Date: Fri, 22 May 2009 11:59:52 -0400 (EDT)
From: Todd R. Hunter 
To: Richard Hills 
Cc: almasci@donar.cv.nrao.edu
Subject: Re: [Almasci] Control of Antenna Focus and Related Topics

Hi Richard,

At the science IPT meeting, you asked for comments on your document on Control_of_Antenna_Focus_and_Related_Topics of 2009 May 19. Of the two strategies you propose, I think that you probably want to implement "A" first. In addition to causing less wear on the focus drives, much of the benefits can be realized even if an accurate readback of the subreflector position is either not available or not yet processed by software in the manner that you suggest.

Some questions come to mind immediately:

Are you thinking that some online software like TelCal would perform the phase corrections, or would it be done offline in CASA? If the latter, then are there existing fields in the ASDM format for 1) indicated subreflector position; and 2) ideal subreflector position? Regardless of the answer to the ASDM question, I think strategy A would be more straightforward to implement, since (to first order) it simply requires the software to:

1) detect baseline-based phase jumps incurred by performing a simultaneous (but generally unequal) focus change on all antennas in the midst of a (relatively) longer observation of the phase calibrator. 2) solve for the equivalent antenna-based phase changes, and 3) take account of these jumps in the temporal phase solution.

In principle, the offline software would not have to know when the focus change occurred, but it would be cleaner if it did. Assuming the stream of indicated and ideal subreflector positions becomes available, then an additional correction to the amplitude due to imperfect achievement of the desired focus position (as judged from the phase change) could be made, as I believe you suggest in the final paragraph of Strategy A?

My main worry about Strategy B is that it makes the data integrity dependent on an accurate and reliable stream of indicated subreflector positions on all antennas, since the subreflectors will be following independent models (at least between the antennas of different design).

Todd

Date: Sat, 23 May 2009 11:33:22 +0100
From: Bojan Nikolic 
To: Richard Hills 
Cc: Todd R. Hunter , almasci@donar.cv.nrao.edu
Subject: Re: [Almasci] Control of Antenna Focus and Related Topics

Dear Richard,

I also think option "A" is the one to go for first.

However, I think it would be useful to do a quick FEM sum on the other antenna designs first. In particular the other designs may not have as stiff a quadripod and/or may not have such good compensation in receiver cabin vs focus position. In scheme A, when moving between phase calibrator and science source it is of course the difference between phase errors in the different antenna designs that is important.

So although Vertex gives 9 micron error for 2 degrees, it may be that the other antennas give -20 micron for the same elevation change, in which case it will certainly be desirable to make a correction based on FEM models.

Regarding the software, although it should be possible to detect in the visibility data the phase jumps when sub-reflector adjustment happens, there is always the possibility of other instrumental effects triggering this, especially before the full array is available and therefore S/N on calibration sources isn't typically as good.

I would suggest from the start this is done by looking at changes in the commanded "Z" position of the sub-reflector -- I imagine these will be written to the archive already. The magnitude of the jump can be determined either from visibilities or recorded position changes. Of course any software procedures that fit antenna phases using more then two points in time should be made aware of these jumps.

Best, Bojan

This topic: ALMA > WebHome > AlmaSci > AlmImcal > MonthlyMeetings > 20May09Agenda > ControlOfAntennaFocus
Topic revision: 2009-05-26, ToddHunter
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