Identify pixels that have an unusually large (or small) number of events.
acis_find_hotpix infile outfile badpixfile maskfile [probthresh]
[regwidth] [clobber] [verbose]
acis_find_hotpix is one of the tools used to find hot pixels and
cosmic-ray afterglows (see acis_run_hotpix). The properties of the
pixels that have an unusually large (or small) number of events are
written to the output file. These pixels may be "hot" or may have
been affected by a cosmic-ray afterglows. The tool
acis_classify_hotpix, which should be run after acis_find_hotpix,
is used to distinguish between these two cases. Pixels that are
already known to be bad or that are inactive are excluded from the
search.
Since astrophysical sources can produce an unusually large number
of events in a pixel, some care is used to try to avoid
misidentifying these events as hot-pixel or afterglow events.
Unlike afterglows or hot-pixel events, events from astrophysical
sources are dithered. Therefore, if the number of events on the
pixels surrounding a suspicious pixel is large compared to the mean
number of events per pixel (see probthresh), the events on the
pixel are most likely associated with an astrophysical source. Such
pixels are not identified as bad and are not written to the output
file. Of course, this means that the tool may not find hot pixels
or afterglows if they are in the dither pattern of an astrophysical
source.
acis_find_hotpix acisf00732_000N002_evt1.fits suspicious_pixels.fits
badpixfile="tmp1_bpix1.fits" maskfile="acisf00732_000N002_msk1.fits"
This example shows the default use of the tool. The file
acisf00732_000N002_evt1.fits is read and searched for
pixels on which there is an unusually large (or small)
number of events. Pixels that are already identified as
bad in the file tmp1_bpix1.fits, which was produced by
acis_build_badpix, or that are marked as inactive in
the file acisf00732_000N002_msk1.fits are excluded from
the search. Information about each suspicious pixel is
written to the file suspicious_pixels.fits. This file
should be used as input to acis_classify_hotpix.
Parameter=infile (file required filetype=input stacks=yes)
The name(s) of the input event data file(s). acis_find_hotpix
can read Level 0, Level 1 and Level 2 files, but users are urged
to use Level 1 files because these files contain all of the
events. Some of the events are excluded from Level 2 files.
The name of the output file. This file contains information
about each suspicious pixel, including the values of CCD_ID,
CHIPX, CHIPY, the number of events on the pixel (S), the
expected number of events on the pixel (R), the number of pixels
used to compute R (N), the probability of obtaining S events for
an expected number R (P), the mean number of events per pixel on
the node (M) and the probability of obtaining R events for an
expected number M (P_EXP). The output file is used as input to
the tool acis_classify_hotpix.
Parameter=badpixfile (file required filetype=input default=NONE)
The name of the input bad-pixel file. Known bad pixels are not
included in the search for new bad pixels.
Parameter=maskfile (file required filetype=input default=NONE)
The name of the mask file that defines the "active" region(s)
of the CCD(s) used for the observation. Typically only the outer
edge of a CCD is inactive. If a pixel is inactive, it is
excluded from the search for hot pixels.
Parameter=probthresh (real not required default=1e-3 min=1e-10 max=0.1)
This parameter specifies the minimum significance of suspicious
pixels. For example, if P is the probability of obtaining S
events on a pixel for an expected number of events R, then the
pixel is identified as suspicious if P < (probthresh / T),
where T is the total number of pixels examined. A pixel is also
suspicious if P > 1 - (probthresh / T). The default value of
0.001 corresponds to 3.09 sigma for a normal distribution. The
one, two and three sigma confidence values are 0.159, 0.0228 and
0.00135, respectively. The default value of this parameter
should be adequate in most cases. Be cautious about using some
other value.
Parameter=regwidth (integer not required default=7 min=3 max=255)
This parameter specifies the size of the reference region used
to determine the expected number of events for a pixel. The
default value of seven means that the forty-eight pixels in a 7
pixel x 7 pixel region surrounding a pixel are used. The actual
number of pixels used is smaller than forty-eight if the region
contains pixels known to be bad, contains inactive pixels or
overlaps the edge of a node. If no events are in the reference
region, then the mean number of events per pixel for the node is
used as the expected number of events. The default value of
regwidth should be adequate in most cases. Be cautious about
using some other value.
If clobber=yes and a file exists that has the same name as the
name of the output file, then the existing file is overwritten.
If clobber=no and a file exists that has the same name as the
name of the output file, then an output file is not created.
Parameter=verbose (integer not required default=0 min=0 max=5)
This parameter determines the amount of messages that is
generated by acis_find_hotpix. If verbose=0, very few messages
are reported. If verbose=5, the largest amount of messages is
produced.
See the
bugs page
on the CIAO website for an up-to-date listing of known bugs.
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