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4.1.3.2 Wide FITS

The FITS BINTABLE standard (FITS Standard v4.0, section 7.3) permits a maximum of 999 columns in a binary table extension. Up to version 3.2 of STIL, attempting to write a table with more than 999 columns using one of the supported FITS-based writers failed with an error. In later versions, a non-standard convention is used which can store wider tables in a FITS BINTABLE extension. The various STIL FITS-based readers can (in their default configurations) read these tables transparently, allowing round-tripping of arbitrarily wide tables to FITS files. Note however that other FITS-compliant software is not in general aware of this convention, and will see a 999-column table. The first 998 columns will appear as intended, but subsequent ones will effectively be hidden.

The rest of this section describes the convention that is used to store tables with more than 999 columns in FITS BINTABLE extensions.

The BINTABLE extension type requires table column metadata to be described using 8-character keywords of the form TXXXXnnn, where TXXXX represents one of an open set of mandatory, reserved or user-defined root keywords up to five characters in length, for instance TFORM (mandatory), TUNIT (reserved), TUCD (user-defined). The nnn part is an integer between 1 and 999 indicating the index of the column to which the keyword in question refers. Since the header syntax confines this indexed part of the keyword to three digits, there is an upper limit of 999 columns in BINTABLE extensions.

Note that the FITS/BINTABLE format does not entail any restriction on the storage of column data beyond the 999 column limit in the data part of the HDU, the problem is just that client software cannot be informed about the layout of this data using the header cards in the usual way.

The following convention is used by STIL FITS-based I/O handlers to accommodate wide tables in FITS files:

Definitions:
Convention:

The resulting HDU is a completely legal FITS BINTABLE extension. Readers aware of this convention may use it to extract column data and metadata beyond the 999-column limit. Readers unaware of this convention will see 998 columns in their intended form, and an additional (possibly large) column 999 which contains byte data but which cannot be easily interpreted.

An example header might look like this:

   XTENSION= 'BINTABLE'           /  binary table extension
   BITPIX  =                    8 /  8-bit bytes
   NAXIS   =                    2 /  2-dimensional table
   NAXIS1  =                 9229 /  width of table in bytes
   NAXIS2  =                   26 /  number of rows in table
   PCOUNT  =                    0 /  size of special data area
   GCOUNT  =                    1 /  one data group
   TFIELDS =                  999 /  number of columns
   XT_ICOL =                  999 /  index of container column
   XT_NCOL =                 1204 /  total columns including extended
   TTYPE1  = 'posid_1 '           /  label for column 1
   TFORM1  = 'J       '           /  format for column 1
   TTYPE2  = 'instrument_1'       /  label for column 2
   TFORM2  = '4A      '           /  format for column 2
   TTYPE3  = 'edge_code_1'        /  label for column 3
   TFORM3  = 'I       '           /  format for column 3
   TUCD3   = 'meta.code.qual'
    ...
   TTYPE998= 'var_min_s_2'        /  label for column 998
   TFORM998= 'D       '           /  format for column 998
   TUNIT998= 'counts/s'           /  units for column 998
   TTYPE999= 'XT_MORECOLS'        /  label for column 999
   TFORM999= '813I    '           /  format for column 999
   HIERARCH XT TTYPE999         = 'var_min_u_2' / label for column 999
   HIERARCH XT TFORM999         = 'D' / format for column 999
   HIERARCH XT TUNIT999         = 'counts/s' / units for column 999
   HIERARCH XT TTYPE1000        = 'var_prob_h_2' / label for column 1000
   HIERARCH XT TFORM1000        = 'D' / format for column 1000
    ...
   HIERARCH XT TTYPE1203        = 'var_prob_w_2' / label for column 1203
   HIERARCH XT TFORM1203        = 'D' / format for column 1203
   HIERARCH XT TTYPE1204        = 'var_sigma_w_2' / label for column 1204
   HIERARCH XT TFORM1204        = 'D' / format for column 1204
   HIERARCH XT TUNIT1204        = 'counts/s' / units for column 1204
   END

This general approach was suggested by William Pence on the FITSBITS list in June 2012, and by François-Xavier Pineau (CDS) in private conversation in 2016. The details have been filled in by Mark Taylor (Bristol), and discussed in some detail on the FITSBITS list in July 2017.


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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