on an existing antenna or on one of the prototype antennas. This
will test basics of the design but the final complex design can only be
tested with an ALMA dewar and prototype receiver system.
This is under investigation at IRAM.
There is some interest in this for the Green Bank
Telescope also. Mike Stennes, receiver engineer there reports:
For an 86 GHz receiver front-end having just
one 5- or 6-stage HEMT amplifier, a 300 Kelvin load would not drive
the amplifier into a nonlinear regime, for bandwidths of 20 GHz or so.
(GkTB is approximately 83 nW for G=30 dB, B=20 GHz, and T=300K).
The CDL 86 GHz amps can output up to 1 microwatt without worry of nonlinearity.
This corresponds to an input temperature of 3,623 K. In my case,
with the GBT 3mm receiver, I need to consider the use of two 86 GHz HEMT
amplifiers. Here, the gain would be 60 dB rather than 30 dB.
The second amplifier would certainly saturate when the feed is looking
at a 300 K load. Therefore we are investigating the use of semi-transparent
vanes or cold loads. The alternative would be to to try continuum
detection at IF (after the first mixer).
ALMA Memo #401 Saturation by Noise and CW Signals
in SIS Mixers by A. R. Kerr (NRAO/Charlottesville, VA) 14/12/2001
discusses some aspects of saturation in SIS receivers.
Proposals include the semi-transparent vane system, and the dual load calibration
system described in the project book. For the former, materials need to
be tested to identify one whose scattering properties are acceptable. Engineering
is apparently soluble though complex and should be carried out on a prototype
antenna. The dual load system provides continuous monitoring of the atmosphere
and an engineering model exists. However, questions about standing waves,
frequency dependence and access/reliability cause some concern; further
investigation should be pursued.
The phase calibration system employs novel elements water vapor radiometry
and fast switching. The radiometry remains in a boutique stage employed
by some few observers for demonstrations. It needs to be developed so that
observers use it in the course of executing normal scientific experiments.
Extension to 183 Ghz is urgent as nothing will highlight the problems implementing
this on ALMA more than a prototype system will. Simulations should be performed
to help guide the hapless observer toward employing the system which will
do his science the most good. Simulation of calibration at one frequency
and transfer to another should be considered. WVRs will be totally useless
for dry fluctuations. There are other instrumental phases that will still
need to be solved for, such as incorrect baselines. Scaling astronomical
phase measured at 3mm to other frequencies ignores some residual instrumental
phase errors that may not scale with wavelength
The IRMA device has sensitivity but details of its implementation remain
unclear how does it respond to particulate water. Some discussion was devoted
to an IRMA sweeping a cone on the radio axis as a seeing monitor.
The photonic calibration system is promising but needs to be demonstrated
in the field. Point 2 is particularly relevant here how does the system
interact with other systems on the telescope (particularly the antenna/receiver
At the ALG meeting of 24 February 2002, Masato Ishiguro
reported on tests recently carried out in Tucson. Wootten's notes
Photonic cal: photodiode measured two weeks
ago in Tucson. measured output power of photodiode integrated
into log periodic antenna with hyperhemispheric lens over range 100
GHz to 1.2 THz using an FTS. The polarization properties change
with frequency owing to use of the log periodic antenna (+/- 25% typical)
so measured in two polarizations. Calibrated with blackbody radiator.
Data is under evaluation, about 1 microwatt at 400 GHz seems about
right, some signal may have been seen to 800 GHz, where coupling between
photodiode and fibre was only 1%, which can be improved. The
tests were considered generally successful, working on data, tests,
Bandpass calibration will be complicated by weak available sources, standing
waves and other problems.
For single antennas in total power mode sideband gain ratio calibration
is to be performed in interferometric mode.
Single antenna calibration was notably lacking in discussion although originally
on the agenda. Calibration of single antennas differs from calibration
of interferometers in critical ways. More attention should be paid
the standing wave patterns in the antenna owing to possible problems with
the dual-load calibration system and the antenna of the photonic calibration.
This requires collaboration between Antenna, Science, System and Receiver
groups, at least. What is an acceptable level?
