[evlatests] Slow focus motion on ea14

[Doug Whiton]

Focus was set to about 1/3 spec velocity.  I set it back to spec.  Not 
sure how long it has been like this.  I thought we had addressed the 
brake faults issue that caused us to slow Focus down.  We will find out 
soon.

Doug


On 2/10/2017 10:11 AM, Ken Sowinski wrote:
> 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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>>
>
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-- 
Doug Whiton
National Radio Astronomy Observatory
Servo/Fiber Group Leader
575-835-7417

"The new ACUs perform magnificently"  Rick Perley