[evlatests] ea25 pointing

[Dave Parker]

The ACU was replaced on Oct 24th, was there any improvement that you 
could notice with that replacement, in case we are looking at 2 problems 
compounding issues.

Dave

On 10/29/2018 10:08 AM, Ken Sowinski wrote:
> Since it underwent a bearing replacement in 2011 ea25 has exhibited
> pointing errors which are both not modeled by the usual pointing model
> and are hard to explain.  This note documents one example of what we
> see in the hope of inspiring an explanation or repair.
> 
> This example is drawn from the pointing run of October 26, 2018;
> other pointing runs could be examined to find similar examples if
> that is useful.
> 
>   Az    El    Azerr Elerr (errors in arc-min)
> 248.4 109.8 -0.09  0.24
> 248.2 109.5 -0.26 -0.37
> 248.1 109.2 -0.47 -0.91
> 
> 334.5  11.2 -0.13 -0.59
> 286.5  13.6 -0.05 -1.15
> 293.2  36.6  0.04  0.20
> 313.0  78.2 -0.03 -0.27
> 
> The first example is especially striking because the large change in
> elevation error occurs when neither Az nor El change by even one
> degree.  The second example covers a larger range of Az and El, but
> still has a change in El error which is large and sudden.
> 
> An examination of the post-fit residuals to the pointing model fit,
> shows that since the bearing replacement the pointing behavior has
> deteriorated, and is limited to azimuths between 240 and 330 degrees.
> This limited range makes it very difficult to explain the poor
> pointing by software errors, FRM faults, loose mechanical structure,
> or hysteresis.  There is nothing left to invoke except for a defective
> bearing.  What ever the mechanism, if must be able to explain erratic
> poinitng changes greater than an arc-minute occurring on azimuth
> scales of a degree or less.
> 
> If a measurement of the bearing run out error confirms this suspicion,
> then the only effective remedy is another bearing replacement.  None
> of our usual solutions to this kind of problem: referenced pointing,
> modeling the error profile, or measuring it with a tiltmeter, can
> correct for errors that change so fast in both time and azimuth.
> 
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