Last modified: 13 Jan 2022


An Image of Diffuse Emission

CIAO 4.15 Science Threads



The procedure used here is intended to make a nice image for a poster or paper and to aid in understanding the morphology of the extended emission. Point sources are identified and removed, filling the gaps with a sampling of the background region. The filled image is then smoothed, with the option to exposure-correct the results.

Care must be taken in the scientific interpretation of the final image as it is highly processed.


To create a smoothed image of diffuse emission, replacing point sources by a local estimate of the background emission.

Related Links:

Last Update: 13 Jan 2022 - Reviewed for CIAO 4.14. Minor tweaks to wording only.


Get Started

Download the sample data: 315 (ACIS-S, NGC 4038/39)

Early dataset

The data used in this thread was taken very early in the Chandra mission. It was not included in the bulk reprocessing metioned in the Watchout page. The keywords required for analysis with CIAO need to be added by running chandra_repro or r4_header_update.

The data is assumed to have been downloaded from the archive - e.g. with download_chandra_obsid - then reprocessed using chandra_repro to use the latest calibration and software updates.

unix% chandra_repro . repro/
unix% fluximage "repro/acisf00315_repro_evt2.fits[ccd_id=7]" images/ binsize=1 bands=broad psfecf=0.393
Running fluximage
Version: 28 November 2018

Using CSC ACIS broad science energy band.
Aspect solution repro/pcadf060414371N003_asol1.fits found.
Bad-pixel file repro/acisf00315_repro_bpix1.fits found.
Mask file repro/acisf00315_000N003_msk1.fits found.

The output images will have 1289 by 1289 pixels, pixel size of 0.492 arcsec,
    and cover x=3536.5:4825.5:1,y=3171.5:4460.5:1.

Running tasks in parallel with 4 processors.
Creating aspect histogram for obsid 315
Creating instrument map for obsid 315
Creating exposure map for obsid 315
Thresholding data for obsid 315
Exposure-correcting image for obsid 315
Creating PSF map for obsid 315

The following files were created:

 The clipped counts image is:

 The clipped exposure map is:

 The PSF map is:

 The exposure-corrected image is:
New in CIAO 4.11

The fluximage script now creates the PSF map needed by wavdetect when the psfecf parameter is specified.

The output of psfmap is an image where the pixel values are the size of a circular region (in arcseconds) needed to enclose a fraction of the PSF at specified energy.

unix% dmstat broad_thresh.psfmap cen-
    min:	0.24236044288 	      @:	( 4081 4155 )
    max:	3.6420326233 	      @:	( 4826 3446 )
   mean:	1.2048927606 
  sigma:	0.77728920391 
    sum:	1328687.0576 
   good:	1102743 
   null:	562645 

The last line, null:562645, indicates that there are pixels in the PSF map with a value of NaN, indicating pixels in the image that are outside the instrument field-of-view, FOV.

We will use ds9 to visualize the psfmap.

% ds9 \
  broad_thresh.img -scale limits 0 2 \
  broad_thresh.psfmap -single \
  -contour -contour smooth 1 -contour levels "0.5 1 2" -contour copy \
  -frame 1 -contour paste wcs red 2 no  -contour close \
  -pan to 4200 3790 physical -view colorbar no  \
  -grid yes -grid grid no -grid type analysis -grid border no \
  -grid title no -grid axes type exterior -grid numerics vertical yes

Each line is described below

  1. The application name
  2. Load the counts image and set the scale limits to 0:2
  3. Load the psfmap in a new frame. Set ds9 to display a single frame at a time.
  4. Generate contours on the psfmap at specific levels using the smooth algorithm. Copy the contours
  5. Switch to counts image and paste the contours from the psfmap image.
  6. Pan the imager to the specified physical coordinates and remove the color bar
  7. Turn on the coordinate grids, disable the grid lines and border
  8. Move the coordinate grid axes to edge of image and rotate labeles.

We have used dmstat and ds9 to look at how the PSF map varies across the image; the contours in Figure 1 show the positions where the ECF is 0.5, 1, and 2 arcseconds (for the chosen energy and ECF fraction).

Figure 1: Spatial variation of the PSF

[Thumbnail image: The diffuse emission is centered on the 0.5 arcsecond radius ECF.]

[Version: full-size]

[Print media version: The diffuse emission is centered on the 0.5 arcsecond radius ECF.]

Figure 1: Spatial variation of the PSF

The contours show the locus of the 0.5, 1, and 2 arcsecond radii for the energy and ECF values used in the mkpsfmap call, namely 1.4967 keV and an ECF of 0.393.

