3.4 XAS header keywords

The header of a XAS file is composed by a sequence of keywords. Programs store in keywords basic parameters of a file, or details of the data processing done. The content of a file header can be accessed programmatically by later programs (in this way constituting a way of exchange of information), and viewed or edited by the user,

Each keyword has a name, a type, a length and (a) value(s).

the name is a possibly meaningful, case-insensitive word, no longer than 8 characters for FITS compatibility. Not all FITS mandatory keywords are used in XAS files (when necessary they can be generated by conversion programs) but if they are the same names are used.
the type is one of character, integer, real or double precision as detailed elsewhere
the length of a character keyword is the string length. For FITS compatibility this is limited to 68 characters (although in principle could be longer).
the length of a numeric keyword is either one for scalar keywords, or the number of elements for array keywords. The total length in bytes cannot exceed 246.
the value is either a string, or an array of one or more numeric values

Keywords in the file usually appear in the following order (which is however irrelevant since entire headers are read in memory at once) :

subset of FITS mandatory or recommended keywords. In particular the NAXIS2 keyword is the number of bins or photons in tabular files (while NAXIS1 and NAXIS2 are the X and Y sizes of an image). Also all the binary table keywords like TFIELDS, TTYPEn etc. are supported.
the original file name without path and type is in FILENAME
the OBJECT and OBSERVER keywords are filled deriving the information from FOT tape directory.
there is a SATELLIT keywords instead of the FITS TELESCOP, the (SAX) instrument name is in INSTRUME and the sub-unit in DETNAM
then accumulation specific and instrument specific keywords follow.
the last keywords are the HISTORY (i.e. the runstring of the command which created the file) and optionally its PARENT files and some COMMENTs.

When a program manipulates a file in place, it may alter some of the keywords, and add some new ones. In particular it will add a new HISTORY section.
If a program reads from a file and produces a new one, it may copy part or all of the original file header and then add its own HISTORY section.

Only the HISTORY, COMMENT and PARENT keywords may appear more than once in a header (they are "duplicatable").

If an HISTORY keyword is longer than the maximum allowed, it is continued on the next line, prefixing the value with a plus sign.

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