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#### 3.8.2 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:
  - BINTABLE columns are those columns defined using the FITS BINTABLE standard
  - Data columns are the columns to be encoded
  - `N_TOT` is the total number of data columns to be stored
  - Data columns with (1-based) indexes from 999 to `N_TOT` inclusive are known as extended columns. Their data is stored within the container column.
  - BINTABLE column 999 is known as the container column. It contains the byte data for all the extended columns.
Convention:
  - All column data (for columns 1 to `N_TOT`) is laid out in the data part of the HDU in exactly the same way as if there were no 999-column limit.
  - The `TFIELDS` header is declared with the value 999.
  - The container column is declared in the header with some `TFORM999` value corresponding to the total field length required by all the extended columns ('`B`' is the obvious data type, but any legal `TFORM` value that gives the right width MAY be used). The byte count implied by `TFORM999` MUST be equal to the total byte count implied by all extended columns.
  - Other `TXXXX999` headers MAY optionally be declared to describe the container column in accordance with the usual rules, e.g. `TTYPE999` to give it a name.
  - The `NAXIS1` header is declared in the usual way to give the width of a table row in bytes. This is equal to the sum of all the BINTABLE column widths as usual. It is also equal to the sum of all the data column widths, which has the same value.
  - Headers for Data columns 1-998 are declared as usual, corresponding to BINTABLE columns 1-998.
  - Keyword `XT_ICOL` indicates the index of the container column. It MUST be present with the integer value 999 to indicate that this convention is in use.
  - Keyword `XT_NCOL` indicates the total number of data columns encoded. It MUST be present with an integer value equal to `N_TOT`.
  - Metadata for each extended column is encoded with keywords of the form '`HIERARCH XT TXXXXnnnnn`', where `TXXXX` are the same keyword roots as used for normal BINTABLE extensions, and `nnnnn` is a decimal number written as usual (no leading zeros, as many digits as are required). Thus the formats for data columns 999, 1000, 1001 etc are declared with the keywords `HIERARCH XT TFORM999`, `HIERARCH XT TFORM1000`, `HIERARCH XT TFORM1001`, etc. Note this uses the ESO HIERARCH convention described at https://fits.gsfc.nasa.gov/registry/hierarch_keyword.html. The name space token has been chosen as '`XT`' (extended table).
  - This convention MUST NOT be used for `N_TOT`<=999.

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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STIL - Starlink Tables Infrastructure Library
Starlink User Note252
STIL web page: http://www.starlink.ac.uk/stil/
Author email: m.b.taylor@bristol.ac.uk