[evlatests] ea01 tilt mystery (fwd)

[Ken Sowinski]

The behavior of ea01 I wrote about a few minutes ago is not new,
There was a similar examples almost exactly two years ago when
the antenna was on E09.  The sense and magnitude of the tilt
change then was about the same as that which I reported for the
event of this month.

Some seasonal change in the antenna?


---------- Forwarded message ----------
Date: Fri, 27 Nov 2009 11:48:44 -0700 (MST)
From: Ken Sowinski <ksowinsk at nrao.edu>
To: evlatests at aoc.nrao.edu
Subject: [evlatests] ea01 tilt mystery

There has been a mysterious change in tilt for ea01 this month.
I document what is known in the hope that someone can explain
what is happening.  the evidence points to something happening
to the antenna on November 4.  Most likely it is unrelated to
the subreflector because it is expressed as a change in tilt,
rather than collimations.  Does anyone remember any work done
to ea01 on that day that can affect its tilt by more than an
arc-minute?

Chronolgy:
Sep 17  ea01 was moved from W16 to W09.  A tilt error was measured
          and applied to the pad.
Sep 23  Another pointing run confirmed and refined the previous tilt
          measurement.

Several pointing runs from Sep 23 to early morning Nov 04 confirmed
correct pointing terms, including tilt, for ea01.

Nov 11  A large tilt error was noticed for ea01 during a collimation
          run.  Collimation errors were small at all bands.
Nov 27  An X band pointing run confirmed tilt errors for ea01.

Additional Information
>From weekly focus checks, is is clear that the high frequency
performance of the antenna deteriorated between Nov 04 and Nov 12.
Examination of pointing results in November suggest that ea01
got worse between Nov 03 and the end of the day on Nov 04.  Nov 04
was a maintenance day.

The tilts for W09 and EA01 in the parameters database have not
changed, nor has the PADAZ term for W09.
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