[evlatests] R-L Phase Differences -- a 'simple' explanation

[Barry Clark]

EW

ea09  N16   0.21       -0.58    -0.37     123   -.030

NS

ea09  N16   -0.65     -1.20     -1.85     1182   -1.08

On 4/4/2022 2:57 PM, Rick Perley via evlatests wrote:
> What are the values for ea09 (at N16), which was the reference antenna 
> I used?
>
> On 4/4/22 14:20, Barry Clark via evlatests wrote:
>> Just for specificity, I'll quote the tilts actually in use.  For VLA, 
>> we have separate tilts for the antenna and the pad, so that in theory 
>> the antenna can be moved from pad to pad without changing the 
>> pointing parameters.  What is used is the sum.  What we are 
>> interested in is the  tilt relative to the vertical at the array center.
>>
>> Ant.   Pad    AntEW tilt  PadEW tilt  SumEW tilt  X   CenterEW tilt
>>
>> ea01   W06    0.51         0.68        1.19    -261      1.33
>> ea03   W18    0.24        -1.95       -1.71   -1722     -2.85
>> ea05   E14    0.52        -5.37       -4.85    1001     -4.31
>> ea06   N06    0.52        -1.20       -0.68      22     -0.67
>> ea22   W12    1.10         0.10        1.20    -859      0.74
>>
>> Ant           AntNS tilt  PadNS tilt  SumNS tilt  Z   CenterNS tilt
>> ea01         -0.44         1.74        2.18    -136      2.09
>> ea03          0.02         3.39        3.41    -900      2.82
>> ea05         -0.19         0.67        0.48    -442      0.20
>> ea06          0.43        -2.19       -1.76     220     -1.62
>> ea22         -0.44         0.60        0.16    -448     -0.13
>>
>>
>> On 4/1/22 10:41, rperley via evlatests wrote:
>>> There is a simple explanation for the R-L phase differentials -- a 
>>> differential tilt between the two antennas.  If the two antennas' 
>>> poles point in slightly different directions, the parallactic angles 
>>> seen by each are different, which results in a different measured 
>>> phase for the two polarizations.
>>>
>>> For the 'RR' correlation, the phase difference is -(delta par 
>>> angle), for the 'LL' correlation, it is +(delta par angle), so the 
>>> R-L difference is twice the difference is parallactic angle between 
>>> the two antennas.
>>>
>>> This effect is independent of band -- it is purely geometrical.
>>>
>>> To show how these differentials vary with source declination, I 
>>> generated plots for the four sources observed:  3C286 (dec = 30.5), 
>>> OQ208 (dec = 28.5), 3C287 (dec = 25.2), and 3C273 (dec = 2.1).  I 
>>> generated curves for a E-W tilt, and a N-S tilt.
>>>
>>> Attached is the resulting plot, showing the predicted R-L phase in 
>>> degrees as a function of Hour Angle, for the four sources. Solid 
>>> lines are for an E-W tilt, Dashed lines for N-S tilt.
>>>
>>> The match to the observed data is extremely good.  (To be fair, the 
>>> match to the largest of the 'even' tilts is extremely good).  But I 
>>> bet that a suitable combination of E-W and N-S tilts would give a 
>>> good fit to almost all of the data.
>>>
>>> There is only one problem -- the amplitude of the tilt required to 
>>> give the size of the observed phase is about 5 times larger than the 
>>> largest measured tilt.  The plots were generated with a tilt 
>>> differential (between the two antennas) of 6 arcminutes.
>>>
>>> So, if this model has any relevance, it begs the question: How do we 
>>> measure the antenna tilts?  Are these tilts different than those 
>>> used in the model?
>>>
>>> Rick
>>>
>>> _______________________________________________
>>> evlatests mailing list
>>> evlatests at listmgr.nrao.edu
>>> https://listmgr.nrao.edu/mailman/listinfo/evlatests
>>
>> _______________________________________________
>> evlatests mailing list
>> evlatests at listmgr.nrao.edu
>> https://listmgr.nrao.edu/mailman/listinfo/evlatests
>
> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.nrao.edu
> https://listmgr.nrao.edu/mailman/listinfo/evlatests
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listmgr.nrao.edu/pipermail/evlatests/attachments/20220404/9b37286e/attachment.html>