[daip] forwarded message from Eric Greisen

[Eric Greisen]

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From: Eric Greisen <egreisen at nrao.edu>
To: Olaf Wucknitz <wucknitz at jive.nl>
Cc: DBCON at aoc.nrao.edu
Subject: Re: DBCON problem
Date: Tue, 17 Jan 2006 11:32:59 -0700

Olaf Wucknitz writes:
 > Hi Eric,
 > 
 > sorry to bother you again. With my current data set (upper and lower 
 > sidebands for different IFs), DBCON does not work correctly if the initial 
 > reference pixel for the channels is not pixel 1.
 > DBCON starts with setting this reference pixel to 1 in both data sets. 
 > This works fine if all IFs are either upper or all lower sideband, because 
 > the CDELT for the FREQ axis has the same value as the channelwidth in the 
 > FQ table then. This value is needed to correct CRVAL when CRPIX is 
 > changed. Unfortunately, if the IFs have different signs of the 
 > channelwidth, the modification is not correct. Instead of just changing 
 > CRVAL, the individual frequency offsets should be modified in the FQ table.
 > It was easy to correct that in my data set, but other user may run into 
 > the same problem. It only happens for mixed sidebands and initial 
 > reference pixels !=1.
 > 

I have fixed this in the 31DEC06 version.  Perhaps it should go in the
31DEC05 as a patch, but that may wait for more patches.

Thanks,

Eric Greisen
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