[evlatests] 2007-04-26_0556_POINT.txt

[Ken Sowinski]

On Thu, 26 Apr 2007, Ken Sowinski wrote:
> On Thu, 26 Apr 2007, Ken Sowinski wrote:
>
>> An historical check of pointing runs shows that the change occurred
>> between pointing runs on March 24 and April 2.  There was a
>> collimation pointing check on April 18, but antenna 14 was not
>> working that night.  The shift has been more or less stable since
>> it was first measured on April 2.
>>
>> The magnitude of the shift is about +0.5 arc-min in elevation and
>> about -.25 arc-min in azimuth.  Does anyone remember anything
>> done to antenna 14 in the week between March 24 and April 2?
>
> Juan Uson reported that he saw funny behavior from antenna 14
> the night of March 29.  He was doing pointing at X and L band
> over several hours and saw an example of a collimation jump
> at X band between two successive pointing trials on the same
> source.  Other EVLA antennas did not do this.

There was a short collimation run on April 27.  The results are
puzzling, but confirm that something happened to antenna 14.
The first pair of numbers below are the measured pointing offsets
for X band and the other bands relative to X band.  The second
pair of numbers are the inferred pointing error that would have
been measured at each band had referenced pointing not been used.
FPA is the approximate feed position angle measured clockwise,
looking down at the feed ring, with zero at the up-side.  The
feeds at the elevation axis will be at 90 (Q band and 270 (X band).
The only interesting feature I see is that Q and X bands, both
on the elevation axis, experienced about the same shift in
pointing.  The easiest way out is to claim that the C band
result is aberrant (Rick's 14C phase problem?) and that the
shift is about the same for X, K and Q suggesting a mechanical
subreflector shift.

Any suggestions?


    Az   El      Az   El    FPA
X  0.3  0.6     0.3  0.6   270
C -0.2  0.6     0.1  1.2   315
K -0.2 -0.2     0.1  0.4    65
Q  0.0 -0.1     0.3  0.5    90