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#### 3.8.1 FITS-plus

When writing tables to FITS BINTABLE format, STIL can optionally store additional metadata in the FITS file using a private convention known as "FITS-plus". The table is written exactly as usual in a BINTABLE extension HDU, but the primary HDU (HDU#0) contains a sequence of characters, stored as a 1-d array of bytes using UTF-8 encoding, which forms the text of a DATA-less VOTable document. Note that the Primary HDU cannot be used to store table data, so under normal circumstances has no interesting content in a FITS file used just for table storage. The FITS tables in the subsequent extensions are understood to contain the data.

The point of this is that the VOTable can contain all the rich metadata about the table(s), but the bulk data are in a form which can be read efficiently. Crucially, the resulting FITS file is a perfectly good FITS table on its own, so non-VOTable-aware readers can read it in just the usual way, though of course they do not benefit from the additional metadata stored in the VOTable header.

In practice, STIL normally writes FITS files using this convention (it writes the VOTable metadata into the Primary HDU) and when reading a FITS files it looks for use of this convention (examines the Primary HDU for VOTable metadata and uses it if present). But if an input file does not follow this convention, the metadata is taken directly from the BINTABLE header as normal. Non-FITS-plus-aware (i.e. non-STIL) readers will ignore the Primary HDU, since it has no purpose in a standard FITS file containing only a table, and it doesn't look like anything else that such readers are usually expecting. The upshot is that for nearly all purposes you can forget about use of this convention when writing and reading FITS tables using STIL and other libraries, but STIL may be able to recover rich metadata from files that it has written itself.

To be recognised as a FITS-plus file, the Primary HDU (and hence the FITS file) must begin like this:

        SIMPLE  =              T
        BITPIX  =              8
        NAXIS   =              1
        NAXIS1  =            ???
        VOTMETA =              T

The sequence and values of the given header cards must be as shown, except for `NAXIS1` which contains the number of bytes in the data block; any comments are ignored.

The content of the Primary HDU must be a VOTable document containing zero or more `TABLE` elements, one for each BINTABLE extension appearing later in the FITS file. Each such TABLE must not contain a `DATA` child; the table content is taken from the BINTABLE in the next unused table HDU. For instance the Primary HDU content annotating a single table might look like this:

       <?xml version='1.0'?>
       <VOTABLE version="1.3" xmlns="http://www.ivoa.net/xml/VOTable/v1.3">
       <RESOURCE>
       <TABLE nrows="1000">
         <FIELD datatype="double" name="RA" ucd="pos.eq.ra;meta.main"/>
         <FIELD datatype="double" name="Dec" ucd="pos.eq.dec;meta.main"/>
         <!-- Dummy VOTable - no DATA element here -->
       </TABLE>
       </RESOURCE>
       </VOTABLE>

The first extension HDU would then contain the two-column BINTABLE corresponding to the given metadata.

The VOTable metadata MUST be compatible with the structure of the annotated BINTABLE(s) in terms of number and datatypes of columns.

Note: This arrangement bears some similarity to VOTable/FITS encoding, in which the output file is a VOTable which references an inline or external FITS file containing the bulk data. However, the VOTable/FITS format is inconvenient in that either (for in-line data) the FITS file is base64-encoded and so hard to read efficiently especially for random access, or (for referenced data) the table is split across two files.

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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