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