This should be a topic to be covered during the ALMA
Modeling of the atmosphere has progressed very well. The effects of ice
particles and liquid water in the higher frequency windows cannot be accounted
for well by the WVR alone. A standalone FTS at array center could provide
a diagnosis for this. When particles are present it is unclear the WVR
will be useful. A temperature sounder operating at 60 GHz was described;
this off the shelf item should be operated in conjunction with the array
Pointing calibration seems well in hand. Wave front tip/tilt problems could
conceivably cause 20-50% amplitude errors at the highest frequencies but
need further investigation. This was identified as a possible implementation
of IRMA. The scanning IRMA system sounds very promising, but is totally
unproven and no more than a concept. Demonstrating this would seem to be
a priority. Using a guide star for pointing sounds a good technique if
calibration of the offset in the optical and radio axes, which is likely
to be frequency and elevation dependent, can be accomplished. This technique
needs to be demonstrated at the the 0.5 arcsec level.
Discussion of polarization calibration was deferred to a subsequent meeting.
The photonic system needs further investigation to know whether it will
1) Receiver calibration needs engineering realization. Materials need to
be identified for vane calibration and incorporated into a working version
Testing of the dual load calibration system should continue.
Status 4 March 2002: Douglas Bock provided
the following report to Cunningham and Radford:
Since the write-up in ASP Conf 217, we have focused
(somewhat sporadically) on trying to make accurate and reproducible measurements
of the coupling coefficient. However, we have
had limited manpower and telescope time to pursue the project. Measuring
the coupling coefficient requires the entire system, since we cannot
tune antennas individually. It also needs good observing weather!
The prototype calibration system is installed
on antenna 6, which uniquely has an NRAO 6-junction 1mm receiver, and so
it should not have
gain compression. However, when we measure the
calibration difference signal (coupled to about 2% of the beam) we find
that it varies with
time at the level of a few percent. The switching
mirror is repeatable; we cannot account for the variation. Perhaps it is
related to the changing sky total power. Meanwhile, measurements of ambient
and nitrogen loads intercepting the entire beam are very stable.
We have also modified our correlator code so we
can measure the calibration signal with some spectral resolution. We see
a standing wave
which may be due to some mismatch in the coupling
to the loads. But before we can investigate the spectral properties in
detail, we need to
track down the problem(s) in our measurement
of the broadband coupling coefficient.
Once we can measure the coupling coefficient reliably,
we want to test the scheme in interferometric mode and try to calibrate
observations. So we need to replicate the system
on a second antenna. There are some mechanical and control issues to be
sorted out on the prototype on the first antenna, but they are not fundamental
to the calibration technique (they just make it somewhat cumbersome to
use). I believe BIMA still has an important role to play in this
part of the project, since we already have in place the necessary telescopes
and control and data acquisition systems. It seems that ALMA may not have
these in time to test this system before a decision on whether to use it
has to be made. An instrumentally inclined postdoc or staff scientist who
can spend significant time at Berkeley and Hat Creek on the project should
be able to push this forward, but unfortunately we do not have that person
here right now. The staff who have been working on this project are all
heavily committed to other projects at the moment.
2) The final calibration design will result from iteration between the
groups constituting the Calibration Group. In particular, the receiver
group should supply a reaction to Memo No. 372. How stable will the
receivers be and how often will these various calibration steps need to
be performed? What level of saturation will be encountered? How stable
will sideband gain ratios be in a tunerless receiver? Although this is
critical for development of a working system, prototype receivers
probably need to be built to push the calibration system toward final design.
We suggest that when the third prototype antenna arrives at the VLA site,
the evaluation receiver from one of the earlier antennas be transferred
to it and the prototype receiver from which they were taken be outfitted
with prototype receivers and calibration system. This is an urgent necessity.
A response to Memo No. 372 is eagerly awaited.
At the face-to-face ALMA/NA DH meeting 25 Feb 2002 the subject of equipping
an ALMA prototype with a prototype receiver front end was discussed.
The first order of priority is to test the prototype antennas, with that
report due January 2004. It is possible that at that time a prototype
of Band 6 might be available but that is probably optimistic.
3) Characterization of the standing wave patterns on the prototype antennas
should be performed and assessed.
4) The WVR and fast switching schemes need further work. The former does
not yet work in a production fashion at 22 GHz. The combination of the
two and formulation on which to employ under what conditions needs to be
understood. Simulations are needed.
A 22 GHz system is in the field at OVRO and one is
under construction at the VLA. Tests with a 183 GHz system have been
underway since August 2001 at the SubMillimeter Array on Mauna Kea.
Fast Switching with ALMA had not been investigated thoroughly in the past;
Holdaway has produced ALMA Memo #403 Fast Switching Phase Correction
Revisited for 64 12 m Antennas (2001/12/17)
5) Effects of refraction on the site need to be investigated. The ASTE
antenna may help considerably in this.
The ASTE antenna has arrived and is being erected
at Pozos Tres.