[evlatests] A remarkable gain change at P-band

[Rick Perley]

I can explain what happened with the changing gains.

The 'digital' gain values were forgotten after the first holography scan.
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In more detail:

The first scan in the SB was a slew to source, at C-band.

The second scan was a 1-minute observation of the calibrator, to set the analog gain.  New gains were implemented very quickly — within about 10 seconds.

The third scan was a 10-second observation of the calibrator, to set the digital gain.   This resulted in PSum values close to 15 for all antennas, all SPWs.  As they should be.

The next two scans were separate observations of the calibrator, followed by the four observations within the holography script:  calibrator, holography setup, holography scan, calibrator.

It was at this point that the Psum values all reverted back to exactly the same levels they were after the second (analog gain setup) scan.

So it is clear that the system 'forgot' the digital gain setting.

Rick
________________________________
From: evlatests <evlatests-bounces at listmgr.nrao.edu> on behalf of Rick Perley via evlatests <evlatests at listmgr.nrao.edu>
Sent: Monday, October 28, 2024 2:12 PM
To: Rob Long <rlong at nrao.edu>; evlatests <evlatests at nrao.edu>
Subject: Re: [evlatests] A remarkable gain change at P-band

All setups were done according to instructions:  After moving to the source at a different band, a 1-minute on-source observation, at P-band, was executed for the T304 gains, followed by a 10-second observation to setup  the digital gain.  These worked as expected (so far as I can tell).

Rick

________________________________
From: Rob Long <rlong at nrao.edu>
Sent: Monday, October 28, 2024 2:08 PM
To: Rick Perley <rperley at nrao.edu>; evlatests <evlatests at nrao.edu>
Subject: Re: [evlatests] A remarkable gain change at P-band


Did the T304s reset attenuators between runs? I'm sure they did when you went to a calibrator. So much interference in P-band, I wouldn't be shocked to see significantly different attenuator settings from run to run.


Rob


On 10/28/2024 1:58 PM, Rick Perley via evlatests wrote:
I ran a holography test last night (to assist Tracy Clarke in her characterizations of the P-band primary beam).  The SB consisted of four single cuts through the primary beam.  The four cuts were:  horizontal, vertical, and two cuts at 45 degree angles.

Each holography cut begins and ends with a short on-source calibrator observation.

A most remarkable thing happened between the first two holography cuts in last night's test:

The amplitude changed, for all antennas.  Some amplitudes went up, others went down.  For some antennas, the change was barely noticeable.  For others, it was as large as a factor of six.  (!).  The changes were not the same in the two polarizations.

The effect is due to a gain chang, and not to any antenna or subreflector position error —it is seen in the switched power monitors —both PSum and PDif.

The PSum values, at the beginning of the run, and through to the end of the first holography observation, were normal at near 14 counts.

This gain change occurred only after the first holography scan — about ten to fifteen minutes into the observation.   For all other beam cuts, the gain remained constant (at the wrong power levels).

Any idea what happened?

Rick



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