[evlatests] Subreflector rotation effects at L, C, X, U Bands

[Rick Perley]

    Ken performed a test at noon today, to see if the subreflector 
rotation which will be used to offset 2nd order sag at K, Ka, and Q 
bands has any effect on the lower frequency bands.  He observed 3C84 (HA 
~ 4) at L, C, X, and Ku bands, with rotation off, then on. 

    No change in gain is seen between the on/off observations at these 
bands.  This is as expected. 
   
    However -- one notable effect is seen.  At C, X, and Ku bands, when 
the rotation has been applied, the visibilities are observed to be low 
(and stable) for the first ~20 seconds of the scan.  After this time, 
the visibilities abruptly rise to the proper levels.  This does not 
happen with the rotation 'off'.  There is no obvious phase effect.  The 
'low amplitude' data are not flagged. 
    The difference in amplitude varies with band:  At L-band, the effect 
is barely discernible.  The amplitude change is ~10, 16, and 18% for C, 
X, and U bands, respectively.  (The quoted values are for EVLA antennas 
for C and X, and for VLA antennas at U band).
    The suspected cause is a delayed command for the subreflector to 
assume the rotated position.  Why would the command be delayed by ~ 20 
seconds?