[evlatests] Polarization Accuracy/Stability

[Rick Perley]

    The PCAL solutions made yesterday reveal puzzling differences in the 
antenna polarizations between the three paths.  Curiously, the B0D0 path 
(8-bit) antenna polarizations are much more similar to the A1C1 path 
than the A1C1 is to A2C2!  In comparing these solutions, we can see the 
same basic structures in most cases.  In general, the cross-polarization 
phases are more similar than the amplitudes (something which I find very 
odd). 

    Given that these solutions were made with a limited timespan of data 
(about 2.5 hours), the parallactic rotation (needed to permit separation 
of antenna a source polarizations) is limited -- we might suspect the 
solutions are faulty.  If so, the three parallel paths might give rather 
different results for the source polarizations. 

    Not so!  Overplotting the three sources polarizations for the three 
independent paths shows astonishing similarity (actually, identicality) 
of derived source polarizations -- both in amplitude and phase (position 
angle).  There are no differences to within the noise.  We might 
interpret this as confirmation that the PCAL program has found correct 
solutions (to within the model it uses for these solutions). 
   
    There appears to be a small but significant linear polarization in 
3C84.   It averages (over the 14.0 -- 16.0 GHz frequency span) at about 
0.4%, with many interesting structures where the polarization rises to 
nearly 1% over 50 or so MHz spans.  The PA is fairly constant over the 
frequency span (I don't know the true angle since we didn't include a 
polarization calibrator for this run). 
    J0303+4716 (the 'medium' strength calibrator) is about 1.5% 
polarized, and also shows variations in its fractional polarization over 
the frequency span. 
    J0349+4609 (the 'weak' calibrator) has very low polarization -- 
about 0.5%. 

    Another, much longer run was taken last night.  This will allow us 
to (we hope) deduce any time variability in the antenna polarizations.  
We will also be able to compare the new solutions to those derived 
yesterday.