[daip] AIPS image scaling effect

[Eric Greisen]

I am perplexed by your findings.  I cannot find anything in the
changes in 2005 and 2006 to account for a difference between those 2
versions of UVFIX or SPLIT.  UVFIX "recent" change was in 2003 to do
shifting nore accurately (before which your test would have failed
badly).  The change in  the 2007 (and patched 2006) affected
uncompressed data and made an error in frequency during position
shifting equal to the reference channel * channel freq increment.

If I understand correctly, you use UVFIX to shift the full vis data to
the position of the target source.  This should bring each channel to
near 0 phase so the fact that SPLIT corrects u,v,w to the apparent
average frequency (ignoring flagging) should not matter.  Then when
you image, the source is not at the expected position exactly but off
by e.g. 0.5 arc sec although it is not smeared out.

UVFIX does take differential aberation into account when computing the
new u,v,w.  But if the phases are made near 0 by the position shift
then it should not matter so I do not understand your remark about
aberation.  You tell UVFIX a shift in RA/cos(dec0) and in Dec and it
does that rather blindly.  Subtleties in the new u,v,w would affect
sources away from the new phase center not sources at the new phase
center.

Note that if you use IMAGR on the averaged data to make an image of a
source way away from the image center then you should expect both
smearing and, if there is any systematic channel-dependent flagging, a
position error.

I wish I had some better answer to explain what is going on, but I do
not.  UVFIX is the best we can do and the phase shifting part is
actually rather simple.  The recomputation of u,v,w is more difficult
but again that affects things away from the phase center not at the
phase center.  I am wondering if your original measured positions from
wide-field imagr might not be in error due to the frequency averaging.
If your shifts are taken from these measurements, then the errors
would propogate.

Eric Greisen