[evlatests] ea20 pointing; was Stellar high-frequency data

[Ken Sowinski]

On Fri, 25 Jun 2010, Rick Perley wrote:

>    2) Antenna 20 is clearly not pointing correctly, with a cyclic
> response of a few minutes.  I think this is known to be a subreflector
> or encoder problem -- it would be good to hear this is being addressed
> as it affects all bands (even L-band!).

I reviewed pointing data for the last few months.  It clearly
has an problem in  elevation that has slowly been getting worse.

Mar 25  OK
Apr 16  OK
Apr 26  did not work
May 02  El rms = 6.2 arc-sec
May 25  El rms = 4.9
Jun 11  El rms = 8,9
Jun 20  El rms = 13.2 !

For the last pointing run the peak-to-peak excursions of
measured El pointing error exceeded an arc minute on a short
time scale.  Either there is a very large periodic pointing
error in elevation, or the subreflector is loose or wandering
back and forth in elevation on a time scale of a few minutes.

A 3 dB tracking test was done over night and will be examined.