[daip] AIPS problem

[Gary A. Welch]

Dear AIPS helper,

Recent comments by Eric Greisen have motivated me to revisit an unsolved problem 
in hopes of at least finding the cause, if not a fix.  First, let me explain 
that I am far from being an AIPS expert, so part of my difficulty might result 
from inexperience.

I have D-array data distributed over 4 days in 1998.   The configuration was the 
same on all days.  The IFs were tuned separately to increase the baseline, and 
then UJOIN-ed at the end of the calibration process.  

The problem shows up in IF 2 data from 2 of the 4 days.  Symptom: Each 
single-channel image in the velocity range covered by IF 2 shows a bull's-eye 
pattern.  The rms on those images is 2-4 times larger than on the images from IF 
1.  Curiously, the pattern seems to reverse from channel to channel, so that 
2-channel averaging significantly lowers its amplitude - in most cases.  After 
averaging, though, I'm still left with a few objectionable images, some of which 
occupy interesting velocity ranges!

Flagging has been ineffective.  I've used TVFLG, and also UVFLG to get rid of 
1-2 occasionally flaky antennas and a few very aberrant phases.  I've looked 
closely at UVPLT plots of amplitude vs baseline.  I've spent a lot of time on 
the flagging issue, although I might have missed something obvious to a more 
experienced AIPS user.  Plots of the BP tables seem to show more "jitter" in the 
phases for IF 2 compared to IF 1, but I cannot judge whether that is 
significant.

What do you advise?

Regards,

Gary Welch