[daip] CVEL & SETJY questions (Doppler tracking correction)

[Michael Bietenholz]

I was helping a student (Alet Dewitt) with AIPS and we were having
some difficulty with using CVEL to apply the manual Doppler tracking.

Your instructions say to use SETJY to set the rest-freq. and then
run CVEL.

We did this, entering the frequency in RESTFREQ.  However, when we
then ran CVEL it caused large shifts (>100 channels; outside of our
observed band)

Trying to track down what was going on , I printed out the SU table
with PRTAB.  Now what is not quite clear to me is what the entries in
the SU table that are made by SETJY are, in particular, what is the
"LSRVEL" is.  The things I would have thought it might be is either
the velocity of the telescopes (ie. array centre) wrt. the local
standard of rest, or effective velocity offset implied by the
difference between the observed centre frequency (once corrected for
earth's motion wrt. the LSR) and the rest-frequency of the line.

As an experiment, I set RESTFREQ to exactly the observed centre
frequencies in SETJY (ie. Header freq + FQ table offset per IF).
SETJY then fills in two *different* values for LSRVEL for the two
IF's, which doesn't seem right in either the above interpretations of
LSRVEL.

So, what we've got I think is observations for a maser, made
with Doppler tracking turned off (because of EVLA).  We just want
to correct for the small time-dependent frequency offsets
caused by the motion of the array.  What do we enter for
RESTFREQ, SYSVEL, VELTYP and VELDEF in SETJY?

Also - we have both continuum calibrator sources and a maser
source - does one apply CVEL to both or just the maser?
(seems like it shouldn't make much diff to the calibrators
which will have smooth spectra)

              thanks,       michael bietenholz