[daip] solve for a complex band pass

[Z.-Q. Shen]

The inputs I used for the BPASS run are something like

calsour='NRAO530'
timer=0 (for all the scans)
docalib=1 (used for solving for the complex bandpass)
gainu=7 (including ACCOR and FRING, but not APCAL of amplitude calibration)
minamper=10
minphser=10
solint=0
bpassprm=1,1 (using ac data)    or,
bpassprm=0,1 (using xc data)
smooth=1,0

I also tried the single calibrator scan about 3 minutes, or restrict
the BPASS to the inner array (without SC,HN and MK), or cut some
edge channels. But unfortunately, the frequency-dependent amplitude
ripple in BP is still there.

  > >BPASS2 11:10:41 Antenna  1  IF  1  corr 2  had      2088 excess closure 
errors

Do I understand correctly that the large number given above (2088)
tells you how bad the bandpass solution is? I suspect such a problem
may be common for the 43 GHz VLBA observations. This may be related
to the low SNR signal for each spectral channel at 43 GHz. Can you
comment on this?

I am wondering if there is any special smoothing function within the
freq band.

-Zhiqiang Shen


>So far as I can tell, BPASS has worked moderately correctly.  Your
>calibrator source appears to be weak and to be resolved so that the
>fringes are even weaker on the longer baselines (more distant
>antennas).  You did not send your inputs to BPASS so it is hard to be
>certain of what all you did.  What is clear is that you set MINAMPERR
>and MINPHSER to fairly small numbers (10) - appropriate to VLA
>bandpasses at longer wavelengths with strong calibrators.  The program
>counts up each baseline-time sample that, when the determined bandpass
>is applied, is not reduced to (1,0) within 10% and/or 10 degrees
>phase.  If all 45 baselines had this then you must have had at least 42
>time samples averaged in the solution interval.  With 0.2 sec
>integrations, that would be only 8 seconds of data - I assume your
>calibrator scans are rather longer but your integration time in the
>data set may also be > than the minimum 0.2.
>
>With a weak calibrator, you should use a solution interval SOLINT=0 to
>do the full scan.  Each time sample is divided by the average of the
>channels so that continuum time changes due to things like atmospheric
>phase and source visbility are fully compensated.  Unfortunately, this
>division also adds noise and a Ricean bias (complex division is phase
>subtraction and actual division by an amplitude which is therefore
>biased away from 0).  Setting BPASSPRM(5) = 1 turns off this division
>and setting BPASSPRM(10) = 1, causes it to normalize the amplitude
>portion of the bandpass.  Doing this requires either a good source
>model or a source sufficiently  unresolved that its visibility does
>not change much over a scan and also good atmospheric phases.  The
>31DEC01 version of AIPS offers more options for dealing with this
>problem but would still require good time stability.
>
>The autocorrelation bandpass is based on an enormous signal and so is
>noise free.  But it does not represent the xc data well on some
>antennas (see PT).  Nonetheless, it may be wise to use it for the
>present and to determine other calibrations.  When the atmospheric
>phases have been calibrated and FRING and ACCOR, then you may be able
>to use a good model for the calibrator rather than channel 0 and then
>do long integrations for a complex bandpass.
>
>Eric Greisen