[evlatests] K-Band tuning and polarization

[Rick Perley]

    An experiment was run last night to check tuning and 
cross-polarization across K-band. 

    A clean point-source (2007+404) was observed for two hours, centered 
on meridian transit.  The parallactic angle rotation of over 100 
degrees, plus the high flux density (about 2 Jy) should ensure decent 
polarization solutions.  Seven or eight short (70 second) scans at each 
frequency were made. 

    Frequencies tuned were (in IF1, IF2 pairs), in GHz:

    18.125, 19.025
    20.025, 21,024
    22.025, 23.025
    24.025, 25.025
    26.025, 26.025 (I wanted 27.025 for BD, but Ken was concerned that 
attempting to do so would crash the entire script). 

    BW = 50 MHz. 

    Antennas 11 (moved, primary parameters not derived), 13 (optical 
power low, no synch. power), and 23 (stuck subreflector) were not 
operational.  None of these was flagged as bad, by the system.
    A number of antennas, which gave good fringes, were flagged as bad 
by the system.  This behavior differed between frequencies and 
polarizations. 

     All other EVLA antennas fringed, but there were some problems:

    1) 21D gave no fringes at 21.025 and 23.025 GHz.
    2) 26B gave no fringes at 23.025 and 25.025 GHz.
    3) Amplitude stability in IF#2 was very poor at 21.025 and 23.025 
GHz -- especially in RCP.  Record-to-record variability of up to 50%.    
Stability was also poor at 25.025 GHz -- but not as unsteady as the 
previous listed frequencies. 

    I QUACKed the first 45 seconds of each scan (as a precautionary 
measure), as some antenna/IFs were slow in getting stabilized. 

    With the small amount of data remaining (about 15 seconds of 
integration, for each frequency, for eight scans), the polarization 
calibration program PCAL was run.  The solutions, referenced to antenna 
18, show that except at the highest frequencies, the 'D' 
cross-polarization terms, for most antennas, are well below the desired 
limit of 5%. 

    No statement of stability can be made from this test.