[daip] BPCOR: spectral index correction

[Olaf Wucknitz]

Hi,

I tried BPCOR for the first time to correct for the spectral index of the 
calibrator or tilt spectral slopes in general.

There seem to be some problems with it, even though I am not entirely sure 
if I understand everything correctly.

1. BIF and EIF don't seem to be used.

2. All gains are corrected with freq**APARM(1). I think only the sqrt of
    that factor should be applied to each antenna.

3. The scaling with frequency uses the reference frequency etc. but not
    the offsets for all IFs. In the case of large separations between IFs,
    this is not entirely accurate.

4. The pivot channel APARM(4) is not properly applied, and the frequency
    calculations don't seem to be plausible generally.
    IF CHNOFQ is meant to be the frequency at the pivot channel, it should
    be

       CHNOFQ = REFFRQ - (REFPIX - CHNPIV) * CHNINC

    rather than

       CHNOFQ = REFFRQ + (REFPIX - CHNPIV) * CHNINC


    And the correction factor should be

                ACOR = (1.0 - ((CHNINC/CHNOFQ)*(CHNPIV-ICHAN)))
      *            ** (0.5*APARM(1))

    instead of

                ACOR = (1.0 - ((CHNINC/CHNOFQ)*(REFPIX-ICHAN)))
      *            ** APARM(1)

    to make it unity at ICHAN=CHNPIV.


As said, my interpretation may not be entirely correct, but in any case 
the result seems to be wrong.

This does not have the highest priority for me at the moment, because I 
can use parseltongue instead to make the corrections.


Cheers,
Olaf