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Spectra

FITS files with the HDU keywords HDUCLASS and HDUCLAS1 set to OGIP and SPECTRUM respectively will be accepted by Asterix as both a general 1-D binned dataset, and in particular as a spectrum. The following mapping exists between FITS header keywords and their equivalents in the current Asterix system.

The comment of the PA_PNT keyword reads "file average of ROLL". The definitions of roll angle and position angle are different in Asterix and checks need to be made to decide what is actually being used here.
Most of the other keywords present in a FITS spectrum are for documentary purposes and have no relevance to Asterix when reading the files. Some of them would be filled in by Asterix writing those files, such as the CREATOR and HISTORY keywords.

The XSPEC fields CHANNEL maps to the spectrum axis, COUNTS to the data and QUALITY to Asterix quality bits. Grouping information will cause a rebinning of the data, errors and energy response on the fly to take place initially, as Asterix does not support grouping -- a situation which could change in the long term.

The good time intervals extension (GTI) maps on to an equivalent structure in the Asterix instrument structure.

Web Master
Wed Oct 8 09:28:40 BST 1997