Header updating when combining images

Frank Valdes wrote on Apr 28, 1998

Q:  A quick question.  I wish to add images together thereby hoping to
improve the SNR.  This is OK using IMALIGN and IMSUM - that is no
problem but how does one do this AND update the header informtion.  I
know that there is the hparams and that it would add the exptimes
together, but I would also need to update the UT time of the 'exposure'
to the middle of the summed image (and hence the airmass using
setairmass) etc.
I guess that by summing images (as opposed to averaging) the exposure
time needs to be summed but the time of image needs to be averaged.

Currently I simply set the middle image as the reference image which is
OK but I was wondering ......


A:  There is no automatic feature that updates the astronomical content of
image headers in IRAF.  The only minimal feature is the ones you see in
IMSUM and IMARITH.  The IRAF tasks that combine or operate of more than
one image to produce a single new output image generally just copy the
first image header they encounter to make the output image header.  The
task may then update a few keywords if it is designed to.

So there isn't anything you are missing and you seem to have a grasp of
what you have to do.  As you said if you get the dependent information
set for the effective image you produce then SETAIRMASS and SETJD can
be used to update other keywords.

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Last post on Apr 28, 1998