[evlatests] interband collimation corrections

[Eugene Cole]

Per coord mtg summary (1/4/2000) dish panels were readjusted on a14 
3/6/97; on a16 1/29/97.

-- Gene

Rick Perley wrote:

>    Chris Carilli has (or should have) the record of when each antenna was
> modified on the basis of holography done since 1993.    My recollection 
> of the procedure is that the holography program provided
> both the subreflector offset and the panel adjustments.   Subreflector and
> panels were adjusted on the basis of this.    The observations made for 
> these adjustments were near 50 degrees in
> all cases.
> 
> Tom Frost wrote:
> 
>> to answer Vivek's question about holography on # 14 & 16;
>>
>> Ramon Gutierrez & I looked thru all the Antenna Mechanic
>> documentation concerning Holography....we could not find
>> any records of holography being done, other than VLA Test
>> Memo No.169 dating from 1993 when all 28 VLA Antennas were
>> done....Ramon thinks that #14 & #16 have not been done since
>> then...
>>
>> Vivek Dhawan wrote:
>>
>>> Sorry - this initially went to only a subset of evlatests.
>>> ------------------------------
>>>
>>> I was talking about the subreflectors all being in slightly
>>> different places, variations of 3-5mm. The corresponding
>>> equivalent displacements of feeds in the focal plane is proportional to
>>> magnification M, so about 1-2 inches on the VLA. (The axial
>>> or focus position shift is  ~M*M/2)
>>>
>>> The interband colimations are not affected - a shift (i.e. across the
>>> optic axis) of the subreflector makes all the bands change colimation by
>>> the same amount. Similarly, the sag under gravity affects all bands the
>>> same, but is accounted for as belonging to the antenna, I believe.
>>> The different subreflector positions can be seen in the different
>>> antennas (Q-band) peaking in gain at different elevations.
>>>
>>> However, a change in subreflector tilt moves the colimations by the
>>> same magnitude at all bands, but the vector points around a circle as
>>> the subreflector is rotated around the bands. This does affect the
>>> interband colimations. So also a change in FRM rotation encoder offset.
>>> These are easiest to see if pointing offsets changes from many bands are
>>> plotted on a circle each at the position of the corresponding feed,
>>> right after a change in the subreflector position.
>>>
>>> I guess the question I have is, was holography used to set the
>>> subreflectors of antenna 14 and 16 (and others) when they were VLA 
>>> antennas, before conversion to evla?
>>>
>>>
>>> V.
>>>
>>
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