MMA Imaging and Calibration Group

Minutes of meeting Mon, 19 October 1998 at 11am ET.

Date: Monday, 19 October, 1998

Time: 11:00 am EDT (9:00 am Socorro, 8:00 am Tucson/CA)

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

Past minutes, etc on http://www.cv.nrao.edu/~awootten/mmaimcal/mmaimcal.html

Agenda

--------

At this meeting we were to discuss configurations. However, Tamara is just back from U. Md. and I don't know that she will want to lead an hour's discussion of this first thing Monday morning. Other items:

MAC Meeting: Review (Al)

Areas of concern are items which may drive cost but whose scientific justification is not clear to all:

(1) Nutating subreflector - do we need one?

Al mentioned the JCMT experiments with the Dutch 'DREAM' mode which Hills plans to follow up with further experiments. The desire was to integrate on source continuously, rather than chop off source and lose that time in addition to ramp-up transit time to the off. In 'DREAM' mode, the subreflector was driven to the points where array integration was desired in triangles until an image was made. Reportedly, jerky motion tended to compromise SCUBA performance, as apparent microphonics from the subreflector. Liszt reported that some DREAM mode data had appeared. Hills planned experiments with smoother, lissajous figure driving of the secondary. Radford pointed out that the JCMT subreflector is unbalanced, leading to freedom of motion but compromise in performance in just this way. We have no immediate plans for bolometers on the MMA, nor do we plan to move the subreflector in creative patterns, as the antennas will be capable of fast precision driving. Bryan is reportedly examining the Holdaway memo on this. We would like to have the Holdaway memo available in some form for discussion at the MAC meeting.

(2) Fast switching - does it drive design unnecessarily?

To some extent, this requirement comes from the desire to obtain short spacings 'on the fly' and to another extent it comes from the desire to move from source to calibrator speedily to calibrate out atmospheric variations. In practical effect, this requirement has steered the antenna design to high Q designs, which have ancillary benefits as well.

DTE pointed out that while the water vapor radiometer was expected to get an improvement in phase tracking by a factor of a few, we really needed to do better than it could provide. Furthermore, in the ~10% of the time during which the 183 GHz line may be saturated at the Chajnantor site, this will be the only reliable means of calibration.

(3) Amplitude Calibration to 1% - what is the 'right' level?

Depending upon source size, as the size of the primary antenna grows, baselines lengthen and the amount of flux recovered gets smaller. For example, IRAM PdBI recovered only ~2% of the flux from C/Hale-Bopp. Seldom do single antennas quote calibration accuracies to anything near the few per cent level. Recently, Sandell reported to one of us that he thought that SCUBA/JCMT could be calibrated to the 5% level, though with some painstaking effort. Sandell has accepted a position at NRAO-GB and will be arriving 1 Nov. I'll ask him to contribute to these discussions.

If a scientific goal is to make accurate maps containing all of the flux, the other 98% of the flux gathered by the single antennas must be very well calibrated. Currently, a BIMA group including Welch and Bock are constructing a calibration package to go behind the BIMA secondary with a goal of increasing the accuracy of one part of single antenna calibration. We need to determine if this is a technology suited to the MMA, and to what level the scientific goals requite us to calibrate.

(4) Correlator--how to sample the IF band (analog vs digital)

The signal might be digitally filtered after 3 level sampling, then resampled to some deep level, say 16 bits before going back to 3 levels for the correlator. This conversion can result in a loss of sensitivity. However, it should result in excellent calibration through an essentially perfect phase response. The system will be stable and identical antenna to antenna, with less maintenance needed, Darrel reported. Darrel favored digital sampling, believing that the gain would be worth the small sensitivity loss. This is an issue mainly for another MMA Division, but we should offer an opinion on whether the sensitivity loss is acceptable. General agreement was that it was, at the meeting.

(5) Antenna size 10m vs 12m

A meeting will occur 4 Nov 98 in Berkeley to determine tradeoffs in antenna size.

LMA Science Meeting (Al)

Bob has suggested holding one in Washington with Congressional staffers invited in Fall, 1999 (after World LMA MOU and URSI Meetings next summer, and just before FY2000 budget).

Simon reported that several radiosondes had flown and that Bryan and he had begun some analysis of the data.

From DivHd meeting: Is 100 M Baseline OK for Phase 1 Test Interferometer Background: Emerson talked on the different elements of staying with an 100 M baseline for Phase 1 of the test interferometer. Please send him comments sometime this week as he would like to get back Escoffier on this issue.

Project Book: Project Book chapters are due to Emerson 23Oct. Authors who have not submitted your chapters, please do so.

Division Head Meeting

Eduardo Hardy reported that all Chilean travel should be arranged through secretaries at the sites, with whom Carlson Wagonlit would communicate to ease planning. This includes hotel, internal air flights, etc.

Bob reported that the MMAOC (NSF) wants to know the procedure for controlling changes to the baseline project. A draft memo describing the formal process will be created and circulated.

Action Items 19Oct98

------

URSI Meeting (Radford)

Travel

------

T. Helfer:07Oct-17Oct (Green Bank/UMD), 20-22 Nov (Chi) 26Nov-09Dec (Tenerife)

A. Wootten: 13Oct-15Oct (U. Md.) 20-22 Nov (Chi)

J. Mangum: 20-22 Nov (Chi)

S. Radford: 20-22 Nov (Chi)