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

[Barry Clark]

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