[daip] question about BPASS (specifically, about the CHN_SHIFT field)

[Leonia Kogan]

Hello Dan,

I have looked at the BPASS code and the subroutine DATBND which makes
application of the band pass correction.

I think I got the picture.

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Shortly the answer is: CHN_SHIFT=-1000 does not effect the result.
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Now the details:

in BPASS:

For each baseline IA1-IA2 the spectrum shift of the first antenna relatively 
the center of the Earth is calculated. The maximum value of this shift 
for each IF and each antenna is stored in AVDELI(IA1, IF). AVDELI is initiated 
to be equal -1000. Because IA1 can not be equal to the last antenna number 
AVDELI(ILAST, IF) = -1000. AVDELI is recorded to the CHN_SHIFT.

in DATBND:

The value of the shift for all antennas is recalculated.
The CHN_SHIFT column is used only in estimmation of the flagging of 
frequency channels at the beginning and end of the spectrum.
IF (the recalculated SHIFT .NE. 0) THEN
   At the beginning the MAX(CHN_SHIFT, RECALCULATED_SHIFT) is used.
   So if CHN_SHIFT=-1000 then the recalculated SHIFT (which is expected 
   to be >-1000) is used for the calculation LEFT_FLAG

   Similar right thing is done in calculation of the right edge of the spectrum.

ELSE
   LEFT_FLAG = CHN_SHIFT
   and the flagging is ignored, if CHN_SHIF=-1000 because there is no 
   a channel <-1000
    
   END IF


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>From owner-daip at kochab.cv.nrao.edu Wed Dec 13 16:21 MST 2000
Date: Wed, 13 Dec 2000 16:21:20 -0700 (MST)
From: Craig Walker <cwalker at aoc.nrao.edu>
To: DAIP at zia.aoc.NRAO.EDU
Subject: [daip] question about BPASS (specifically, about the CHN_SHIFT field)
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Could someone answer this?  I presume CHN_SHIFT has something to do with
the shifting that can happen with CVEL, but it is likely that Lebach is
dealing with continuum data and not using CVEL.  In any case, I don't have
a clue why the number should be -1000 occasionally when it is normally 
much less than 1.

Craig




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>From del at gyro.harvard.edu Wed Dec 13 16:15 MST 2000
Date: Wed, 13 Dec 2000 18:14:59 -0500 (EST)
To: aipsmail at cv3.cv.nrao.edu
Subject: question about BPASS (specifically, about the CHN_SHIFT field)
Cc: cwalker at zia.aoc.NRAO.EDU, dgg at leo.gsfc.nasa.gov,
        michael at polaris.phys.yorku.ca, ransom at polaris.phys.yorku.ca
From: Daniel Lebach <dlebach at cfa.harvard.edu>
Content-Type: text
Status: RO


Hello,

I have noticed that for certain non-VLBA sites, when I run BPASS, I
sometimes get values of -1.0D+03 for the CHN_SHIFT field (indicating
some kind of problem?) even though weights (WEIGHT field) and phases
(REAL and IMAG fields) look fine (nonzero weights and phases) and I
know of no problems with the data.  Can you explain why this
happens?  Need I be concerned?

In the most recent experiment I looked at (BR063C, obs. on 1999
Sept. 18-19), I got CHN_SHIFT values of -1.0D+03 for the VLA most of
the time but not all the time, depending on the epoch and timerange.
For example, when I specified one particular epoch via TIMERANGE and
ran BPASS, I got reasonable values of CHN_SHIFT for the VLA (about
1.9D-02 for all elements), but for a different epoch I got CHN_SHIFT
values of -1.0D+03, and when I used TIMERA 0, I got CHN_SHIFT values
of -1.0D+03 at all epochs for the VLA, including the epoch for which
it was not that value with a single-epoch timerange.  I found that
very worrisome.  Just how is this field used, anyway?  The weights
and phases looked fine in all cases.

-- Dan Lebach

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| Daniel E. Lebach         ! Harvard-Smithsonian Center for Astrophysics |
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