[evlatests] C band pointing run

[Ken Sowinski]

                   C Band EVLA Pointing Run
                         Sept 7, 2005

A C band pointing run was performed the night of September 6 using
all VLA antennas as well as 14 and 16.  The goal was to verify the
changes to tilt made as a result of the last X band pointing run and
to see if the funny azimuth behavior in 16 persisted at C band.
All the elvation fits, VLA and EVLA antnnas, we funny with the antennas
pointing too low at lower elevations.  The best fit was achieved
whem limiting the lower elevation to 25 degrees and allowing the
sag term (cos(El)) to take up the slack.  The solutions for all antennas 
suggest sag change of about 0.5 arc-min.  The changes reported below for
EVLA antennas were calculated with these constraints.  The origin of
this elevation behavior probably lies in some combination of a broken
dew point meter and an abnormal atmosphere last night.

The answers are puzzling.  Simple facts.  Tilt in both antennas
changed as expected given the changes made.  Antenna 16 seems to be
pointed well at C band except for collimation terms in both axes.
Antenna 14 seems to be OK in elevation except for a small collimation
correction, but is totally wrong in azimuth.

The less simple facts.  Antenna 14 azimuth pointing seems to be bistable.
This is what was seen in 16 at X band last week, but is not seen in
16 this week.  The jump occurs once during the second excursion OTT
and remains through the end of the run.  The very last pointing trial
suggests that pointing may have returned to the pre-jump state.  The
difference between the two states is about 2.1 arc-min and it appears
to be a change in collimation not encoder offset.  There is no hint of 
anything funny in elevation at the time that the azimuth pointing changes
state.  There is also no evidence that the offset is related to wrap.  
Last week antenna 16 seemed to change state during each OTT excursion; 
this week antenna 14 did so only once.  Any explanation for this bizarre
behavior is welcome.  There is a pointing run scheduled for Thursday
night.  We should try hard to have both antennas working at X band at
that time.

Antenna 16
There were not as many successful trials as expected, probably because
the amplitude scale for 16 is much too low.
El collimation change   +1.6
EL Tilt change          not significant
Az collimation change   -0.4
Az offset change        -0.6   **compare this with X band**
Az E-W Tilt change      -0.9   Az/El tilt difference still there!
The azimuth offset is not easily compared with the X band data because
antenna 16 was misbehaving at the last X band pointing run.

Antenna 14
El collimation change   +0.3
Azimuth ?????

All the collimation changes were applied to the 6 GHz pointing terms
at about 2200 UT, Sep 7, 2005.