[evlatests] antenna 28 pointing still bad

[Ken Sowinski]

On Tue, 18 Nov 2008, Bob Broilo wrote:

> 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.

Some calculations to see how important this is.

>From .001" -> 2 arc-sec, the radius the .001" is acting over is
given by 10^-2/R = 10^-5 radians, or R = 100 inches.

Does .01 inch measured non-repeatability account for 0.5 arc-min
pointing error?
.01/100 = 10^-4 rad or 20 arc-sec,
which is getting close.

Bob sees this between 110 and 140 degrees, but the pointing "bump"
is seen between 100 and 250 degrees of azimuth.