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 = n       /n= numeric (integer) value for the lower boundary      
    CHANMAX = m       /m= numeric (integer) value for the higher boundary

 where m & n are specified in unbinned ('raw') channels.

 For those experiments which derived files could be created selecting 
 on diffent channel coodinates (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

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.

a) The memo number OGIP/93-003 defines the keywords MINCHAN and MAXCHAN
   for the channel boundaries.
b) 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
c) 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
    PHAMIN
    PHAMAX
   in the case of HRI instruments.