HFWG Recommendation R10:

    On the Keywords and definitions denoting the channel and energy boundaries

    (Approved: 1994 Jul 06)

    Typically, in the FITS header of a 'derived' file (such as an image, a lightcurve or a spectrum), it is desirable to have keywords which carry the information of the channel or energy boundaries/ranges used to select the data. It is recommended that channel and/or energy boundaries be recorded using the following keywords.

    Channel boundary:

    The keywords to define lower and higher channel bounderies are:

    • CHANMIN - a numeric (integer) value, m, for the lower boundary
    • CHANMAX - a numeric (integer) value, n, for the higher boundary
    where m and n are specified in unbinned ('raw') channels.

    For those experiments which derived files could be created selecting on diffent channel coordinates (e.g. PHA or PI), the coodinates type should be specified using the keyword

    • CHANTYPE - a character string giving the type of channel system CHANMIN and CHANMAX refer to. Currently, the only values allowed within the OGIP are:
      • CHANTYPE = 'PHA' - for observed Pulse Height Analyser channels
      • CHANTYPE = 'PI' - for (converted) 'Pulse Invariant' detector channels

    Energy boundary:

    To define the energy range of the data in a derived/product file, the keywords recommended are:

    • E_MIN - a real giving the lower energy boudary
    • E_MAX - a real giving the higher energy boundary
    • EUNIT - character string giving the physical units in which E_MIN and E_MAX are measured (which must conform to the rules given in OGIP memo OGIP/93-001, George & Angelini 1994

    It should be stressed that E_MIN and E_MAX give the approximate (or nominal) energies of the lower boundary of the lowest channel, and the upper boundary of the highest channel included in the dataset. Any conversion between channel numbers and nominal energies should be performed using the 'EBOUNDS' extension on the appropriate detector response matrix.

    Deprecated Alternatives:

    Within the OGIP a number of files and /or documents have used or defined different keywords to specify energies or channels boundaries. Below is a list of those different/old definitions. These old keywords should no longer be used unless they are required by existing instrument-specific software. It is strongly suggested that any such software be updated to handle the above keywords as soon as possible:

    1. The memo OGIP/93-003 (Angelini et al 1993) defines the keywords MINCHAN and MAXCHAN for the channel boundaries.
    2. The previous versions of the ROSAT standard processing software (SASS) created images containing the following keywords: XS-MINCH, MINPI, MINPHA, XS-MAXCH, MAXPI, MAXPHA, XS-CHAN, PICHANS, PHACHANS.
    3. The latest version of the ROSAT standard processing software (SASS) creates images where the above 9 keywords have been replaced by PIMIN, PIMAX in the case of PSPC instrument, and PHAMIN, PHAMAX in the case of HRI instruments.