[daip] solve for a complex band pass

[Eric Greisen]

Z.-Q. Shen writes:
 > The inputs I used for the BPASS run are something like

     They were about what I suspected

 > 
 > docalib=1 (used for solving for the complex bandpass)
 > gainu=7 (including ACCOR and FRING, but not APCAL of amplitude calibration)

       I am more a VLA person and so thought this not needed - but
correcting fringe rates across the band and with time before averaging
for channel zero and then for the scan average may be a very good idea.

 > minamper=10
 > minphser=10

These are a bit low.

 > solint=0

You could try -1 and do all of the data for a single solution.  This
may be adequate for the vLBA and your S/N does not lead me to expect
that you will achieve b=very high spectral dynamic range anyway.

 > smooth=1,0

One could also do wider smoothing for improved S/N.  I do not know how
much this would help.  The actual bandpass probably does not have
narrow features so smoothing over say 5 channels may not hurt.

 > 
 > 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.

      In the plot I saw this "ripple" appears to be just noise.

 > 
 >   > >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?

       This number is the sum of all time samples and all baselines
for which the corrected data differred from (1,0) by more than 10%.
If you had 100 minutes of 0.2 second data, then you would have 30000
times 45 such samples and 2088 failures would be a failure rate of
0.15 per cent which would be very good.  If you have 3 minutes of
data, then the failure rate is 5.2 % which is worrisome but not a
disaster.  If you have 3 minutes of 1 second data then the failure
rate is 25% which is not good.

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

       There is also the task CPASS.  I hesitate to recommend it
becuase I am not sure that the fitting of the functions is really
proper, but it was designed to fit a function to the complex bandpass
using a smaller number of parameters than the number of channels.  In
CPASS, set BPASSP(3) 1, BPASSP(5) 1.  This does the scan average first
and then divides by a channel 0 determined from the average which is
better than dividing each record by its channel 0.

Eric Greisen