ALMA/US Imaging and Calibration Group

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

Date: 13 Mar 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.

--------

Agenda

--------

News -- Wootten

Configuration PDR Summary (by Pierre Cox)

The important progress which has been made in the configuration group during the last six months was presented at the Configuration PDR which was held in Grenoble just after the ASAC meeting. Reviews were given of the zoom spiral and donut/double ring designs and the advantages and disadvantages of both concepts were exposed in great depth, in particular on the issues of operational flexibility. The studies do confirm that both designs have similar performances in imaging capability. Further work needs to be done on the compact E configuration, the extended A+ configuration and on the incorporation of the ACA. For these future studies, the PDR Configuration panel provided a series of recommandations including: the optimization of the single configuration; the design of the 14~km array should be optimized for the resolution and hybrid configurations should be investigated; the design of the compact configuration should be aimed at obtaining maximum surface brightness; the configuration schemes should be as flexible as possible; and the ACA configuration should be studied on a basis of 12 to 16 antennas with a diameter of 7-metre well adapted to te 1.25~D contraint. Further details can be found in the panel recommandations of the Configuration PDR.

ASAC Report I haven't seen this yet. The telecon is tomorrow, with the agenda here .

--------

Compact Configuration -- Kogan, Wootten

Leonia poses a question about roads, compact configurations, etc.

Related to this, Darrel posed a question to the Antenna group:

Question: on both the Vertex and EIE antennas, can an antenna, sitting on its pad, be picked up by the transporter from all azimuth angles? Or, does a transporter have to approach a given pad from a specific direction? This came up in the context of the specific antenna configuration in the close-packed arrangement. It affects where openings between antennas need to be left, for antennas at the middle of the close packed array to be accessed.

Kinglsey responded:

The answer depends on the transporter not the antenna. If the transporter is able to fully straddle the antenna and allow it to rotate fully in azimuth then the answer is yes. If the transporter constrains this motion the range will be limited to about +/- 30 degree range. Current concepts and design do permit the full motion of the antenna in azimuth. We should know more from ESO/Lund next month when they deliver the transporter requirements document. Keep in mind that the trade off will be the transporter width that will also be a concern for close packing.

Torben responded: As it looks now, the answer to your question below is yes. You can approach the antennas from any side, but it is not part of the specification so until CDD it is formally still open.

It seems to me difficult to proceed with configuration design until this is settled. Darrel: Should we insist on it becoming an antenna spec, or should we warn the configurations group that they can't assume it will be possible to approach an antenna from any angle?

I think it should be a spec. I don't see how configuration design can proceed otherwise.

--------

ACA, Radiosonde - Holdaway

--------

Correlator -- Wootten

investigations by Ray Escoffier and John Webber show that reprogramming of the correlator will allow more efficient correlation. In this mode the 3 bit initial quantization and 4-bit requantization after FIR filtering, results in a gain in efficiency from 88% for the nominal baseline correlator to 95% in this more efficient mode, according to calculations by Brent Carlson. Some tradeoff in correlator capacity must be made to run in this very efficient mode; these are detailed in the table at http://www.cv.nrao.edu/~awootten/mmaimcal/CorrelatorTable4bits.html I would like to make a point of clarification. At the meeting, I reported that investigations by Ray Escoffier and John Webber show that reprogramming of the correlator will allow more efficient correlation. There was some question as to exactly what this efficiency would be. In this mode the 3 bit initial quantization and 7-bit requantization, with 3 or 4 bits passed along to the correlator after FIR filtering, results in a gain in efficiency from 88% for the nominal baseline correlator to 95% in this more efficient mode, according to calculations by Brent Carlson. Some tradeoff in correlator capacity must be made to run in this very efficient mode; these are detailed in the table at this location.

Note that this results in a correlator satisfying point b(i) of the recommendations, of achieving full 3bit efficiency.

Detail: Brent Carlson performed some simulations to quantify these efficiencies. He has some preliminary numbers that he emailed Webber before incorporation into a memo:

1. 3-bit quantization and correlation; no FIR filters; threshold step = 0.65 sigma; odd output encoding (-7,-5,-3,-1,1,3,5,7); expected correlation coefficient=0.5; 10^8 samples; correlator output converted to normalized correlation coefficients by dividing by the geometric mean of lag 0 autopower. Efficiency is 0.962 (3.8% SNR loss).

2. Same as 1. only 4-bit/15 level with integer encoding and threshold step = 0.374 sigma. Efficiency is 0.987 (1.3% SNR loss).

3. 3-bit initial quantization (8-level; odd encoding; 0.65 sigma thresh. step) and 4-bit requantization (15-level; integer encoding; 0.374 sigma thresh. step); WIDAR filtering/processing with -12.5 dB cutoff filters (for minimum additional SNR loss) and filter gain compensation; 12-bit LUT output in the FIRs; 10^8 samples; lag 0 power normalization; expected correlation coefficient=0.5. Efficiency is 0.943 (5.7% SNR loss).

The efficiency in 1. x the efficiency in 2. is 0.962 * 0.987 = 0.949 (5.1% SNR loss) which agrees reasonably well with the efficiency found in 3. (5.7% SNR loss).

I am also running tests with a correlation coefficient of 0.1 and the results thus far are in good agreement with the above results.

In the ALMA correlator case where you are using 4 x FPT n=3, 2-bit multipliers to do 4-bit correlation, the 4-bit output encoding must be odd encoding--which may give a slight (<~0.1%) efficiency increase (according to a curve in Hovey's master's thesis on 14-level correlation we have here).

Perhaps that largest source of error in these results is that I just produce normalized correlator coefficients by dividing the raw correlator data by the lag 0 autopower. This does not take into account the slight up-turn in the curve of raw coefficients vs normalized coefficients. This, however, should make the above efficiencies slightly better than presented.

--------

Site

Steve has identified further times of microclimatic events from the CBI logs.

  • 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?

    ----------------------------------------------------------------------

    --------


    Action Items 1 Feb2000

    DECISION: Configurations--PDR upcoming?

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

    --------

    Travel

  • Wootten - 14-113 Feb MMAOC

  • ------