Report of Calibration Strategy Review Committee

on

ALMA Calibration Strategy Preliminary Design Review

Cambridge, England, 21-22 June 2001

Committee Members:

ALMA Division Heads/IPT Leaders

Jaap Baars (jbaars@eso.org), Bob Brown (rbrown@nrao.edu), Darrel Emerson (demerson@nrao.edu), Stephane Guilloteau (guillote@iram.fr), Tetsuo Hasegawa (tetsuo.hasegawa@nao.ac.jp), Kohno Kotaro (kotaro@nro.nao.ac.jp), Morita Koh- Ichiro (morita@nro.nao.ac.jp), John Payne (jpayne@nrao.edu),

Simon Radford (sradford@nrao.edu), Gie Han Tan (ghtan@eso.org), Wolfgang Wild (wild@astro.rug.nl), Al Wootten (awootten@NRAO.EDU)

External Reviewers:

Mel Wright (wright@astro.berkeley.edu), Anne Dutrey (Anne.Dutrey@obs.ujf- grenoble.fr), Bob Sault (Bob.Sault@atnf.csiro.au), Peter Schilke (schilke@mpifr- bonn.mpg.de)

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 desired accuracy.

General comments

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.

4) Although the SSR group actively pursues examination of the observing modes for ALMA they should do so in close collaboration with the Calibration Group.

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% at Band 7 and below, and 3% above Band7. Another possibility which was mentioned was 1% * (nu/100 Ghz) but the committee does not currently endorse this. At 3mm standing waves may compromise achieving this; work should be directed at understanding this aspect of calibration.

Specific details:

Absolute 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.

Receiver calibration

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.

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.

Phase calibration

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. For instance, at high frequencies dispersive effects may play a role in the atmospheric phase, with a different character than instrumental phases.

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.

Photonic calibration

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 system).

Bandpass calibration

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

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?

Atmosphere

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 weather station.

Pointing

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.

Polarization

Discussion of polarization calibration was deferred to a subsequent meeting. The photonic system needs further investigation to know whether it will be sufficient.

Actions Needed

1) Receiver calibration needs engineering realization. Materials need to be identified for vane calibration and incorporated into a working version for testing. Testing of the dual load calibration system should continue.

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 Guilloteau's Amplitude Calibration Strategy Memo–how stable will the receivers be and how often will these various calibration steps need to be performed? The front end group proposed to measure the stability of a prototype receiver to determine this; Payne suggested 1E-3 as a guess. What level of saturation will be encountered? The Calibration Group should propose a lower limit for the saturation temperature as this number is not now known. How stable will sideband gain ratios be in a tunerless receiver? The front end group proposed the working assumption that this would be quite stable. 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.

A prototype receiver needs to be developed and installed on one of the prototype antennas; the project should present sufficient resources to enable this.

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.

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.

5) Effects of 'anomalous' refraction on the site need to be investigated. The ASTE antenna may help considerably in this.