[daip] VLBA bandpass calibration in AIPS (fwd)

[Craig Walker]

I have used some data taken a year ago, but just recently
post-processed to investigate the amplitude calibration issue.

First, a quick review of the meanings of the bpassprm settings:

bpassprm(5) -2 => Divide by "channel 0" determined as the
                   average after time averaging.  Used if
                   have good phase stability.
              0 => Divide by "channel 0" determined record by
                   record.
              1 => Do no normalization before determining solutions.
                   Can still normalize with bpassprm(10)=3
bpassprm(10) 2 => Normalize amplitude solution portion of bandpass
                   using channels in ICHANSEL.
              4 => Normalize amplitude solution portion of bandpass
                   using all channels (ignored ICHANSEL, I think).
ichansel          Channels for normalization.
                   Default is inner 75%

I split off my main calibrator scan to make tests easy, and ran
with a variety of combinations of these parameters.  The results
are summarized in the following table.

Summary:
(Scale is calibrated cross correlation amplitude of IF 1 on KP-LA)
(BP amp is the eyeball amp of the central channels in the BP table.

Ver  bpassprm  ichansel  AmpSlopes  PhaseSlopes  Scale  BP amp
      (5) (10)
  1   -2    4     1,64      flat        flat       1.52   1.12
  2   -2    4      0        flat        flat       1.52   1.12
  3    0    4     1,64      flat        flat       1.52   1.12
  4    1    4     1,64      sloped      flat       1.52   1.12
  5    1    2     1,64      sloped      flat       1.87   1.00
  6    1    2      0        sloped      flat       1.87   1.00
  7    0    2      0        flat        flat       1.87   1.00
  8    0    2     1,64      flat        flat       1.52   1.12
  9    1    4      0        sloped      flat       1.52   1.12

Assuming ICHANSEL=0, Sasha's "new" is bpver 9, "old" is bpver 7, and
"newold" is bpver 6.  So I agree "old" and "newold" should give the
same results, while "new" should be different at the ~23% (24% close
enough) level.  I think it mainly boils down to whether or not the
normalization was over all channels, or the inner 75%, the default
with ICHANSEL=0.

bpassprm(5)= 1 (no pre-solution normalization) doesn't seem to work
very well.  Not sure why, but it ends up with small amplitude slopes.
Perhaps it is some sort of coherence issue with moving phases?  The
slope is about 3% across the band, so it's not too scary.

Note 1.12**2 =1.25 is about the scale difference reported here and in
Sasha's note.  It is certainly within the eyeball amplitude errors.

Here that the actual eyeball bandpass table values for the central
channels are either about 1.12 or about 1.00.  I think the 1.12 result
is appropriate when adding all across the band while the 1.00 is
appropriate for the center.  Breaking the channels into far edge,
inner edge, and center channels and eyeballing a spectrum that has
1.12 in the center, I get an average of roughly:
(16*0.75 + 1.04*8 + 1.12*(64-16-8))/64 = 1.017
This is probably within the eyeball errors of 1.0 - the expected
value.  So the normalization across all channels is ok when it
happens.  Note 1.12**2 =1.25 is about the scale difference, certainly
within the eyeball amplitude errors.  So I think most of what matters
here depends on whether the average was over the whole band or the
inner 75% (the default with ICHANSEL=0 when ICHANSEL is used).

It seems that full band averages are done when bpassprm(10)=4 as
advertised.  When bpassprm(10)=2, ICHANSEL is supposed to be obeyed
which seems to be the case except in the one instance when no
pre-solution normalization (bpassprm(5)=1) is done - another case of
that setting leading to strange results.  After seeing these results,
I cannot recommend bpassprm(5)=1.

I use the parameters of BP 1 (bpassprm(5)=-2 and bpassprm(10)=4) and I
see no reason to change after this little study.

Note that the Tsys values are measured as an average over the whole
band, both during the observations and when measuring the Ta/Ts values
used to determine the gains.  So I think it is important to use the
full band normalization.

Currently the gains are measured using the legacy BBC power detectors.
Those have different bandpass shapes then the PFB filters.  I'm not
100% convinced right now, but I think this does not matter if the Ts
values used for calibration are measured in the same filters as are
used for the interferometer data.  The gain is really a fractional
increase in power for a given source flux density and I think that
will be the same regardless of the filter shape when measured over the
whole filter.  The bandpass calibration just allocates the power to
the right places in the band.  The Tsys with the data, at least for
the more recent data, is measured in the same filters as used for the
interferometry.

My thinking on this subject changed somewhat from what I have said in
the past and what I put in some of the earlier emails in this thread.
I used to think the fact that the gains were measured with different
filters really mattered.  My current thinking is that it does not
matter.

I have taken the liberty to add the VLBATESTS group to this email as I 
think the results might be of interest to others working on the VLBA.

Cheers,

Craig


On 06/25/2014 01:59 PM, apushkar at mpifr-bonn.mpg.de wrote:
> 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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     R. Craig Walker            Array Operations Center
     cwalker at nrao.edu           National Radio Astronomy Observatory
     Phone  575 835 7247        P. O. Box O
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