[daip] AIPS and BPASS

[Eric Greisen]

Leonid Petrov wrote:
> Dear Eric,
> 
>   Thank you for your detailed answer.
> 
>> bpassp(5) = -1.  With 0, the data of each record are normalized in amp 
>> and phase which can introduce a Ricean bias since the expeted amplitude 
>> in the case of no signal is not zero.  VLBI often uses all channels so 
>> bpassp(10) = 1 or 4 rather than 2.
> 
>   May I ask you a favor to shed a light how AIPS normalizes bandpass?
> We found that applying bandpass increased the fringe amplitude by 5.8%
> in our case of VLBA observations of strong sources. The phase of the bandpass
> did not exceed 0.08 rad, so it is bandpass amplitude that caused the change
> in the fringe amplitude. The average value of the bandpass that Yuri
> generated with AIPS was not 1.

I already did in the above.

If BPASSP(5) 0 as in the example you sent, then for each data record the 
subroutine DIVCHZ is called.  It averages the vis with $APLSUB/AVGCHN 
and then divides that into the vis (subscript 1 is real, 2 imaginary, 3 
weight).

If BPASSP(10) is set, subroutine NORMBP is called.  It averages up the 
amplitudes and then divides them into the bandpass or does a complex 
average if requested.  It does this on the bandpass values them selves 
which are antenna based only.

BPASS is found in $QPGNOT and I believe the code to be readable with the 
guides given.

5% is a possible result of BPASSP(5)=0 with somewhat noisy data.

ERic Greisen