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#### 4.1.1.1 FITS

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 from a multi-extension FITS file, this is indicated by giving an identifier after a '#' at the end of the table location. The identifier can be in one of two forms:

- The numeric index of the HDU. 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.
- The name of the extension. This is the value of the EXTNAME header in the HDU, or alternatively the value of EXTNAME followed by "-" followed by the value of EXTVER. This follows the recommendation in the FITS standard that EXTNAME and EXTVER headers can be used to identify an HDU. So in a multi-extension FITS file "cat.fits" where a table extension has EXTNAME='UV_DATA' and EXTVER=1, it could be referenced as "cat.fits#UV_DATA" or "cat.fits#UV_DATA-1". Matching of these names is case-insensitive.

You can select which extension to use interactively 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.

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Next: Column-oriented FITS
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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk