ALMA/US Imaging and Calibration Group

Agenda for meeting Tuesday, 13 Feb 2001 at 4:00pm EST.

Date: 13 Feb 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

ALMA - Tom Leher

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

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-- Wootten

Table 10.3 Selected correlator modes

# of Digitizers	Bandwidth/ Digitizer	Cross-pol Products?	Channels/ Product 2 bit X 2 bit	Channels/ Product 2 bit X 4 bit	Channels/ Product 4 bit X 4 bit
8	2 GHz	Yes	64	--	--
8	2 GHz	No	128	--	--
8	1 GHz	No	256	128	64
8	500 MHz	Yes	256	128	--
8	250 MHz	No	1024	512	256
4	2 GHz	Yes	128	64	--
4	1 GHz	No	512	256	128
4	500 MHz	Yes	512	256	128
4	250 MHz	No	2048	1024	512
2	2 GHz	Yes	256	128	64
2	1 GHz	No	1024	512	256
2	500 MHz	Yes	1024	512	256
2	250 MHz	No	4096	2048	1024

This table is meant to update that in Chapter 10 of the Project Book for the specific case of operation of the ALMA Baseline Correlator in a 4 bit 16 level mode. The default personality of the correlator is as defined in Table 10.1, reproduced above. The 4bit mode could be available through the downloading of a new ‘personality' into the correlator chip, an operation which could be accomplished very rapidly (seconds) in the array. Under this new personality, the correlation format line above would be changed to 4 bit 16 level and operation would be restricted to bandwidth per digitizer of 1 Ghz or less, and at each mode half the number of channels would be available as would be available using the default personality. For many scientific applications, a gain in sensitivity is achieved with very little cost in throughput. John explains further:

Normal multiplication = multiplier is 4-level X 4-level

2 X 4 multiplication = multiplier is 4-level X 16-level

4 X 4 multiplication = multiplier is 16-level X 16-level

This is accomplished in the following manner. In normal mode, the multipliers in every chip are used to multiply the two most significant bits coming out of each FIR filter (which carries 7-bit precision in its final stage). In 2 X 4 mode, half the elemental multipliers work on normal data, and half work on the two most significant bits from one antenna and the two next most significant bits from the other antenna, but at the same delay values as the normal multipliers for that particular channel. In 4 X 4 mode, 4 different cross- products are calculated for the same delay values:

"high" bits X "high" bits

"high" bits 1 X "next highest" bits 2

"next highest" bits 1 X "high" bits 1

"next highest" bits 1 X "next highest" bits 2

In either 2 X 4 or 4 X 4 modes, it is up to software to reassemble the various products from the accumulators which service them in order to get the full-precision product.

It is because some multipliers must be assigned to the extra cross-products that the frequency resolution decreases.

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Polarization

Johan Hamaker has provided

Intensity measurements with feeds in p.a. 0 and 45 degrees.

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Receiver Optics

James Lamb has written the ALMA Receiver Optics Report which may be accessed at his URL.

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Site

Steve has identified times of microclimatic events from the CBI logs. There has been no collection of Pampa la Bola data since September. Radford has made a preliminary reconnaissance of the data we have. Seiichi will get JP data when he visits in March.

By examining the site data retrospectively, can we find a signature for these events?

Try to predict events since Steve's.

Look carefully at 225 GHz sit and stare (was it working?) during those events.

What else?

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Configurations, PDR

ACA documents from Min and Morita, agenda.

Mark pointed out that he had looked at your strawman E configuration and was pleased to find that there was very little shadowing for declinations between -70 and +40 for +/- 2 hours from transit. However, partly (we guess) because of the necessity of getting transporters into the center, it was perhaps not optimal for homogeneous array operation. He feels that this strongly suggests that the ACA may be necessary for ALMA to work well as a homogeneous array. We noted that your E configuration design was a strawman and that not a lot of effort had gone into determining its properties.

Other comments, from Steve mostly I think: The memo should be made a little more forceful in its conclusions.

The memo states that the 8m array produces better images. However, the 6m mosaic pattern may not be optimal. There is more redundancy in the 8m pattern. In practice better results may be achieved if another ring of pointings were made, outside the single ring used in the simulation. I think this is similar to what Tamara found in her experiments, though I don't think she ever wrote this up.

We asked the same question about Morita's simulations. Mark said that Morita used a very large rectangular grid of pointings.

We discussed how the ACA really impacted the configuation discussion. The general agreement was that it did so because one might design the E array differently if one knew that there would be an ACA. There is also the question of what the ACA configuration itself would be.

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Action Items 1 Feb2000

DECISION: Configurations--PDR upcoming?

DECISION: Implementation of 183 GHz WVR? 22GHz at OVRO, VLA?

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Travel

Wootten - 14-113 Feb MMAOC

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