The FITS BINTABLE standard (FITS Standard v3.0, 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:
N_TOT is the total number of data columns to be stored
N_TOT
inclusive are known as extended columns.
Their data is stored within the container column.
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.
TFIELDS header is declared with the value 999.
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.
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.
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.
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.
XT_NCOL indicates the total number
of data columns encoded.
It MUST be present with an integer value equal to N_TOT.
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).
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.