[daip] AIPS and BPASS

[Eric Greisen]

Yuri Y. Kovalev wrote:
> Dear Eric,
> 
> I have started recently to analyze the BPASS application for VLBA data 
> in order to make sure that no amplitude shift is introduced in my data 
> after BPASS calibration.
> 
> Here is an example of how I calculate the complex bpass for the MOJAVE 
> 2cm VLBA data:
> timer=0; calsour='2200+420''
> docal 2; bpassprm 0; bpassprm(10) 2; bpassprm(11) 2; bpver 0;clr3nam
> gainuse 0;outver 0; bif 0; eif 0;solint -1; smodel 0; doband -1
> SPECINDX=0;SPECURVE=0
> runwait('bpass')
> 
> It seems [can be wrong, still continue analyzing], I do see a shift in 
> the amplitude of at least some of my VLBA data by about 5%. It 
> apparently depends on the 8/16/32 MHz filter and correlation parameters.
> Here are my questions, if I may bother you with them:
> - do you see anything clearly wrong in the setup of bpass above?
> - after bpass application, I lose the first and last channel in every 
> IF. Is it a default behavior of bpass? Why? Can I run bpass and keep 
> the first and last channel in every IF? This would indeed be my 
> preference.
> 
> Best regards,
> Yuri
> 
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How stable are the phases within each scan?  If they are stable, 
bpassp(5) should be 1.  If amplitude is stable, but phase is not,
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.

In VLBI, the bandpasses are shifted in frequency after they are 
determined to correct for the Earth's rotation.  This causes the 
outermost spectral channels to have no calibration in most cases, often 
more than 1 channel at each edge.  Note that with most systems, the 
outermost channels are also affected by aliasing in the FFTs and so are 
contaminated and noisy and not really worth keeping.

Eric Greisen