[evlatests] ea08 pointing

[Ken Sowinski]

On Mon, 12 Jan 2015, Ken Sowinski wrote:

> ea08 behaved very badly in the pointing run this weekend.  It seems
> to be a recent develpment; short pointing runs to fix moved antennas
> last week did not clearly show ea08 as very bad.
>
> The trouble is all in the elevation axis.  It looks like there
> are random large offsets in the pointing determination.  Unrelated
> to azimuth, time, elevation, direction of elevation motion, or anything
> else I have been able to think of.  'Large' means +/- 0.5 arc-min.
>
> Among the possibilities are
>
> 1.  It just wasn't happy that day and will look fine if we do it
>     again.
>
> 2.  Something loose at the subreflector; but not an error in the
>     rotation zero point or setting.
>
> 3.  Periodic el encoder error.  This should be easy to check when the
>     snow abates.

This is it.  A 3 dB tracking test revealed a very large periodic
encoder error.  The amplitude varies by about +/- 20 percent.

> 4.  Something unrelated to pointing which affects correlated amplitude
>     in a way to make it look like an elevation pointing error.  Unlikley,
>     but listed for completeness.
>
> I write this mainly in the hope of triggering a memory of something
> done to ea08 recently that could have bearing on this.