[daip] VLBA bandpass calibration in AIPS

[Yuri Y. Kovalev]

Dear Eric,

In Aug 2013 I contacted you about issues related to BPASSPRM values as
applied to VLBA data sets.
Since 2013-Dec-15, we have reduced our MOJAVE datasets with ICHANSEL = 0,
BPASSPRM = 0,0,0,0,1,0,0,0,0,4,1 following on your suggestions. A keyfile
containing the setup can be found at
http://www.vlba.nrao.edu/astro/VOBS/astronomy/may14/bl193ai/bl193ai.key

We've found that with these values, the amplitudes are about 20% lower than
they should be.
We've verified this with near-simultaneous observations with OVRO at the
same observing frequency.

Re-reducing the latest dataset with the BPASSPRMS we used prior to
2013-Dec-15: ie.. ICHANSEL = 0, BPASSPRM = 0,0,0,0,1,0,0,0,0,2,1   gives
amplitudes consistent with OVRO.  We found that in that dataset, using
either BPASSPRM(5) = 0 or BPASSPRM(5) = 1 made less than a 1% difference in
the amplitudes.

So using all of the channels in BPASS normalization (i.e. BPASSPRM(10) = 4)
doesn't appear to be advisable for VLBA data, as the resulting
normalization yields significantly low amplitudes.

I wonder if you could take a look at this issue and comment on it? Do you
find problems in the code? Do you find problems in the algorithm? Did we
make a mistake somewhere?
It seems like (my guess) there might be something to do with the way how
weights of the spectral channels are being used.

To help you, attached please find two BPASS solution plots.
For BPASSPRM(10)=4 (flux seems off my 20% for the current MOJAVE frequency
setup): BPASS4.PS.gz
For BPASSPRM(10)=2 (flux seems OK for the current MOJAVE frequency setup):
BPASS4.PS.gz

Best regards,
Yuri
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