FITS binary and ASCII table extensions can be read. Normally, TOPCAT will load all TABLE or BINTABLE extensions in a given file. If only a single extension is required, this is indicated 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 named "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.
You can select which extension to use more conveniently than by specifying the HDU numbers if you use the Hierarchy Browser to load the table.
If the table has been written using TOPCAT's "fits-plus"
output format (see Section 4.1.2.1) then the metadata will be
read in from the primary HDU as well.
For normal FITS files, header cards in the table's HDU header will be made available as table parameters (see Appendix A.3.2). Only header cards which are not used to specify the table format itself are visible as parameters (e.g. NAXIS, TTYPE* etc cards are not). HISTORY and COMMENT cards are run together as one multi-line value.
If the table is stored in a FITS binary table extension in a file on local disk in uncompressed form, then the table is 'mapped' into memory - this generally means fast loading and low memory use.