[evlatests] Slow focus motion on ea14

[Ken Sowinski]

On Thu, 9 Feb 2017, Rick Perley wrote:

>    A test last weekend had all the antennas changing band every 30 seconds.
>
>    In general, results were very good.  But ea14 has a special problem.  It 
> was clear that the visibility amplitudes and phases took much longer to 
> stabilize upon change of band than any other antenna.  Careful review of 
> these showed that the amplitudes stabilized well before the phases.  This 
> rules out subreflector rotation as the cause, and points the finger at 
> *subreflector focus*.
>
>    Only a few band combinations were affected.  My test runs the bands in 
> order as the appear on the secondary focus:  L to X to C to S to Ku to K to Q 
> to Ka.
>
>    The effect was seen only in the C to S and S to Ku transitions.  The 
> former took 25 seconds for the subreflector to stabilize,  the latter took 40 
> seconds!  The S-band focus position is significantly different than the 
> higher frequency ones, probably accounting for the extra time.  So either the 
> focus motion on ea14 is especially slow, or the S-band focus on ea14 
> especially far from the normal (or the value in the tables is especially 
> wrong ...)

Focus setting, neglecting the elevation dependent term.  Nominal
speed of the focus drive is 0.5 cm/sec.

C   -0.5 cm
S   -4.0
Ku   0.5

>    If a focus motion is to blame, we would expect the L to X transition to 
> also suffer the problem -- but this is hidden as I do referenced pointing 
> first (taking nearly 3 minutes).
>
>
>
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