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:
- The memo
OGIP/93-003 (Angelini et al 1993)
defines the keywords
MINCHAN and MAXCHAN
for the channel boundaries.
- 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 .
- 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.
|