[evlatests] antenna 28 pointing still bad

[Bob Broilo]

Dave Alderman and I got some time to check gross mechanical parts of
Antenna 28.

The guy wires are within specification tightness.  Measuring the
frequency of oscillation with an accelerometer, the wires are: NE
3Hz, NW 3Hz, SE 2.5Hz, and SW 2.75Hz.  Spec is 2.5Hz +/- 0.5Hz.

The subreflector has a sloppy bearing or loose bolts.  It can be moved
back and forth with hand pressure, and moves ~3/16" to ~5/16" (approx
5-8mm).  Dave Alderman feels that it moved more in the East-West
direction than up-down.  I didn't note what band position it was in.
The antenna mechanics plan to check the bolts tomorrow.

I will set up the theodolite and see how the subreflector moves with
elevation.  If it moves from side to side instead of just sagging, or
sags in a non-linear fashion, it will probably have to be repaired
before the pointing can improve.

We then checked the azimuth and elevation bearings.  We checked the
elevation bearings with a dial indicator while moving the antenna in
various ways.  The elevation bearings are very repeatable, to less
than 0.001".  We checked the azimuth bearing for sloppiness in various
places by using a dial indicator between the pedestal and the yoke.
There is an area from 110 degrees to 140 degrees where the bearing
does not repeat if the direction of rotation is changed.  The maximum
difference was 0.01" for a degree after the direction changed and
0.005" up to three degrees from the direction change.  In 1995 I had
calculated the elevation error from the azimuth bearing slop to be up
to 2 arcseconds per 0.001" and up to 1.3 arcseconds azimuth error per
0.001".  The slop occured over the pedestal room door.

The plan is to re-torque the bolts on the subreflector tomorrow, then
check the subreflector behavior optically and see if it must be
removed and repaired.

Bob.