Chandra X-Ray Observatory
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Last modified: January 2016

AHELP for CIAO 4.9


Context: tools


Create exposure-corrected images and exposure maps for an observation.


fluximage  infile outroot [bands] [xygrid] [binsize] [asolfile]
[badpixfile] [maskfile] [dtffile] [units] [expmapthresh] [background]
[bkgparams] [random] [parallel] [nproc] [tmpdir] [cleanup] [clobber]


The fluximage script creates exposure-corrected images of an ACIS or HRC observation given an events file and one or more energy bands. It automates the creation of the aspect histogram, instrument, and exposure maps. The output image size can be automatically chosen to cover all the data in the observation, or it can be set to match an existing image. For HRC-I observations, the HRC background files from the Chandra CALDB are used - if installed - to subtract off the particle background and so reduce the ring artifacts that can be seen in HRC-I data (this can be turned off by setting the hidden background parameter to "none").

For recently-processed data the script can be run with only two parameters: for example

unix% fluximage evt2.fits out/

creates a number of files in the directory out/, including the exposure-corrected image, out/broad_flux.img (ACIS) or out/wide_flux.img (HRC).

Example 1

unix% download_chandra_obsid 1843
unix% cd 1843
unix% chandra_repro indir=. outdir=repro
unix% fluximage . fluxed/
unix% ds9 fluxed/broad_flux.img

Here we download ObsId 1843 from the Chandra archive (an ACIS observation), reprocess it using chandra_repro, and then create an exposure-corrected image of the data using the CSC broad band (0.5 to 7 keV with an effective energy of 2.3 keV) in the directory fluxed/ (this directory will be created if it does not exist). As the infile parameter is set to a directory, the script searches for any files that match the pattern "./repro/*evt*", "./primary/*evt*", then "./*evt". In this case the script picks repro/acisf01843_repro_evt2.fits.

The output image contains all ACIS ccds that were on in the observation and has been binned to 8 ACIS pixels (so an output pixel scale of 3.936 arcseconds on a side). The events file is used to locate all the necessary files (which, due to chandra_repro, have all been copied to the repro/ directory). Finally ds9 is used to view the exposure-corrected image, the name of which includes the energy range, effective energy used and the binning factor.

Note that reprocessing your data - whether via chandra_repro or manually - is not required to use fluximage. It does however ensure that the necessary keywords are included in the events file (old data may be missing one or more of the keywords used to identify the ancillary data files) and that the latest calibrations have been applied.

Example 2

unix% fluximage repro/acisf01843_repro_evt2.fits fluxed/

This is a repeat of the previous example except that the event file to use is listed explicitly as the infile parameter value.

Specifying the exact file to use is necessary when using interleaved-mode observations, since there are two event files in the directory. For instance:

unix% fluximage 10551/repro/acisf10551e1_repro_evt2.fits out/e1
unix% fluximage 10551/repro/acisf10551e2_repro_evt2.fits out/e2
unix% ds9 out/e1_broad_flux.img out/e2_broad_flux.img &

Example 3

unix% fluximage repro/ fluxed/ binsize=4 clobber+
unix% ds9 fluxed/broad_flux.img

Here we repeat the analysis but use half the bin size (4 instead of 8, giving a pixel size of 1.968 arcseconds on a side). The clobber mode has been set to yes to ensure the output from the previous example does not call the script to fail.

Example 4

unix% fluximage "evt2.fits[ccd_id<4]" acisi/ binsize=2

Here a Data Model filter is used to restrict the output to the ACIS I array (ie ccd_id values of 0 to 3). The output image axis range is calculated so that it just covers the data in the input events file (after applying any Data Model filters).

