[daip] VLBA bandpass calibration in AIPS (fwd)

[apushkar at mpifr-bonn.mpg.de]

Dear Colleagues,

We studied the effect of different bpass schemes by applying them
to the same data (BL193AI session) and then using corresponding
imaging results. In particular, we compared flux scales using
total flux density values taken as a sum of all cc components
for each of 22 sources imaged for three versions described
earlier by Yuri (and using his nomenclature)
- new    (bpassprm(5)=1; bpassprm(10)=4)
- old    (bpassprm(5)=0; bpassprm(10)=2)
- newold (bpassprm(5)=1; bpassprm(10)=2)

Median flux scale shift between 'old' and 'new' (see attached plots)
is about 24%, while between 'old' and 'newold' is less than 1% (there
are two outliers due to additional low-amplitude data flagging before
imaging 'newold' version).

Cheers,
Sasha



On 20.06.2014 09:30, Yuri Y. Kovalev wrote:
> ---------- Forwarded message ----------
> Date: Mon, 16 Jun 2014 16:21:51 -0600
> From: Craig Walker <cwalker at aoc.nrao.edu>
> To: Yuri Y. Kovalev <yykovalev at gmail.com>
> Cc: Eric Greisen <egreisen at nrao.edu>, daip at nrao.edu
> Subject: Re: [daip] VLBA bandpass calibration in AIPS
>
> 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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