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

[Rick Perley]

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
>>
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