The script will reflect any previous filtering applied to the file, so the following will create the same output (the use of clobber ensures that the existing data will be over-written):

unix% dmcopy "evt2.fits[ccd_id<4]" acisi.evt
unix% fluximage acisi.evt acisi/ binsize=2 clobber=yes

Example 5

unix% fluximage evt2.fits fimg bands=csc,broad

In this example four separate exposure-corrected images are created: one each for the CSC soft, medium, hard and broad energy bands. As with the previous example the names of the ancillary files - such as bad-pixel file and aspect solution - are read from the header of the events file.

Since the outroot does not end with a / character then the the output files will be created in the same directory as the events file, with names like fimg_soft_flux.img.

It is also possible to run the script once for each band; so the following is equivalent to the single run above:

unix% fluximage evt2.fits fimg bands=soft
unix% fluximage evt2.fits fimg bands=medium
unix% fluximage evt2.fits fimg bands=hard
unix% fluximage evt2.fits fimg bands=broad

Example 6

unix% make_instmap_weights wgts.txt "*" \
paramvals="gal.nh=0.45;pl.gamma=1.7" emin=0.5 emax=7 ewidth=0.25
unix% fluximage evt2.fits fimg/ bands=wgts.txt
unix% ds9 fimg/band1_flux.img

In this example we use a weights file (wgts.txt), created by make_instmap_weights, so that the exposure map is the weighted sum of a range of energies rather than evaluated at a single energy. The output files will be labelled "band1", so the exposure-corrected image will be called "fimg/band1_flux.fits". The energy range used to create the images is calculated from the contents of the weights file, so in this case 0.5 to 7 keV.

Example 7

unix% fluximage evt2.fits fimg bands=csc,0.5:7:3,wgt1.dat,wgt2.dat

Here we combine the three different forms for specifying an energy band - using a label ("csc"), an explicit range ("0.5:7:3"), and weights files ("wgt1.dat" and "wgt2.dat") to create 6 exposure-corrected images. The files using the weights file "wgt1.dat" are labelled "band1" and those using "wgt2.dat" are labelled "band2".

Example 8

unix% download_chandra_obsid 6202
unix% cd 6202
unix% chandra_repro indir=. outdir=repro
unix% fluximage repro/ fluxed/
unix% ds9 fluxed/wide_flux.img

Here we use a HRC-I observation, for which the default "bands=default" setting is the same as setting "bands=wide". The "wide" band uses all events (since the spectral response of HRC-I is so broad), and evaluates the exposure map at an energy of 1.5 keV. The default binning value for HRC data is 32. If the Chandra CALDB package "HRC background event files" is installed then the appropriate background file will be subtracted from your data in order to ring artifacts that can be seen in HRC-I data.

When background subtraction is used, the BEVTFILE keyword is set to the the name of the background event file; for example:

unix% dmkeypar fluxed/wide_flux.img BEVTFILE echo+

To turn off this step, set the background parameter to "none"; e.g.

unix% pset fluximage background=none
unix% fluximage repro/hrcf00144_repro_evt2.fits fluxed2/

Example 9

unix% fluximage hrcf00144_repro_evt2.fits fluxed/ bands=30:200:1.1

Band limits can also be given for HRC observaitons, but the lower and upper limits are in PI channels rather than energy. Here we specify the PI limits to use for the images (30 to 200), with the exposure map created at 1.1 keV. The final image is called fluxed/30-200_flux.img and uses a bin size of 16 HRC pixels (2.1088 arcseconds).

Example 10

unix% fluximage evt2.fits fi_ bands=csc xygrid=acisi/broad.img

Here we use multiple bands (csc is a short form for "soft,medium,hard"), write files to the current directory with a prefix of "fi_" (rather than to a separate directory), and use the xygrid parameter to make the data match the sky area covered by the acisi/broad.img (in this case the result of a previous run of the script but it can be any file which can be processed by the get_sky_limits script). The binsize parameter is not used in this case.

Example 11

unix% pset fluximage xygrid="3428.5:5476.5:2,6874.5:7898.5:2"
unix% fluximage evt2.fits zoom

Here we explicitly give the grid for the output images.

