[evlatests] Subreflector Rotation Trick

[Rick Perley]

    Ken and I ran a test early Saturday morning, to test if the 
'subreflector rotation trick' works.  This 'trick' intends to correct 
for the sag of the quad legs by rotating the subreflector.  In essence, 
it compensates for the vertical drift of the secondary focus position 
from the feed horn due to gravitational sag by redirecting the secondary 
focus position. 

    Three sources were observed two times each at both K and Q bands.  
The three sources were chosen to lie at low (20 degree elevation), 
medium (50 degree) and high (80 degeree) elevations. 

    For each source and band:

    a) A 'blind' observation was made of the source.
    b) Primary referenced pointing was done to measure the beam offset. 
    c) A pointing determination was done, with the corrections applied, 
to ensure the pointing error was removed.
    d) The 'subreflector rotation trick' was applied.
    e) A pointing determination was done, to measure both the amplitude 
of the on-axis gain, and the beam position error. 

    The pointing solution program is very convenient for analysis, as it 
provides both the observed forward amplitude (uncorrected for pointing 
error) and the apparent pointing position. 
    For the K-band observations, all pointing observations were done at 
K-band.  For the Q-band observations, the primary pointing correction 
was done at X-band, with the following pair of determinations done at 
Q-band. 

    Results:

   1) The referenced pointing program works well at all bands and 
elevations. 

    2) Rotating the subreflector to compensate for the gravitational sag 
improves the forward gain for some antennas, and degrades it for 
others.  The antennas for which improvement is noted are:  1, 11, 15, 
18, 21, and 24.  In some cases, the improvement is spectacular -- nearly 
a factor of two at Q-band.  Improvement, if it occurs,  is always seen 
at both bands.  The antennas for which the rotation degrades performance 
include 14, 16, 19, 25 and 26, plus nearly all of the VLA antennas -- 
and these degradations occur at both bands. 

    3)  Two additional characteristics lead me to believe that the 
improvement/degradation noted above has essentially nothing to do with 
the rotation being applied:

       a)  It is noted that with the rotation applied, significant 
pointing errors were introduced in nearly all antennas -- independent of 
whether the antenna gain increased, decreased, or remained the same.  
Thus, even for antennas whose gain nearly doubled at Q-band with the 
rotation applied, there was a ~15 arcsecond residual pointing error! 

    b) There is little dependence of the improvement, or loss, of gain 
upon elevation.  Plotting the fractional gain improvement or loss 
against elevation shows the expected trend (the improvement declines 
with increasing elevation) -- but the change between 20 and 80 degrees 
elevation is much small than the offset. 

    I conclude that basic optical misalignments at K and Q bands are 
overwhelming any improvements due to rotating the subreflector to offset 
gravitational sag.  To make sense of all this, we must first properly 
align the systems at a high elevation.  There is some dynamic time 
tonight, which I plan to use for this purpose, using 9 x 9 holography.