[evlatests] Holography scan phase inversions

[Rick Perley]

    As noted in my earlier report, we have two different phenomena to 
contend with.  This report deals with the intriguing 180 degree phase 
inversions associated with holography. 

    In previous tests, there was confusion about what happens at the 
beginning and ends of holography scans, since we invariably are near a 
null, or in a sidelobe, when we turn around. 

    The test from last night avoided this by scanning only within the 
3dB points of the primary beam.  In this case, the phases *must* remain 
nearly equal to their center-beam value. 

    I did two 6-raster holography runs:  The first scanned in azimuth 
(left-right) , reversing direction each time, and always scanning 
through the center of the beam.  There were 11 stopping points (each of 
10 seconds duration) in each cut.
    The second run also scanned in azimuth, but only in one direction 
(so that at the end of each cut, we returned to the same point that we 
started with before). 

    Frequencies/BW were:  1485/25 for AC, 1275/12.5 for BD. 

    Data were dumped with 410 ms averaging. 

    Results:

    A) The 10-second phase jumps discussed in the previous message were 
seen throughout, in both IFs, and all antennas.  Out of the 2*6*11 = 132 
holographic pointings, about 10 to 13 (depending on antenna) of these 
showed these jumps.  The phase jump values were not always the same for 
any antenna-IF.  If somebody wants a more complete description, I can 
provide it. 

    B) No EVLA antenna in IFs A or C (at 1485 MHz) showed the 
holographic scan phase jump.  The only jumps seen were of the 10-second, 
single pointing, variety. 
    C)  All EVLA antennas in IFs B and D (1275 Mhz) showed numerous 
major jumps, nearly always associated with an entire holgraphic scan.   
The unique characteristic of this jump is that it is *always* of 
magnitude 180 degrees. 

    Closer inspection shows that there may be an instance or two where 
the 180-degree jumps is not uniquely associated with an entire 
holographic scan.  For this run, the calibrator observations were also 
affected by the phenomenon -- one of the three calibrator observations 
was in the 'inverted' state (at 1275 MHz only).