[evlatests] antenna 28 pointing still bad

[Jim Jackson]

I've been looking at various archive data plots of LO/IF system 
parameters plotted against antenna elevation.  I can see no 
significant or unexpected electronic behavior of the Front Ends or 
LO/IF system. It also seems that Servo has exhausted their box of 
tricks on this antenna.

At this point I think electronics troubleshooting has gone as far as 
it can until the appropriate folks have had a chance to do some close 
mechanical inspection and testing of Antenna 28.

Jim Jackson




At 10:27 AM 11/14/2008, Ken Sowinski wrote:
>On Tue, 11 Nov 2008, Ken Sowinski wrote:
>
> > Meanwhile I can say two things about how the antenna is
> > misbehaving.  These signatures amy be a consequence of the
> > azimuth encoder tracking error, or be independent problems;
> > it is hard to distinguish.  1) The measured azimuth pointing
> > is proportional to elevation and varies about an arc-min
> > between elevations of 15d and 115d (over-the-top).  It is
> > *not* fit by the usual perpendicularity term.  2)  The
> > measured elevation pointing error is bistable; between about
> > 70d and 150d it is close to zero, elsewhere it is about
> > -0.5 arc-min.  Neither of these two signatures can be fit
> > by any terms in the standard pointing model.  I have plots
> > illustrating these if anyone wants to look at them on
> > Wednesday.
>
>The above statement was based on a five hour pointing run several
>nights ago.  Since then the az eoncoder has been replaced.
>A three hour pointing run last night shows the antenna to be no
>better and the two symptoms described above are still there.
>
>A more careful examination of the data taken for the pointing
>runs suggests that the antenna misbehaves at low elevations.
>There were many, but not all, scans below 20 degrees in which
>the amplitude response for antenna 28 was essentially zero, or
>more than an order of magnitude smaller than the other antennas.
>There is weak evidence that the degradation is not caused by
>pointing.  For one scan there was sufficient response on one
>baseline to see that the pointing offsets were not large at all.
>
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