ACIS Event-Selection Criteria
Analyses of ACIS event-list (i.e. _evt1.fits, _evt1a.fits, and _evt2.fits)
data leads to the conclusion that the following criteria may be used, if
necessary, to filter the event-list data.
1. Exclude GRADE = 1, 5, and 7.
Events that have GRADE = 1, 5, and 7 are generally associated with cosmic rays,
whereas events that have GRADE = 0, 2, 3, 4, and 6 are generally
associated with X rays.
2. Exclude ENERGY > 10000.
The ACIS detectors are essentially insensitive to X rays that have energies >
10000 eV. In general, the events that have ENERGY > 10000 eV are associated
with cosmic rays or with X-ray events that have been piled up.
3. Exclude CHIPX = 256, 257, 512, 513, 768, and 769.
Cosmic-ray events that intercept the edge of a node in the frame-store
region can have their event islands truncated because the node boundaries
are separated by several pixels in the frame-store region. As a result,
these columns contain a relatively large number of cosmic-ray events that
would otherwise not be telemetered to the ground. Several of these events
pass the GRADE and ENERGY selection criteria.
4. Exclude STATUS not equal to 0
Some events have a STATUS bit set if they have unusual chip coordinates, have
unusual pulse-height amplitudes, occur on bad pixels, have unusual bias-map
values, or have unusual over-clock values. Several of these events pass the
GRADE, ENERGY, and CHIPX selection criteria. (This selection does not need to
be performed on _evt2.fits files, because it is performed automatically when
producing the _evt2.fits files.)
5. /nfs/wiwaxia/h1/houck/destreak_s4.pro
Do da. Do da.
This page (http://space.mit.edu/~gea/acis_event_selection.html) was posted on
November 10, 1999 by Glenn E. Allen.