The FITS Extension   

An important feature of the well-known FITS data format (which was originally designed as a convenient container for the interchange of astronomical images between sites) is its ``FITS header''. This, in essence, is a sequence of character strings each of which contains the name of a keyword, an associated value and (optionally) a comment.

Although rather few of the keywords that appear in a FITS header have standardised meanings, the freedom that this gives makes it a convenient place to store information about which the reader or writer may have little knowledge. A special NDF extension mirroring the properties of a FITS header can therefore provide a useful ``airlock'' or ``staging post'' for interchanging specialist information between general purpose conversion utilities (for which the information is meaningless) and specialist utilities (for which it has meaning).

To satisfy this requirement a FITS extension, equipped to hold FITS header information, may be added to an NDF. By convention, it consists of a 1-dimensional (HDS) array of _CHAR*80 character strings which holds a sequence of header records according to FITS formatting rules (including the final `END' record).