I sent this email on June 19 but never received any reply. In adition to
what stated at that time I must say that when the average value of the
image is close to 0, so there are positive and negative pixels, there is
no way to recover the result, as multiplying by -1 obviously does not
help. Please let me know?

Thanks,
Leonardo Vanzi

-------- Original Message --------
Subject: imcombine
Date: Mon, 19 Jun 2006 00:39:43 -0400
From: Leonardo Vanzi <lvanzi@eso.org>
To: iraf@noao.edu

Dear Iraf expert,

I happened to notice that the option "scale" in imcombine does not
produce the correct sigma image when the input images have negative
counts. Using scale=none, median or mean I always obtain the same result
as sigma image. The same is not true when the counts are positive, in
this case I obtain results that make more sense. Is there any bug
reported with this respect? Of course I can fix this by multiplying my
images by -1.
I am running IRAF 2.12.2 on Mac OS version 10.3.9.

Thanks a lot for your help,
Leonardo

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Question answered in http://iraf.net/phpBB2/viewtopic.php?t=85389