`acisreadcorr' handles the out-of-time events from a bright
source. While ACIS reads out, it is still taking data. Photons detected
during the readout are clocked out in the `wrong' row and so have
incorrect CHIPY values. For a bright source, you get a streak all
along the column (on both sides of the source, since some events are
from the previous exposure).
The user specifies the sky coordinates of the source, and tolerances
dx, dy in chipx and chipy. The program calculates the chip coordinates of
the source as a function of time (using the aspect solution) and
flags those photons within dx/2 of the source's CHIPX at the photon
arrival time, but further than dy/2 away from the source's CHIPY
(so that the source itself is not flagged).
We want to both remove the streak from the image (for cosmetic reasons
and so that it doesn't affect source detection) and to retain the
streak photons, since in principle they have more accurate timing
information and are not affected by pileup.
However, removing all photons from the given coordinates just leaves a
black streak instead of a bright one - we want instead to leave the
background photons where they are. For this reason, the user can input a
background PI spectrum in PHA file format.
The program attempts to leave in the streak as
background a set of photons consistent with the given PI spectrum and
distributed evenly in time. Note that if there is another bright source
in the streak or if dy is set too small, so that part of the primary
source is in the region being corrected, the photons from that source
will probably be the ones (erroneously) grabbed by the background
algorithm.
It is important that the data subspace (specifically, the good time
intervals) of the event file correctly reflect the data. In certain
old level 1 files (evt1), the good times include long periods where no
data was being taken, and the background algorithm will make an incorrect
estimate of the rate; a workaround is to time filter the file on
an appropriate time range, although in general it is probably advisable
to just use the level 2 file as input to this tool.
If you set verbose=1, you will get a report on the results of the
background estimation as a function of PI bin, allowing you to judge
how successful that part of the algorithm has been.
Events flagged as in the region but not part of the background are adjusted
so that their CHIPY coordinates are set to that of the source. The
algorithm then moves a rectangle of "excess" streak photons to a
line (constant aspect-adjusted CHIPY, variable CHIPX) near the center
of the source PSF. If you run this program, you may want to filter
out the affected photons before doing accurate PSF work.
The event times of the moved photons are also crudely corrected to give "improved"
photon arrival times. Whether these times are actually an improvement
in the current release has not been well tested.
Most importantly, the STATUS values of both the "background" and "source"
photons affected are modified, so that the user can isolate them and
see what has been done. Bit 22 is used for background and bit 21 for source
(counting from 0 at the right of the bit string, least significant bit).
To see the photons tagged as background,
dmcopy "fix.fits[status=10xxxxxxxxxxxxxxxxxxxxx]" flag_bkg.fits
To see the photons tagged as out-of-time source events,
dmcopy "fix.fits[status=01xxxxxxxxxxxxxxxxxxxxx]" flag_src.fits
The
Remove the ACIS Readout Streak thread shows an example run of the acsireadcorr tool.
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