ALMA/US Imaging and Calibration Group

Agenda for meeting Tuesday, 2 Jan 2001 at 4:00pm EST.

Date: 2 Jan 2001

Time: 4:00 pm EDT (2:00 pm Socorro, 2:00 pm Tucson)

Phone: (804)296-7082 (CV SoundStation Premier Conference phone).

Past ImCal minutes, etc on MMA Imaging and Calibration Division Page

"NSF is also pleased to be playing a leadership role in developing the international partnership for ALMA--the Atacama Large Millimeter Array." --remarks of Dr. Rita Colwell, Director, NSF, at the Dedication of the Green Bank Telescope, August 25, 2000.

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Agenda

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News -- Wootten

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Use Cases -- Mangum

We should discuss some of these use Cases in more detail. A list is available. Let us begin with going over:

Observe Single Field

Reduce Single Field

Setup Single Field Comments on Mark's and Crystals comments on these from last time.

A. Wootten to restart the matter of special observing modes.

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ASAC Meeting -- Wootten

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Special Polarization Meeting Thursday -- Wootten

Crutcher has proposed a special meeting on polarization on Thursday. We will make a proto agenda. Dick's discussion guide:

Polarization observations have been defined by the ALMA Science Advisory Committee as being one of the important science drivers for the ALMA telescope(ref: March Report). The polarization science given the highest importance was the mapping of linear polarization, both over ALMA telescope primary beams and over multi-pointing mosaic fields. Hence, both interferometer and single-dish polarization must be addressed. The March 2000 ASAC report proposed the requirement for 0.1% polarization mapping fidelity after calibration. This requirement of 0.1% polarization fidelity after calibration does not lead directly to specifications on antennas, receivers, and software, since the combination of instrumental polarization induced by receivers and antennas both play a role, along with the procedures for calibration. In order to insure that the polarization requirements will be met, it is essential that coordinated planning across these areas take place.

The present ALMA Project Book does not address how the science polarization requirements will be met. Indeed, things seem to be going the way they usually do with radio telescope design. One discovers that to make polarization work well will compromise other things, so polarization is given low priority and polarization science is compromised. ALMA examples include adopting linear rather than circular polarization feeds, having all feeds oriented identically rather than half rotated at 45 degrees (which would allow use of software to derive optimally all 4 Stokes parameters simultaneously), and having all feeds off-axis (which will introduce significant primary beam instrumental polarization). All of these design considerations have sound justifications in order to optimize performance for Stokes I observations, but at the expense of polarization observations. There does not appear to be any plan for insuring that the potential of ALMA for polarization observations will be met.

The following design features should be reviewed to see whether it might be possible to give some weight to polarization in the ALMA design.

1. Having the linear polarization response of half of the antennas at 45 degrees to the rest would provide an established method for measurement of all four Stokes parameters. This arrangement would require measuring 4 correlations, which would reduce the allowable maximum bandwidth to be correlated if only Stokes I is desired. Is this loss of proven polarization capability worth the tradeoff of higher bandwidth? If so, could it be recovered by having half of the receiver units rotatable through 45 degrees?

2. Having all receivers off-axis ignores the recommendation that one (at least) receiver be optimized for polarization observations. Off-axis receivers will produce instrumental polarization that is dependant on position within the primary beam, making it more difficult to map polarization over large fields of view. Different antennas, particularly those of different sizes, will have different instrumental polarization characteristics. Is it possible to have a prime polarization receiver channel (345 GHz) on axis?

3. Even if the ALMA design were most carefully optimized for polarization, there will a variable instrumental polarization response over the primary beams of the antennas. To have any realistic hope of being able to do polarization mapping, it is essential that the polarized beam patterns be stable over time, temperature, antenna pointing, etc. In order to calibrate polarization science maps, the polarized beam of the antennas will have to be measured by mapping the polarized beam pattern at the 0.1% level on the spatial scale of the synthesized beam. This will be a time consuming operation - to be practical, it will have to be done once (or infrequently) and applied over extended periods of time. This will require that the beam pattern be stable and repeatable. Is this a formal design consideration for the antennas? How will this be tested before an antenna design is accepted?

4. How will the polarization calibrations be done? Darrel Emerson presented a design for a bandpass calibration system at the Berkeley ASAC meeting that could also be used for polarization calibration of the receiver. This would consist of an amplitude and phase stable signal that would be broadcast into the receiver, so one could essentially continuously calibrate the gain and phase difference between the two nominally orthogonal receptors on each antenna. If both receiver gain and phase can be very accurately calibrated, it should be possible to derive all four Stokes parameters from orthogonal linear feeds with common orientation, with acceptable loss of fidelity in the Stokes parameter derived from $X - Y$. Is this a firm component of the ALMA plan?

5. Provision for testing the polarization characteristics of the two-antenna test array should be made. This will give the opportunity to test the system hardware and software for interferometer and single-dish polarimetry and identify problems before additional contracts are signed. What is the plan for a full testing of the ALMA polarization system to insure that the science polarization goals will be met?

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Richard M. Crutcher

Professor & Chair, Astronomy Department

Chief Application Scientist, NCSA

University of Illinois

1002 W. Green Street

Urbana, IL 61801

Voice: 217/333-9581

Fax: 217/244-7638

Dick Crutcher suggested that we have a polarization teleconference. This will mostly discuss his concerns about the design of ALMA diverging from polarization friendliness, but it will also alert us all to the operation of ALMA as a system to measure polarization, what is lacking, what can be remedied, what cannot. Not a bad idea, I think. He will work on talking points, which I will pass around and we can discuss next Tues. I suggest that we have invited:

Butler

Brogan

Holdaway

Mangum

Myers

Radford

Wootten +?other ImCal folk interested?

Glendenning

Napier

Lamb

Payne

Wild

Emerson

Kawabe

D'Addario

Guilloteau

Crutcher

New Year/URSI/Configuration telecon constraints suggest it occur 1230pm EST Thursday 4 Jan 2001.

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Report - Wootten

Another monthly report has been assembled!!

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Configurations

Steven calculated fidelity images for the CYG, MPD and SDC examples for the B resolution at the same time as the C array ones posted earlier, so has posted them for perusal at his site.

Details for the Configuration PDR

1) Date and Place

Date: February 26 and 27

Location: IRAM, Grenoble

2) Attendant List (PLEASE COMPLETE)

Bryan Butler

John Conway

Frederic Gueth

Stephane Guilloteau

Steven Heddle

Mark Holdaway

Leonia Kogan

Jerome Pety

Koh-Ichiro Morita

Simon Radford

Francois Viallefond

Adrian Webster

Al Wootten

Min Yun

+ ASAC members to be confirmed (M.Ishiguro + others)

Possible invited members / reviewers (to be contacted urgently)

Lee Mundy

Dave Woody

Eric Keto

...??

I would like to have also some representative from the logistic (D.Hoffstadt if possible).

3) Documents

To be ready 2 weeks in advance for distribution, specially to the outside members. We already have a lot, but are weak on operation issues. Gross distribution

- Doughnut concept (Yun / Butler)

- Spiral concept (Conway)

- Multi-configuration aspects (Viallefond)

- Compact array issues (Morita / Guilloteau)

- Site issues (Radford / Butler)

- Operation concepts (Wootten / Guilloteau) (oops, have to work here...)

- Cost issues (Hofstadt / ? )

Goal of the meeting: select a baseline design to continue with detailed implementation work ...

Koh-Ichiro Morita has made a summary of my recent imaging simulations for ACA. The summary may be seen at this URL. Any comments or questions are welcome.

Brief summary: