[evlatests] u,v,w, and UVFIX

[Gustaaf van Moorsel]

During the transition meeting yesterday I reported on comparing
results of UVFIX on recent data affected by wrong u,v, and w
values to results of the proposed fix to the archive as put in
place by John Benson.  The outcome was that although there was
a large degree of agreement between the UVFIX results and John's
results, there still was a difference which could not be ignored.

During the meeting it was pointed out that UVFIX, by default,
corrects for differential aberration, and that maybe John's fix
does not.  So I re-ran UVFIX, this time turning off the differ-
ential aberration correction (by setting UVFIXPRM(18)=1 for you
AIPS users).

The result is a much closer agreement as shown in the following
table:

           data sets               u       v       w
UVFIX(dr)    - raw data         0.1359  0.1240  0.1821
UVFIX(dr)    - corrected data   0.0045  0.0031  0.0000
UVFIX(no dr) - corrected data   0.0014  0.0000  0.0000

where: UVFIX (dr) is data after applying UVFIX with
        the default setting of UVFIXPRM(18), which means that
        the differential aberration correction was applied

        UVFIX(no dr) is data after applying UVFIX with
        UVFIXPRM(18) set to the non default value 1, which
        turns off differential aberration correction

        raw data is data affected by the u,v,w bug introduced
        when the Modcomps were retired, and not corrected by
        any means

        corrected data is raw data to which John Benson's
        proposed correction has been applied

        u,v,w shows the rms difference (in klambda) between
        the two data sets in column 1

The results strongly suggest that John Benson's fix does not
correct for differential aberration.  And that may be the
correct thing to do (if u,v, and w are defined with respect
to the tangent point), in which case we should consider
changing the UVFIXPRM(18) default in UVFIX.

Note that setting UVFIXPRM(18)=1 also increases the similarity
between the resulting images.  The amplitude of the difference
between various images is shown in the following table.  The
point source itself is 11 mJy.

           data sets             amp (mJy)    %
UVFIX(dr)    - raw data            4         36
UVFIX(dr)    - corrected data      0.2        2
UVFIX(no dr) - corrected data      0.02       0.2


Gustaaf