[evlatests] Antenna 21 at K-band

[Ken Sowinski]

On Wed, 25 Apr 2007, Rick Perley wrote:

> But antenna 21 is clearly 'special' -- the amplitudes started of very
> weak -- about a factor of 10 below normal, then steadily rose, reaching
> normal levels about 35 minutes after the start, then steadily declined,
> ending up about a factor of 2 below what they should be.
>
> A pointing problem is possible -- the elevation of the beginning was 72
> degrees (HA of +1.5 hours), with steady decrease to 58 degrees at the
> end of the run.   But the factor-of-ten loss (100 in power) indicates a
> large offset of an arcminute or so -- which would not be sufficient to
> cause the X-band data to be lost (in an earlier run), unless the
> pointing then was much worse.

There was a pointing problem.  The pad it was moved to, DE9, has
a large inherent tilt which was accounted for after the move as
encoder offsets.  Thus the antenna pointed well in once place and
poorly everywhere else.  The pad tilt has been adjusted and the
encoder offsets returned to their previous values and the antenna
is functioning again.  The clue to recognize this was the large
and unexpected change in the elevation collimation term after the
move.