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1Most OGIP Calibration Memos of general
community interest will eventually appear as articles in Legacy, but are
also available on request from Pat Tyler, Code 668, NASA/GSFC, Greenbelt, MD
20771, USA.
2A primary goal of the HEASARC is to standardize the format of
similar files from different instruments. Besides strictly being the incorrect
method in the general case, the mapping of the PHA data from the observed to
some 'standard' channel grid cannot be performed correctly for low resolution
detectors.
However, in the case of certain past and present missions, unfortunately the
use of PI channels may be unavoidable since the necessary information
(i.e., to construct & use BCFs) has been lost or is unavailable from
the h/w team. However, in such cases, the data describing the entire
instrument response should be stored in the same format as the RMFs described
below, (see also Section 7).
PHA datasets in which use a PI channel grid are denoted by the CHANTYPE = PI
keyword within the PHA file (see OGIP/92-007, Arnaud, George & Tennant
1992).
[3]A variable-length FITS array requires at least 12 bytes
of disk storage space: 4 bytes for the length of the vector, 4 bytes for the
offset address, and 4 bytes for the value of each element. Thus using
variable-length format for an array will lead to a saving of disk-space only if
the maximum number of elements in that array is greater than 3.
[4]Due to always having to read the offset value first,
before reading the value of a given element within the array, there is a
significant performance inefficiency in using variable-length vectors.
[5]This is denoted by the CHANTYPE = PI keyword within the
PHA file (see OGIP/92-002, Arnaud, George & Tennant 1992). Examples include
the current versions of the ROSAT PSPC and Einstein IPC datasets
within the HEASARC archive
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Last modified: Monday, 19-Jun-2006 11:40:52 EDT
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