[daip] Newsletter article for comment

[Eric Greisen]

New VLBA/VLBI Calibration Procedures in AIPS

For the past year the AIPS group has been developing procedures to
simplify the reduction of VLBA data and, in many cases, other VLBI
data as well.  These procedures are contained in the RUN file VLBAUTIL
available in the 31DEC01 release of AIPS.  They include procedures to
load, ``fix'', calibrate, and examine data, up to and including fringe
fitting.  Using these procedures, it takes only a few hours to take a
typical twelve-hour continuum phase-referencing experiment from
loading to imaging with most of the time being spent loading the data
and fringe fitting.  These procedures not only simplify the inputs to
tasks, but run the variety of ``bookkeeping'' tasks that need to be
performed for a calibration step.  For example, the fringe-fitting
procedures fringe fit and then apply the calibration by looping
through the sources.

The procedures can be separated into three categories: procedures that
should be run for all experiments, special case procedures (multiple
subarrays, multiple frequencies and polarization data), and data
examination procedures.  In the first category there are procedures to
load, remove redundant calibration tables, determine a-priori
amplitude corrections, determine instrumental phase corrections, and 
fringe fit the data.  The special case procedures will find subarrays,
copy different frequencies to separate files, fix polarization
labelling, correct parallactic angles, and calibrate cross-polarized
delays.  To examine data there are procedures that print the antenna
and scan information for the experiment, plot the calibration tables
verses time, and plot the cross-correlation spectrum.

For a detailed description of all the procedures, see AIPS
Memo No. 105, "AIPS Procedures for Initial VLBA Data Reduction",
by Ulvestad, Greisen and Mioduszewski available at the AIPS web site:
http://www.cv.nrao.edu/aips under documentation.