ALMA/US Imaging and Calibration Group

Minutes for meeting Tuesday, 22 May 2001 at 4:00pm EST.

(Agenda in black, comments from notes on meeting (minutes) in blue.

Date: 22 May 2001

Time: 4:00 pm EDT (2:00 pm Socorro, 1:00 pm Tucson)

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

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

ALMA - Tom Lehrer

The loveliest girl in Vienna
Was Alma, the smartest as well.
Once you picked her up on your antenna,
You'd never be free of her spell.

Her lovers were many and varied
From the day she began her - beguine.
There were three famous ones whom she married,
And God knows how many between.

Alma, tell us,
All modern women are jealous,
Which of your magical wands
Got you euros and dollars and yens?

The rest.

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Agenda

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News -- Wootten

AAS Meeting -- I will be there Sunday, Monday and Tuesday. Brochures and a new display will be there. I am still trying to find pieces of ALMA for display. But the delivery room here was piled high today with ALMA freebees for the AAS meeting. 5000 ALMA suckers (no lemon), 2000 ALMA shades and 2000 ALMA globe yo-yos.

News - proposal, etc.

The charge from the EACC to the EAEC is: "Prepare a draft recommendation on the definition of the three-way ALMA, for discussion at the next EACC meeting 12-13 June 2001. The recommendation should define a three-way project with a value per partner for Phase 2 of 220 M USD (FY2000). The recommendation should also consider what could be accomplished with an additional 10 percent funding."

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Fast switching -- Dispersive submillimeter phase -- Holdaway

Mark has been working on a re-evaluation of fast-switching, with ALMA parameters.

Dispersive phase is of concern to both fast switching and radiometric corrections. Dispersive phase plot one and Dispersive phase plot twoare for 0.5 mm PWV for opacity, delta PWV = 0.012 for the phase (this is about 1 degree of phase on the site testing interferometer, which are among the best conditions for 300 m. We can, of course, do better with fast switching, but this will dominate the error budget at submillimeter wavelengths. So, I think we need to do SOMETHING about the dispersive phase for submillimeter observations, but it looks like we don't have an extremely demanding problem -- ie, a first or second order effort will probably get the residual dispersive phase errors less than other phase errors in the system.

Mark's Executive Summary:

Dispersive phase is not a problem in the millimeter, but must be dealt with in the submillimeter.

Radiometric Phase Cal:

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With a good transmission model, we can convert the radiometrically-inferred PWV columns above each antenna into a non-disperive delay PLUS the dispersive phase (which will depend upon things like T and P, so we need to have the atmospheric model in place at the time we make the corrections -- ie, in near-real time).

Radiometric handles dispersion OK, but fails to handle dry air turbulence, which is invisible to the radiometer but will affect the interferometer at some level (what level? Dave Woody and Larry D'Addario speculate).

Fast Switching:

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Fast switching will calibrate at 90~GHz and observe the target source at other frequencies, scaling the 90~GHz solutions to the target frequency.

As the dry air fluctuations are non-dispersive (or very nearly so), we can treat them with fast switching.

However, fast switching has potential problems with dealing with the somewhat dispersive wet phase. Fast switching does not give us the absolute phase or the absolute water vapor column above the antennas, so the transmission model's estimate of how dispersive the phase is does not help us.

Question:

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Can we be clever and combine these two techniques so that we can handle both dry air turbulence and dispersive phase in the submillimeter? Intuition says: "Yes". I am working out the details.

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ASAC The next ASAC meeting was held 17 May at 1415UT. The proto-agenda has links to documents. We will discuss them also.

Please see my comments on SSB/DSB operation of ALMA.

Thoughts from Project Scientists: