Remove the ACIS Readout Streak

Get Started

Sample ObsID used: 1712 (ACIS-S, 3C273)

File types needed: evt2; asol1

Figure 1 shows the prominent readout streak on the S3 chip, which will be removed by running this thread. There may also be streaks on the S4 chip, which are due to a flaw in the serial readout of the chips; use the tool destreak to remove those streaks.

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Figure 1: ACIS readout streak

Image displaying the prominent readout streak on the S3 chip.

Get Source Location

acisreadcorr uses the source position to determine which events may have a misplaced CHIPY value. There are two ways to determine the source position:

1. With ds9:

   unix% ds9 acisf01712N003_evt2.fits &
   

   From the ds9 display (Figure 2), we can determine that the source is at (4084.0,4085.5).

   

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   Figure 2: Getting the source location from ds9

   The source location is determined from the white cross at the center of the frame.

2. With dmstat:
   Here we use the virtual file syntax to find the maximum number of photons in the field. It follows that this is the source location:

   unix% punlearn dmstat
   unix% pset dmstat centroid=no sigma=no
   unix% dmstat "acisf01712N003_evt2.fits[bin sky=8]"
   EVENTS_IMAGE
       min:        0             @:        ( 4.5 4.5 )
       max:        3862          @:        ( 4084.5 4084.5 )
      mean:        0.43529605865 
       sum:        456441 
      good:        1048576 
      null:        0 
   

   The source location is given as (4084.5,4084.5).

Create a Background Spectrum

Removing all the photons from the affected area would leave a black streak instead of a bright one. We wish instead to leave the background photons where they are. For this reason, it is possible to input a background PI spectrum. The tool then attempts to leave background photons - consistent with the given PI spectrum and distributed evenly in time - where the streak once was.

First, we need to select a background region. This can be from any area of the chip that doesn't contain any bright regions; Figure 3 shows the region used. For help on rotating regions in ds9, see this FAQ.

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Figure 3: Region for background spectrum

Image displaying a background region selected from an area of the chip that doesn't contain any bright sources.

Save the region:

• Region -> File Format-> Ciao
• Region -> File Coordinate System -> Physical
• Region -> Save Regions... -> Save As "bkg.reg"

The resulting file will look something like this:

unix%  more bkg.reg 
# Region file format: CIAO version 1.0
rotbox(4410.92,4353.0555,389.55054,66.92414,24.719781)

The tool dmextract is now used to create the spectrum of this region:

unix% punlearn dmextract
unix% pset dmextract infile="acisf01712N003_evt2.fits[sky=region(bkg.reg)][bin pi=1:1024:1]"
unix% pset dmextract outfile=bkg_pi.fits
unix% dmextract
Input event file  (acisf01712N003_evt2.fits[sky=region(bkg.reg)][bin pi=1:1024:1]): 
Enter output file name (bkg_pi.fits): 

The contents of the parameter file may be checked using plist dmextract.

NOTE: if your observation was taken with a subarray defined, read the Observations Using Subarrays caveat before proceeding.

The background spectrum should be binned to be the same size as your event file. If you were to bin this example to 1:1023:1, for instance, it may cause photons with PI=1024 to be attributed to the source erroneously. Changing the binning is only a concern if you have filtered your event file such that there are no longer 1024 channels of PI.

Run acisreadcorr

Now we have all the necessary information to run acisreadcorr:

unix% punlearn acisreadcorr
unix% pset acisreadcorr infile=acisf01712N003_evt2.fits
unix% pset acisreadcorr outfile=acis_1712_corr_evt2.fits
unix% pset acisreadcorr aspect=pcadf077378077N003_asol1.fits
unix% pset acisreadcorr x=4084.5 y=4084.5
unix% pset acisreadcorr dx=5 dy=50
unix% pset acisreadcorr bkg=bkg_pi.fits
unix% acisreadcorr 
Input dataset/block specification (acisf01712N003_evt2.fits): 
Output dataset/block specification (acis_1712_corr_evt2.fits): 
Aspect file (pcadf077378077N003_asol1.fits): 
Sky X position (pixels) (4084.5): 
Sky Y position (pixels) (4084.5): 

In some cases, there will be more than one asol1.fits file for an observation. All the files must be input to the aspect parameter, either as a comma-separated list or as a stack.

The dx and dy parameters may need to be adjusted. The default value for dx (2 pixels), or a number close to it, should be fine for on-axis point sources. dy can be estimated by drawing a circle in ds9 and finding the diameter (see Figure 4). These values determine the width of the region to be cleaned and how close to get to the source, respectively.

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Figure 4: Choosing dy for the observation

dy, the number of rows to be avoided around a source in a run of acisreadcorr, can be estimated by drawing a circle around the source in ds9 and finding the diameter.

The contents of the parameter file may be checked using plist acisreadcorr.

Examine the Results

The newly created file can be viewed in ds9:

unix% ds9 acis_1712_corr_evt2.fits &

as seen in Figure 5. The streak has obviously been removed and "filled-in " by the background photons.

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Figure 5: Event file with streak removed (used background spectrum)

Event file after processing with acisreadcorr; the streak has been removed and "filled-in" by the background photons.

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Figure 6: Original and new event files, side-by-side

The original event file (left) is compared with the new event file output from acisreadcorr (right).

Figure 7 shows an image of a nearly identical acisreadcorr run. The only difference is that a background spectrum was not supplied to the tool (i.e. bkg=""). The streak has been removed and a stripe of zero counts replaces it.

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Figure 7: Event file with streak removed (didn't use background spectrum)

Event file after processing with another run of acisreadcorr, this time without supplying a background file to the tool; the streak has been removed and a stripe of zero counts replaces it.

dmcopy may be used to see which photons were flagged by the tool. Status bit 22 marks photons identified as background and bit 21 marks out-of-time source events. In the following filters, the bits are numbered from the right, starting at zero:

unix% dmcopy "acis_1712_corr_evt2.fits[status=xxxxxxxxxx10xxxxxxxxxxxxxxxxxxxxx]" \
      flag_bkg.fits

unix% dmcopy "acis_1712_corr_evt2.fits[status=xxxxxxxxxx01xxxxxxxxxxxxxxxxxxxxx]" \
      flag_source.fits

Displaying these event files side-by-side produces Figure 8.

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Figure 8: Photons flagged by acisreadcorr

The background photons are on the left and the out-of-time source photons are on the right. The right frame is set at zoom=4.

Caveats

1024 / (subarray delta y)

1024 / 128 = 8

unix% dmkeypar bkg_pi.fits BACKSCAL echo+
0.00038847826236539

calculate new value:	0.00038847826236539 * 8 = 0.00310782609892312

unix% dmhedit infile=bkg_pi.fits filelist="" operation=add \
      key=BACKSCAL value=0.00310782609892312 unit="[pixel]" comment="Fractional area"

unix% dmkeypar bkg_pi.fits BACKSCAL echo+
0.0031078260989231