FITS BINARY and ASCII tables can be read. Unless told otherwise, TOPCAT will display the first TABLE or BINTABLE extension in a FITS file that it is given. If a later extension is required, this is specified by giving the extension number after a '#' at the end of the table location. The first extension (first HDU after the primary HDU) is numbered 1. Thus in a compressed FITS table called "spec23.fits.gz" with one primary HDU and two BINTABLE extensions, you would view the first one using the name "spec23.fits.gz" or "spec23.fits.gz#1" and the second one using the name. "spec23.fits.gz#2". The suffix "#0" is never used for a legal FITS file, since the primary HDU cannot contain a table.
Compressed FITS files, or ones referred to as URLs, may be used.