[evlatests] R-L delays -- HUGE!

[George Moellenbrock]

Right, VLBACPOL makes the R-L delay measurement for you, too, which
is probably what you want, at least in the long run.  I gather it produces an
SN or CL table with the same delay installed for L (only) for all antennas.

It is interesting to note that over moderate bandwidths a standing-wave
contribution to the D-terms can look like an R-L delay contribution.  It is not
yet clear to me how important it is that these be disentangled for optimized
polarization calibration....  Maybe this has some relevance to the discussion
of the origin of the large R-L "delay" offset....

-George

On Fri, Jul 31, 2009 at 12:14 PM, Michael Rupen<mrupen at aoc.nrao.edu> wrote:
> As Rick, Vivek, and I just discussed, it's probably worth having a look
> at the description of VLBACPOL in the AIPS cookbook (also the EXPLAIN
> file) to see how AIPS wants you to do this.  As I recall most of that
> is to do with deriving the cross-hand pol'n, so if you've already
> measured that CLCOR might be all you need.  VLBACPOL should
> walk you through how to set that up as well.
>
> Cheers,
>
>         Michael
>
>> Rick-
>>
>> To correct the R-L delay, you apply the same delay offset to L (only)
>> for *all* antennas, in the manner in which you have always done
>> so with CLCOR.  This on top of the per antenna L offsets (the big
>> component compensating for the offset with R comes out in the L
>> differences, of course).
>>
>> -George
>>
>>
>> On Fri, Jul 31, 2009 at 11:29 AM, Rick Perley<rperley at nrao.edu> wrote:
>>>
>>>    I'm going through the full polarization data of 3C286, taken at
>>> L-band on 3C286 (a strongly polarized source).
>>>
>>>    There are some remarkable delay issues:
>>>
>>>    1) Delays within the RR side are generally where we have seen them
>>> -- typically 20 nsec.  Two antennas are especially large -- 2 (-77 nsec)
>>> and 9 (-88 nsec), w.r.t antenna 1.
>>>    2) Delays within the LL side are much larger, with seven antennas
>>> having delays more than 50 nsec, and one being more than 200 nsec!!
>>>    These are easy to calibrate out, and I did so.
>>>
>>>    But the biggest surprise comes from the delays in the R-L
>>> correlators -- the time delay between the two polarizations.  After
>>> removing the 'parallel-hand' delays, I find the RL and LR correlators to
>>> have a 305 nsec delay error -- of opposite sign in RL to LR (as it
>>> should be), and the same for all baselines (as it should be).
>>>
>>>    Question to AIPS-experts:  I don't know how to get AIPS to correct
>>> for the delay between R and L.  The phase correction ('POLR' in the
>>> program CLCOR) is not the correct operation.  Can AIPS do this?
>>>
>>>    Other than this -- the X-hand amplitudes look good, and are at about
>>> the right level.
>>>
>>>
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>>>
>>
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