Identify and Remove Point Sources

1. Create an Image of the Region

Throughout this thread, we will use the image created by fluximage, namely images/broad_thresh.img, and you may proceed to source detection with wavdetect; however, should you want to select the region and energies you are interested in, it may done using dmcopy and an events file - e.g.

unix% dmcopy "repro/acisf00315_repro_evt2.fits[energy=300:7000][bin x=4004.5:4404.5:1,y=3625.5:4025.5:1]" \

Note that the get_sky_limits tool can be used to find the sky range covered by an image if you want to match an existing image:

unix% get_sky_limits images/broad_thresh.img 
Running: get_sky_limits
  version: 12 September 2012
Checking binning of image: images/broad_thresh.img
  Image has 1289 x 1289 pixels
  Pixel size is 1.0 by 1.0
  Lower left (0.5,0.5) corner is x,y= 3536.5, 3171.5
  Upper right (1289.5,1289.5) corner is x,y= 4825.5, 4460.5
  DM filter is:
  mkexpmap xygrid value is:

We use ds9 to view the image, as shown in Figure 2:

unix% ds9 images/broad_thresh.img -scale log -zoom 0.5 -smooth -invert

Figure 2: Diffuse emission plus point sources

[Thumbnail image: The filtered and binned event file is displayed in ds9.]

[Version: full-size]

[Print media version: The filtered and binned event file is displayed in ds9.]

Figure 2: Diffuse emission plus point sources

The ACIS-S3 chip is displayed.

2. Source detection with wavdetect

We can now run wavdetect to identify the point sources. Note that here we have chosen to increase the ellsigma parameter from 3 to 4, since we wish to make sure we excise as much of each source as possible from the image. This value should be modified to suit the requirements of your analysis. Similarly the scales setting should be adjusted to match the data.

unix% punlearn wavdetect
unix% pset wavdetect psffile=broad_thresh.psfmap
unix% pset wavdetect regfile=sources.reg 
unix% pset wavdetect ellsigma=4
unix% wavdetect broad_thresh.img sources.fits sources.scell sources.image sources.nbkg
wavelet scales (pixels) (2.0 4.0): 
Image of the size of the PSF (broad_thresh.psfmap): 

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

From inspection, one can see that many of the sources detected by wavdetect are not point sources, but clumps of diffuse emission. Scientific judgment must be used to modify or delete regions before saving the final region file. The Using the Output of Detect Tools thread shows how to display and modify source lists.

Here we manually remove those detections which do not look like point sources. In this example, the modifed source list has been saved as sources_mod.reg:

unix% head sources_mod.reg
 Region file format: DS9 version 4.1
global color=blue dashlist=8 3 width=1 font="helvetica 10 normal roman" select=1 highlite=1 dash=0 fixed=0 edit=1 move=1 delete=1 include=1 source=1
Ellipse(4399.06,3337.57,18.9642,13.2067,34.1611) #  
Ellipse(4304.93,3440.89,14.3973,9.63261,54.2142) #  
Ellipse(4409.26,3504.19,14.3911,9.85874,48.463) #  
Ellipse(4292.44,3614.14,7.73868,5.04972,57.2064) #  
Ellipse(4332.65,3659.78,5.62888,4.18144,64.6945) #  
Ellipse(4103.68,3663.62,9.55326,4.91821,43.1674) #  
Ellipse(4270.7,3663.2,3.15498,1.53822,68.7819) #  

The source list can contain overlapping regions since this case is handled by the roi tool used in the next section.

Figure 3: Original and modified source regions

[Thumbnail image: The source detections we are going to remove from the image are shown as green ellipses; red ellipses show those detections we have decided to ignore.]

[Version: full-size]

[Print media version: The source detections we are going to remove from the image are shown as green ellipses; red ellipses show those detections we have decided to ignore.]

Figure 3: Original and modified source regions

The green ellipses show those regions we have decided to use (sources_mod.reg; i.e. these will be removed from the image). The red ellipses are the sources from the original wavdetect sourcelist (sources.reg) that we have decided to ignore.

The image was displayed using the following command:

unix% ds9 broad_thresh.img -smooth radius 2 -smooth -scale log

We end the section by using dmmakereg to convert the modified source list into FITS format, as required by the roi tool used in the next section:

unix% punlearn dmmakereg
unix% dmmakereg "region(sources_mod.reg)" sources_mod.fits wcsfile=broad_thresh.img
unix% dmlist sources_mod.fits cols
Columns for Table Block REGION
ColNo  Name                 Unit        Type             Range
   1   POS(X,Y)             pixel        Real8          -Inf:+Inf            Position
   2   SHAPE                             String[16]                          Region shape type
   3   R[2]                 pixel        Real8(2)       -Inf:+Inf            Radius
   4   ROTANG[2]            pixel        Real8(2)       -Inf:+Inf            Angle
   5   COMPONENT                         Int2           -                    Component number

dmmakereg uses the wcsfile to convert the region from celestial coordinates (RA,Dec) to physical coordinate. It only does this if it needs to. If the input region is already in physical coordinates then the wcsfile is ignored. Simple numeric values without any annotation are treated as being in physial coordinates. A region in celestial coordinates will be recognized either by the use of colon separated sexagesimal coordinates, with the string fk5 preceeding the regions, or with the special "d" following the values to indiciate decimal degrees. More examples are shows in the DM filtering help file.

In this example, the region is already in physical coordinates so no conversion is performed.

3. Create Source and Background Regions

We now use the roi tool to create the source and background regions since it is much-more capable than the previous solution ( and This requires three stages:

  1. Using roi to create source and background regions for each source;

  2. combining the regions into a format that dmfilth can process;

  3. and finishing with an optional validation and modification phase.

The roi tool takes care of combining overlapping source regions and excluding neighbouring sources from background regions.

First we run roi, where the output files (there will be one file for each source) are written to the sources/ sub-directory:

unix% mkdir sources
unix% punlearn roi
unix% pset roi infile=sources_mod.fits
unix% pset roi outsrcfile=sources/src%d.fits
unix% pset roi bkgfactor=0.5
unix% roi
Input src list (sources_mod.fits): 
Input field of view region (): 
Input streak region (): 
Output source list (sources/src%d.fits): 
Background radius computation method (add|mul|area) (mul): 
Background radius (0:) (3): 
unix% pget roi num_srcs
unix% dmlist sources_mod.fits counts

The contents of the parameter file may be checked using plist roi. Note that we set the bkgfactor parameter to a small, positive value to ensure that the outer radius of the source region is smaller than the inner radius used in background region, since dmfilth can error out when these two boundaries are the same (Figure 4).

Figure 4: Source and background region created by roi

[The inner radius of the background region is just larger than the radius of the source region.]
[Print media version: The inner radius of the background region is just larger than the radius of the source region.]

Figure 4: Source and background region created by roi

Since we set bkgfactor in the call to roi, we have ensured that there is some separation between the inner radius of the background (the green area) and the source region (the blue area). This is done to avoid potential problems in the dmfilth run below, since there may be pixels that appear in both source and background regions due to pixellation issues if the two radii are the same.

Also shown is the area excluded from the background region due the presence of a neighbouring source (the dashed ellipse).

The roi tool

In this example we have barely scratched the functionality of roi. For instance, since the emission we are interested in is all contained within the S3 chip, we do not have to worry about the edges of the chip, and so we have not bothered to set the fovregion parameter. Similarly, as there are no bright sources in the field we do not use a streakregion file, such as created by acis_streak_map.

Please see the ahelp page for roi for more information on this tool.

The parameter file for roi is updated to contain the number of source created; here we have 58. This is smaller than the original number of sources (60) since roi has combined sources that overlap. This behavior can be controlled by changing the tool's group parameter.

The output of roi is a set of FITS region files, each containing a region representing the source (SRCREG) and background (BKGREG) for each source:

unix% ls -1 sources/ | head -5
unix% dmlist sources/src\*.fits blocks
Dataset: sources/src1.fits
     Block Name                          Type         Dimensions
Block    1: PRIMARY                        Null        
Block    2: SRCREG                         Table         5 cols x 1        rows
Block    3: BKGREG                         Table         5 cols x 2        rows


Dataset: sources/src14.fits
     Block Name                          Type         Dimensions
Block    1: PRIMARY                        Null        
Block    2: SRCREG                         Table         5 cols x 2        rows
Block    3: BKGREG                         Table         5 cols x 8        rows

Dataset: sources/src17.fits
     Block Name                          Type         Dimensions
Block    1: PRIMARY                        Null        
Block    2: SRCREG                         Table         5 cols x 1        rows
Block    3: BKGREG                         Table         5 cols x 3        rows


From the dmlist output, we see that a number of sources can have multiple rows in their SRCREG block - e.g. src14.fits - or more than two rows in the BKGREG block (src14.fits and src17.fits); you expect a minimum of two as we have an ellipse minus the original source ellipse. In Figure 5 we show the source and background regions in src14.fits:

Figure 5: Looking at the output of the roi tool

[Thumbnail image: The roi tool has combined sources which overlap (left frame) and excluded neighbouring sources from the background region of another source (right frame).]

[Version: full-size]

[Print media version: The roi tool has combined sources which overlap (left frame) and excluded neighbouring sources from the background region of another source (right frame).]

Figure 5: Looking at the output of the roi tool

This image was created by saying

unix% ds9 -zoom 2 -smooth -scale log broad_thresh.img -region sources/src14.fits \
     broad_thresh.img -region "sources/src14.fits[bkgreg]"

and shows a source region on the left which has combined two sources, and on the right its background region, which has multiple sources removed from it; these overlapping regions can be seen in Figure 3.

The dmfilth tool used in the next section requires two files in ASCII region format: one for the sources, and one for the backgrounds. To do this we use the splitroi script, which is part of the CIAO scripts and modules package (although, as described at the end of this section, you can use sed, or similar tools, to create these files for you.

The script takes two arguments; the first is a list of files to use, using the shell's syntax for specifying regular expressions, and the name to use as the start of the output files (".src.reg" and ".bg.reg" are appended to this value):

unix% splitroi "sources/src*.fits" exclude

We now have two ASCII region files - exclude.src.reg and - that can be used by dmfilth. To view them in ds9 you need to convert them to FITS format using dmmakereg; e.g.

unix% dmmakereg "region("
unix% ds9 broad_thresh.img -scale log -smooth -region

creates Figure 6.

Figure 6: Viewing the background regions

[Thumbnail image: Each source is now surrounded by an annulus describing the region which should be used by dmfilth.]

[Version: full-size]

[Print media version: Each source is now surrounded by an annulus describing the region which should be used by dmfilth.]

Figure 6: Viewing the background regions

Here we see the annuli used as the background for each source. For regions where the source density is high - making it hard to see what is going on - the individual region files created by roi can be used (e.g. Figure 5).

An alternative approach is to use the stack support in dmfilth and create files containing lines that say region(src1.fits) and region(src1.fits[bkgreg]) for each file; in teh following the region() syntax is needed to tell dmfilth that each line represents a region file rather than describing an actual region, as it is in the splitroi approach. For example:

unix% ls -1 sources/src*fits | sed 's/.*/region(&)/' > src.reg
unix% ls -1 sources/src*fits | sed 's/.*/region(&[bkgreg])/' > bg.reg
unix% head -3 src.reg 
unix% head -3 bg.reg

At this point you can inspect these region files to select which ones should be used and which ones removed. For this example we do not make any changes.

4. Fill in the Holes (dmfilth)

The tool dmfilth offers several options for extrapolating over regions (see the ahelp file for more information.) Here, we use the POISSON method, which assigns pixel values to the source region by sampling the Poisson distribution whose mean is that of the pixel values in the background region. This has been chosen for didactic purposes and may not be the best choice for your purposes (it assumes that the diffuse emission at the source has the same surface brightness as in the background regions which is questionable here for some sources, looking at Figure 6).

We run dmfilth using the region files created in the previous sections:

unix% punlearn dmfilth
unix% pset dmfilth infile=broad_thresh.img
unix% pset dmfilth outfile=diffuse.img
unix% pset dmfilth method=POISSON
unix% pset dmfilth srclist=@exclude.src.reg
unix% pset dmfilth
unix% pset dmfilth randseed=0
unix% dmfilth 
Input image file (broad_thresh.img): 
Enter output file name (diffuse.img): 
List of sources to fill in (@exclude.src.reg): 
List of background regions ( 

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

unix% ds9 diffuse.img -smooth -scale sqrt -invert

The output file is shown in Figure 7. As expected, the point sources are no longer visible.

Figure 7: Image of the diffuse emission

[Thumbnail image: The point sources have been removed from the data.]

[Version: full-size]

[Print media version: The point sources have been removed from the data.]

Figure 7: Image of the diffuse emission

The locations where point sources were removed have been replaced by sampling the local background. This image can be compared to Figure 2.

Smooth the Image

The image file is smoothed via the tool aconvolve. A gaussian is used for the kernel specification (kernelspec) and the kernel is normalized by the area (normkernel).

For this data, we define the kernelspec as lib:gaus(2,5,1,7,7). This means the Gaussian:

  • has 2 dimensions;
  • is embedded in an array 5 sigma in size;
  • is normalized to 1;
  • has a sigma of 7 pixels along each axis.

Users will have to experiment with different kernelspec definitions to find the optimal one for the dataset.

unix% punlearn aconvolve
unix% pset aconvolve infile=diffuse.img
unix% pset aconvolve
unix% pset aconvolve kernelspec="lib:gaus(2,5,1,7,7)"
unix% pset aconvolve method=fft
unix% aconvolve 
Input file name (diffuse.img): 
Kernel specification (lib:gaus(2,5,1,7,7)): 
Output file name ( 

The contents of the parameter file may be checked using plist aconvolve and Figure 8 shows the smoothed image,

unix% ds9 -invert -scale sqrt

Figure 8: Smoothed image of the diffuse emission

[Thumbnail image: Brighter features of the object are distinct in the smoothed image.]

[Version: full-size]

[Print media version: Brighter features of the object are distinct in the smoothed image.]

Figure 8: Smoothed image of the diffuse emission

The image has been smoothed with a gaussian kernel.

Exposure-Correcting the Image (Optional)

From this point, it is also possible to incorporate an exposure map in order to create an exposure-corrected image. Unless there are significant exposure variations across the field, this will not make a difference in the final image; exposure-correcting the data used in this thread did not have a visible effect on the output.

If you used fluximage then you have an exposure map - in this case broad_thresh.expmap, otherwise follow one of the Exposure Map threads.

Use dmimgcalc to divide the unsmoothed image (diffuse.img) by the exposure map:

unix% punlearn dmimgcalc
unix% dmimgcalc diffuse.img broad_thresh.expmap diffuse_exp.img div
warning: CONTENT has 1 different values.
warning: DETNAM has different value...Merged...

Smooth the exposure-corrected image (diffuse_exp.img) with aconvolve, as in the Smooth the Image section:

unix% punlearn aconvolve
unix% aconvolve diffuse_exp.img "lib:gauss(2,5,1,7,7)" method=fft
unix% ds9 -invert -scale mode 99.5

In Figure 9 we show that, for this example, there are no major large-scale differences in morphologies in the two smoothed images.

Figure 9: Smoothed, exposure-corrected image of the diffuse emission

[Thumbnail image: Brighter features of the object are distinct in the smoothed image.]

[Version: full-size]

[Print media version: Brighter features of the object are distinct in the smoothed image.]

Figure 9: Smoothed, exposure-corrected image of the diffuse emission

The left image is the smoothed counts image, the right side shows the smoothed exposure-corrected image. Both data sets were smoothed with the same gaussian kernel.

Parameters for /home/username/cxcds_param/wavdetect.par

#   parameter file for wavdetect
#   input
        infile = broad_thresh.img Input file name
#   output
       outfile = sources.fits     Output source list file name
     scellfile = sources.scell    Output source cell image file name
     imagefile = sources.image    Output reconstructed image file name
   defnbkgfile = sources.nbkg     Output normalized background file name
#   scales
        scales = 2.0 4.0          wavelet scales (pixels)
#   end of wtransform parameters
#   wrecon parameters
#   PSF size parameters
       psffile = broad_thresh.psfmap Image of the size of the PSF
      (regfile = sources.reg)     ASCII regions output file
#   output options
      (clobber = no)              Overwrite existing outputs?
     (ellsigma = 4)               Size of output source ellipses (in sigmas)
     (interdir = ${ASCDS_WORK_PATH} -> /tmp) Directory for intermediate outputs
#   wtransform parameters
#   optional input
     (bkginput = )                Input background file name
  (bkgerrinput = no)              Use bkginput[2] for background error
#   output info
  (outputinfix = )                Output filename infix
#   output content options
    (sigthresh = 1e-06)           Threshold significance for output source pixel list
 (bkgsigthresh = 0.001)           Threshold significance when estimating bkgd only
     (falsesrc = -1.0)            Allowed number of false sources per image
   (sigcalfile = ${ASCDS_CALIB}/wtsimresult.fits -> /soft/ciao/data/wtsimresult.fits) Significance calibration file
#   exposure info
      (exptime = 0)               Exposure time (if zero, estimate from map itself
      (expfile = )                Exposure map file name (blank=none)
    (expthresh = 0.1)             Minimum relative exposure needed in pixel to analyze it
#   background
      (bkgtime = 0)               Exposure time for input background file
#   iteration info
      (maxiter = 2)               Maximum number of source-cleansing iterations
     (iterstop = 0.0001)          Min frac of pix that must be cleansed to continue
#   end of wrecon parameters
#   run log verbosity and content
          (log = no)              Make a log file?
      (verbose = 0)               Log verbosity
#   mode  
         (mode = ql)         

Parameters for /home/username/cxcds_param/roi.par

        infile = sources_mod.fits Input src list
     fovregion =                  Input field of view region
  streakregion =                  Input streak region
    outsrcfile = sources/src%d.fits Output source list
    radiusmode = mul              Background radius computation method
     bkgradius = 3                Background radius
      num_srcs = 58               Number of sources output
        (group = group)           Make 1 srcregion per group or per individual source
    (targetbkg = all)             Make background around all sources or just target?
    (bkgfactor = 0.5)             Amount offset or multipled by src to be excluded by bkg
  (bkgfunction = add)             Add bkgfactor or multiply bkgfactor
       (maxpix = INDEF)           Maximum number of pixels when intersecting
       (fovres = 1)               Pixellation resulolution for fov check
    (streakres = 0.25)            Pixellation resulolution for streak check
(ignore_streaksrc = yes)             Ignore streak region for source list ?
      (evtfile = )                Event File (# counts per region)
      (clobber = no)              Remove existing output files?
      (verbose = 0)               tool verbosity
         (mode = ql)

Parameters for /home/username/cxcds_param/dmfilth.par

## DMFILTH --  fill in the hole
        infile = broad_thresh.img Input image file
       outfile = diffuse.img      Enter output file name
        method = POISSON          Interpolation method
       srclist = @exclude.src.reg List of sources to fill in
       bkglist =  List of background regions
     (randseed = 0)               Seed for random number generator
      (clobber = no)              OK to overwrite existing output file(s)?
      (verbose = 0)               Verbosity level
         (mode = ql)              

Parameters for /home/username/cxcds_param/aconvolve.par

# aconvolve.par file
        infile = diffuse.img      Input file name
       outfile =   Output file name
    kernelspec = lib:gaus(2,5,1,7,7) Kernel specification
# auxillary outputs
  (writekernel = no)              Output kernel
   (kernelfile = ./.)             Output kernel file name
     (writefft = no)              Write fft outputs
      (fftroot = ./.)             Root name for FFT files
# processing parameters 
       (method = fft)             Convolution method
        (edges = wrap)            Edge treatment
        (const = 0)               Constant value to use at edges with edges=constant
          (pad = no)              Pad data axes to next power of 2^n
       (center = no)              Center FFT output
   (normkernel = area)            Normalize the kernel
# user specific comments
      (clobber = no)              Clobber existing output
      (verbose = 0)               Debug level
         (mode = ql)              


16 Dec 2004 updated for CIAO 3.2: minor changes to csmooth parameter file
21 Dec 2005 reviewed for CIAO 3.3: no changes
01 Dec 2006 updated for CIAO 3.4: uses aconvolve instead of csmooth for smoothing, updated image to match
16 Jan 2008 updated for CIAO 4.0: kernel parameter removed from aconvolve and wavdetect
29 Jan 2009 updated for CIAO 4.1: images are inline
06 May 2009 check the version of the CIAO scripts package instead of the individual script
05 Feb 2010 updated for CIAO 4.2: ObsID 315 file version and corresponding changes to source detections
13 Jan 2011 reviewed for CIAO 4.3: no changes
09 Jan 2012 reviewed for CIAO 4.4: the thread now assumes that the data has been processed by chandra_repro, uses fluximage to create the necessary images, updated to use the new interface to wavdetect, and use the new tool roi and script splitroi to generate the source and background regions rather than the deprecated and routines; added Figure 9 showing the exposure-corrected version of the image.
24 Jul 2012 added clarifying remark to Identify and Remove Point Sources section.
03 Dec 2012 Review for CIAO 4.5; cleanup a bit of "new in ciao 4.4" related stuff.
25 Nov 2013 Review for CIAO 4.6. Highlighted fact that early mission data needs chandra_repro to update headers for parameter block keywords.
27 Jun 2014 updated file names and tool syntax
17 Dec 2014 Reviewed for CIAO 4.7. No changes.
03 Jan 2016 Review for CIAO 4.9. No changes.
19 Mar 2018 Added a note about region coordinate systems and added wcsfile to the dmmakereg command.
10 Dec 2018 Updated for CIAO 4.11; fluximage can now be used to create the psfmap.
28 Mar 2019 Recreated psfmap figure with ds9.
13 Jan 2022 Reviewed for CIAO 4.14. Minor tweaks to wording only.