[evlatests] Some off-center imaging tests/problems

[Ed Fomalont]

Hello all,

DATA BASE:

---I've been looking at the data base:  X0217O1a.55024.720578472225.ms

---2 hours at x-band:  256 channels in 4 IF's, at frequencies

       Frequencies:   8.4798,  8.3518, 7.9678, 7.8398
       Channel separation = -0.5 MHz

       THE CONSECUTIVE FREQUENCY CHANNELS GO DOWN in FREQUENCY!

    egs.  IF1, channel 1 freq = 8.54375 GHz, channel 127.5 = 8.47975

       WAS THIS THE INTENDED SETUP?

--- 1/3 of the time 0217+738 at phase center
--- 2/3 of the time 0217+738 offset 60" to the east,
     interleved.

REDUCTIONS:

--- Moved to AIPS via casa exportuvfits (also tried CASA)

--- Standard editing, calibration, in AIPS.

     Most antennas had a large delay giving 1 to 2 turns over each IF.
     Very little flagging needed, good amplitude, phase stability
     based on on-axis data.  I did the same reduction for CASA.  But,
     somehow the large delay in each IF confused CASA so that the
     calibrated data (applycal) was not usable.  I'll follow this up
     later.

--- Image of 0217+738 looks good.  1000:1 dynamic range.  Should be
     much better, but this is a detail for now.  Normalized to 1 Jy.
     Since this source was self-caled, this is not surprising.

     ATTACHED source.pdf

--- After applying 0217+738 calibration, image of 0217/offset very
     poor.  After changing the sign of the phases, at least this
     image is centered where it should be, but ...

     ATTACHED offset.pdf

--- In order to compare 0217/offset data with what is expected, I
     made a model data set of 1 Jy, 60" offset from the phase center
     using the same data coverage.
     This image is not as good as that of the real source at the
     phase center.  It should be better since it has no NOISE.  Hence,
     there may be slight errors in the data base effecting the phase
     that task UVMOD has calculated.

     ATTACHED model.pdf

--- Compared phases of 0217/offset with phases associated with the
     model, a source 60" from the phase center.

     ATTACHED PHASE.pdf
     yellow = model phases
     blue = observed phases

--- Used EA02 as antenna reference.  There is good agreement between
     model phases and measured phases for all antennas
     EXCEPT FOR EA24 and EA28???

--- Application of the blue phase to the offset data base produced a
     good quality image at the phase center.  Hence, there is no
     indication of closure errors.


BOTTOM LINE:

--Main point:

     Calibrated phases for the offset source for eight out of
     ten antennas source look good!   The general amplitude and
     phase stability over the two hours was excellent with very
     little editing needed (quack and a bad record near the beginning)

     BUT, EA24 and EA28 have slowly changing large phases offsets for
     the offset position, but were calibrated to zero for the onaxis
     source.  So, this is a differential problem.
     Any technical reasons for this, for example a large instrumental
     phase change versus antenna pointing?  This could be caused by a
     large antenna position error for these two telescopes of ~60 meters
     (see below) These offsets are sufficient to produce the crummy
     image.

--Secondary points that need follow-up

     a. The model source of perfect data is not as good as the real
        source model.  Somewhere, the aips task UVMOD is getting
        somewhat incorrect information on how to move a source 60".
        Will check antenna file and other methods like uvfix.  However,
        this might be consistent with a large antenna-offset error,
        causing the model phases to be in error.  To produce an offset
        of 180 deg phase for a position change of 60" requires an
        antenna error of ~1700 wavelengths, or only 60 meters.  Check
        that the correlator has not changed these two antenna positions.

     b. CASA had problems calibrating the large delay error.  Both the
        bandpass and gain calibrations were in good agreement with the
        AIPS cals, but somehow applycal couldn't calibrate the data
        properly.  Needs more investigation.
        Perhaps, the negative increment of frequency/channel is a
        problem.


Cheers,  Ed

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