With the advent of new correlators for the EVLA, VLBA and ALMA, a
number of changes have been happening in AIPS lately.  In the process,
some bugs were introduced, and have been fixed.  This message
describes one change in particular that users should be aware of and a
few bugs that might have caught VLBI users.

CHANGE OF TERRESTRIAL REFERENCE FRAME

The change is a move to a right-handed coordinate system for stations.
The puts AIPS in agreement with the ITRF rather than an ancient
left-handed system (left over from MarkII days?) and is therefore the
right thing to do.  But it does introduce compatibility issues during
the transition.  In practice, the change means that the sign of BY in
the AN table is reversed.  This change was made in the 31DEC09 version
of AIPS on Sept. 20.  If the 31DEC09 (or 31DEC10) version of AIPS sees
an AN table of the old type, identified by knowing the approximately
correct coordinates of many stations, it converts the table to the new
convention so backward compatibility is good.  The problem comes when
a data set with the ITRF convention is read by an older version of
AIPS, be it 31DEC08 or earlier, or even a 31DEC09 version that has not
had a midnight job run since Sept. 20.  The older versions will not
recognize that the coordinates are different and any task that needs
station elevations or positions will get them wrong.  This includes
UVFIX, any plotting routines doing elevation or parallactic angle on
an axis, CLCOR (any operations that need to know the antenna location
or pointing direction), UVFLG elevation flagging, CALIB elevation
cutoff for gain normalization, TECOR, ANTAB with opacity corrections,
DELZN, PCAL, LPCAL, CVEL and probably others.  In other words, it will
almost certainly affect your processing.

Users can get into trouble during the transition in a couple of ways.
If they use both new and old versions of AIPS in their processing, the
recent version will convert the AN table, but the old version won't
know about it.  So, for now, it is best not to mix versions.  The
processed FITS files in the VLBA archive are produced using the latest
AIPS.  Therefore, files made since around Sept 20 will have the new
antenna convention.  If those are downloaded and processed in an older
AIPS, there will be problems.  A procedure, ANFIX8 is now available
from a midnight job for 31DEC08 or from the 31DEC08 patch info on the
web that will convert back to the frame needed by the 31DEC08
AIPS. Alternatively, one can download the individual job files and run
the procedures (VLBALOAD, VLBAFIX) to read and combine them.  By now,
the 31DEC09 version has become "NEW" and is the latest stable version
while 31DEC10 is "TST".  If one stays with those two, there should not
be a problem.

ANTENNA COORDINATE BUG IN FITLD

There was a bug in the implementation of the above change that caused
the old convention to be used for any station that did not appear in
the first correlator job file.  This will affect observations for
which some station, typically Mauna Kea, starts late.  The processed
archive files (but not the FITS IDI files for each job) are subject to
this bug.  Users should check their AN tables carefully to see if a
fix is needed.  For example, all VLBA antennas should have the same
sign of BY.  A procedure to fix affected data (ANCHECK) will be
available with the midnight job after Dec 1.  Processed files in the
archive are subject to this bug.  They will be fixed eventually, but
meanwhile be careful with such files.  If you schedule calibrators on
all antennas at the start of your run, you may not be subject to the
problem.  This bug affects AIPS 31DEC09 with midnight jobs run between
Sept. 20 and Nov. 26.  Processed archive files made during this date
range are suspect.

POLARIZATION CALIBRATION BUG

The application of polarization calibration (DOPOL=1) was entirely
broken in 31DEC09 AIPS versions updated between Sept. 20 and
Nov. 17. The data are modified, but not correctly.  Any application of
polarization calibration done with an affected AIPS should be redone
after running a midnight job.  Note that the derivation of the D terms
is probably ok if your antenna coordinates are ok.

POLARIZATION EVPA CORRECTION BUG

The POLR option in CLCOR, used to adjust the absolute polarization
position angle, was broken between Sept. 20 and Nov 26 in the 31DEC09
AIPS.  The CL table is corrected properly, but the AN table may not
be.  The task made an error in determining the number of IFs in the
table and might only modify some, but not all IFs.  This leads to
IF-dependent errors in adjusting the angle when the polarization
rotation is applied. The fix is to update AIPS and re-apply the
rotations with CLCOR.  This requires zeroing the angle in the AN table
or deleting the AN table and copying an unmodified one, that hopefully
you saved early in your processing, in its place.  Then rerun the
polarization calibration.

AIPS MAINTENANCE

Note that, even if you are using an older AIPS, it is useful to
occasionally run a midnight job or be sure you get any recent
patches. These patches correct bugs that have been found that are
believed to potentially cause significant problems for user's data.
For example, recent patches for 31DEC08 AIPS have corrected errors in
the use of the ICHANSEL adverb in BPASS and in the frequency put in
the AN table by FITLD when rearranging the frequency order from
correlator FITS IDI files.

We apologize for any disruption caused by these issues.

Sincerely

Craig Walker and Eric Greisen
Revised Dec. 7, 2009.