[evlatests] Holography phase clue

[Walter Brisken]

Rick has reported unusual phase behavior of at least two kinds in 
holography data on EVLA to VLA baselines only.  Clearly either the VLA or 
the EVLA is misbehaving, but which?  I learned a couple of interesting 
things:

I took 30 minutes of data ~5:30 to 6:00 PM this evening in a "hybrid" mode 
where the VLA antennas (excepting reference antennas 6, 9, and 12) 
participated in a series of raster scans, entirely within the primary 
beam.  All of the available EVLA antennas ran a different script which 
made them behave like reference antennas (ie, they pointed at the 
holography source with zero offset the entire time).  An integration time 
of 0.4 seconds was used.  Rasters all went through the center of the beam 
and alternated directions.

The EVLA to Reference antenna baselines have _no_ phase jumps and have 
steady amplitudes of 0.5 bogoJy, the same as VLA-VLA baselines.

Oddly enough, however, I did see phase jumps on antenna 4!  There were 8 
phase jumps in 30 minutes: 6 during the 20 minutes on calibrator scans and 
2 during the 10 minutes of raster; none correspond to scan changes.  All 4 
IFs jump together.  IFs A/C jump always between two states about 90 
degrees apart.  IFs B/D have a different sized jump, but again between two 
states.  I _think_ this is a different issue than Rick's, but he should 
compare his phase jumps against other reference antennas!

The correlator apparently only wants to dump baselines that have exactly 
one reference antenna.  Under the conditions of this experiment the 
correlator thought the EVLA antennas were not reference antennas, so a 
slightly unexpected variety of baselines were omitted during the 
holography scans.  This means that if we attempt to do EVLA-only 
holography, or VLA holography using an EVLA reference antenna the EVLA 
reference antennas will need to be added to the list of reference antennas 
in the ROT files (and I'm not sure that's enough to make it work). 
Perhaps we'll graduate from the VLA system before this is necessary.

I've put VPLOTs of uncalibrated data on various baselines on the EVLA 
tests page: 
http://www.aoc.nrao.edu/evla-internal/techdocs/tests/testresults/20060817.wb/


-Walter