[daip] Aips calibration seems to have changed

[Roderick Johnstone]

Hi,

We have been reducing some GMRT data for some weeks now, progressively
cleaning and calibrating the data. After changing the reference antenna
(to look for a better resoution) and then moving back to the original
reference antenna the data have become much worse than they were.

For example, when using the wiper task in IMAG vs REAL, we were getting
down to a nice circular region of uv data points with a dispersion of
about 15. Now the data are no longer circularly symmetric and have a
dispersion of about 40.

(To do the calibrations we are using a script which takes the original
data, does all the bandpass and calibrations using the specified
reference antenna, applies the latest flag table which is saved as a
.txt file and spits out a uv data set with the new corresponding
calibration tables....so it does not modify the original data set in any
way, it just deletes and re-computes the tables attached to it).

We have done a lot of tests and are unable to get back to the original
good data, so need some expert advice on where to look for the problem.

It seems like AIPS is no longer giving the previous calibrated data we
had. We've tried reloading the uv data set into aips, using previous
flag tables, calibrating manually without the script, but nothing works.

Right before re-running the script with the new reference antenna, we
had to shut down aips (which had been open for some weeks) and
reboot the computer, but we cant see how that could be the problem.

The reduction has all been run with the DEC11 version of aips on Linux
x86_64 (Fedora 14). The midnight job has not been run since starting the
reduction so we are pretty sure that aips itself has not changed.

We are keen to find out what went wrong, rather than just start over
because it takes a long time to flag these data and don't want it to
happen again, so would appreciate some pointers.

Many Thanks

Roderick Johnstone (sysadmin and maintainer of our aips installation)
Julie Hlavacek-Larrondo (student actually doing the work)