[daip] question regarding FXPOL

[Eric Greisen]

ryan ransom writes:

 > I have a couple of questions regarding the AIPS task FXPOL.  The new
 > version writes a new file while fixing the image header as well as
 > fixing and/or copying some extension tables.  For some reason, at
 > least for my data set, the AT table was not appended to the new file.
 > Is the AT table important?  Is there a reason why the AT table would
 > be dropped?

         The AT is a correlator form of the AN table.  It is actually
used only in FITLD but had been carried along in case some of its
extra information might someday be useful.  Dropping it makes sense.

 > 
 > Also, could you briefly state what the differences are between the new
 > version of FXPOL which writes a new file, and the old version which (I
 > believe) only updated the existing data file?  Can the new FXPOL write
 > the new file over the existing file so that extra disk space is not
 > required?

        The new program can handle essentially any polarization
combination thrown at us by the various vlb correlators around the
world.  The old program was able to change only the simplest cases and
did not even handle all that the VLBA correlator was capable of giving
us.  Disk is now cheap and the cost of FXPOL failing in the middle
while overwriting your data is having to reload and start over.  There
may be real problems in table writing when overwriting a file.  If
FXPOL allows you to overwrite, then you can certainly try it.  But it
is a new program and there may be unanticipated problems (and its
author no longer works for us).

Eric Greisen