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Hi,

I'm new to IRAF, and am reducing images using mscred. When trying to use mscskysub, it gives: "Warning: Cannot open file (unavail)," and I'm unable to open the outputs. Why is this happening?? I get the same warning when mscdisplaying my input images, but they still open. I"m using IRAF version 2.16 I believe (from Ureka) on Mac OS X 10.10.4.

My first guess is that the warnings refer to the bad pixel masks that might be referenced by the 'BPM' keyword of the image header. Tasks like MSCDISPLAY can use this automatically, and if you set the 'bpmask' parameter to a null string and the message goes away then it is the most likely cause. Similarly, MSCSKYSUB has a 'mask' parameter that does the same thing (sort of), although it doesn't necessarily explain why you don't get output.

'Cannot open file' messages may also relate to permissions, but I'm assuming you have write permission in the current directory and haven't logged in through another users login.cl file (where you would not then have write permission on the uparm directory). In rare cases the fact that Ureka is a 32-bit only system can cause problems on 64-bit file systems where files essentially aren't visible, however in this case you wouldn't be able to open the images at all.

Thank you, you're first guess was right and the warning went away. However, I'm now having trouble with aligning and stacking the images...when using imalign this is what I get:

immatch\$this->_split2($m[0]) imalign
Input images (test_3.fits,test_2.fits,test_1.fits):
Reference image (test_3.fits):
Reference coordinates file (t3_coord.txt):
Output images (t3.fits,t2.fits,t2.fits):
Warning: Reference not found
ERROR: No shifts were calculated.

But the reference is there and I can open with mscd.
Thanks again for your help!!

The "Reference not found" means the image 'test_3.fits" couldn't be opened by the IMCENTROID task. If these are MEF files (since you're using MSCRED it seems likely), then you'll need to specify the particular extension to be used (e.g. "test_3.fits[1]").

See also https://iraf.net/irafdocs/astrom.php for the write-up on creating/fixing Mosiac WCS images as well as various tasks in the MSCFINDER subpackage. I think the task you're looking for is MSCTPEAK.