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In mscred when I attempt zerocombine the response is "No data to combine." I can display the mosaic image without problem. The fits look like classic bias images. Initially when I tried zerocombine the response was that I first MUST do an overscan and trim. I did these successfully. As stated above I now get the no data message. The headers seem correct with the imagetype = zero. I am using mac osx (tiger). Any suggestions? Thanks

The message you report often means the files are failing to identify
themselves as "zero" exposures. You can work around this by setting
"ccdtype" in ZEROCOMBINE to "". However you can check how the
exposures are being identified using CCDLIST. Note that ZEROCOMBINE calls CCDPROC so you'll need to check the CCDPROC parameters to do only the processing you need or set zerocombine.process=no to combine only the raw data). Hope this helps.Cheers,
-Mike

Mike, Thanks that worked! David Emory