[daip] VLBA bandpass calibration in AIPS

[Craig Walker]

Yuri,

Eric showed me your mail about calibration.  As the keeper (other than 
operations) of the gains, I have some comments.

The gains are measured with essentially analog signals from the legacy 
BBCs.  They have no way of selecting partial bandpasses.  The Tsys 
values that you are using depend a bit on when in our transition to the 
RDBE the data were taken.  But for the recent data, the measurements are 
made in the RDBE.  They are also made over the full bandpass with no 
selection of a partial bandpass.  When everything was in the legacy 
system, to use the Tsys and gains to calibrate, you needed to use the 
full bandpass which was accomplished by normalizing over the full 
bandpass.  If you did the calibration with a normalization over only 
part of the bandpass, the average goes higher than for the full bandpass 
and does not correspond to what was used to derive the antenna gain.

With the RDBE, it gets a bit mixed up and is probably not quite as 
accurate.  We have not made the full transition yet for the calibration 
observations.  So the gains still come from the legacy BBCs.  But the 
Tsys and actual data come from the RDBE.  With the PFB personality, the 
bandpasses have significantly softer edges than with the BBCs.  This 
might cause there to be an offset in the calibration, even with what was 
thought to be proper normalization.  My concern is that, getting a 20% 
effect from this seems to me to be a bit large.  You might try some 
experiments on the magnitude of the differences with full band vs center 
band normalizations with the legacy system (older data sets) and with 
the RDBE_PFB.

The DDC personality has sharper filters and so will likely be more like 
the old system.  But the filters are not the same as the BBCs so even 
that is likely to have some offsets.

Of course, if trying to understand the absolute calibration, we would 
also need to understand the calibration of the OVRO data.

We are close to having the tools to make the gain measurements with the 
new systems, but we are still having some rough edges and have not 
started to do it systematically.  Those measurements, as currently 
configured are made using the DDC.  We have not really thought carefully 
about what to do with DDC vs PFB data.

Cheers,

Craig


On 06/16/2014 03:10 PM, Eric Greisen wrote:
> either BPASSPRM(5) = 0 or BPASSPRM(5) = 1

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     R. Craig Walker            Array Operations Center
     cwalker at nrao.edu           National Radio Astronomy Observatory
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