Description

The acisreadcorr tool 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 acisreadcorr tool.