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
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).
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.
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.
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