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Hi, my name is Deborah and I'm an undergraduate student at Austin Peay State University, trying to teach myself how to use IRAF.
Right now, I'm attempting to create fringe frames, but every time I try to run ccdproc on the fringe images I have selected, I receive the error "Warning: FXF: must specify which FITS extension (n21370.fits)" for example, with the filename changing for each separate image.
I realize this may be very vague, but I'm not too sure how else to describe it.

Thanks in advance for any help!

The error message implies your data are multi-extension FITS images and it is telling you that you need to specify which extension to use, e.g.

cl\$this->_split2($m[0]) imstat foo.fits[1]

instead of

cl\$this->_split2($m[0]) imstat foo.fits

where the number in the square brackets is the (zero-indexed) extension number. Exactly which extensions to use will depend on the FITS file layout, but generally speaking you should probably be using the MSCRED external package (or NFEXTERN) which will deal with the extensions automatically for you.

See the documentation on using Mosaic with generic CCD data at https://iraf.net/irafdocs/generic.php