Example 12

unix% fluximage repro/ fluxed/ parallel=no

In this case, by setting parallel to no, only a single processor will be used.

Example 13

unix% fluximage repro/ fluxed/ nproc=-1

The nproc parameter allows you to control the number of processors that are used; in this case (nproc=-1), all-but-one of them (if your machine has 4 processors then nproc=3 could be used instead).

Example 14

unix% fluximage evt2.fits fimg badpixfile=caldb

Here we use the bad-pixel file in the CALDB, rather than the observation-specific bad-pixel file (the value of the BPIXFILE keyword in evt2.fits).

Example 15

unix% fluximage evt2.fits fimg bands=0.5:6:2 parallel=no
unix% ds9 fimg_0.5-6_flux.img

Here a manually-specified band is used - 0.5 to 6 keV with an effective energy (that is, the energy used to calculate the exposure map), of 2 keV.

The computation only uses a single processor since the parallel parameter is set to no.

Example 16

unix% fluximage evt2.fits out badpixfile=bpix1.fits asolfile=@asol.lis

In this example most of the ancillary data files are specified explicitly rather than read in from the header of the events file. The aspect solution is read in from the stack of files listed in the file asol.lis (see 'ahelp stack' for more information on this syntax) and the bad-pixel file is taken from bpix1.fits.

As the maskfile is "", the parameter events file evt2.fits will be used to determine the file by using the MASKFILE parameter.

Example 17

unix% fluximage "evt2.fits[sky=box(4096,4096,600,600)]" acisi/

Here the exposure-corrected image is created using pixels that are a quarter the width of the default ACIS pixel. A spatial filter has been applied to restrict the calculation to a small area around the aim point of the observation, since this is where the Chandra spatial resolution is high enough to make this analysis worthwhile.

Example 18

unix% fluximage evt2.fits qlook_ units=time

Here we use the units parameter to create an exposure map which has units of seconds, so the exposure-corrected image will have units of count/s. In this case the monochromatic setting used as part of the bands setting is ignored; that is bands=broad and bands=0.5:7:1 will produce the same output (since they both use an energy filter of 0.5 to 7.0 keV).


name type def min max reqd stacks
infile file       yes  
outroot string       yes  
bands string default        
xygrid string          
binsize real INDEF 0      
asolfile file         yes
badpixfile file          
maskfile file          
dtffile file          
units string default        
expmapthresh string 1.5%        
background string default        
bkgparams string [pi=300:500]        
random integer 0 0      
parallel boolean yes        
nproc integer INDEF        
tmpdir string ${ASCDS_WORK_PATH}        
cleanup boolean yes        
clobber boolean no        
verbose integer 1 0 5    

Detailed Parameter Descriptions

Parameter=infile (file required)

The events file to use.

The events file can include Data Model filters to restrict the area used, or the chips processed; for example


This file is used to determine the location of the ancillary files - such as aspect solution and bad-pixel file - if they are not explicitly given. If you have used chandra_repro to reprocess your data then all the necessary files will be in the same directory as the reprocessed events file.

Using a directory name

If the infile parameter is set to a directory name - e.g. "repro" or "." - then the following search is made, where dirname represents the directory name used:

  • dirname/repro/*evt*
  • dirname/primary/*evt*
  • dirname/*evt*

The script will display which file it finds. If there are no matches, or multiple matches in the same directory then the script will fail. DataModel filters - such as "[ccd_id=0:3]" - can not be used when only a directory name is given.

Parameter=outroot (string required)

Root name for the output files.

If the parameter ends in "/" then the output files are placed in this directory (which will be created if necessary), otherwise it is prepended to the file names, with a "_" added. So, with bands=broad and outroot=fi/ the exposure-corrected image is called fi/broad_flux.img whereas with outroot=fi the file is fi_broad_flux.img.

An empty outroot value, or one set to ".", will cause the output files to be written to the current working directory with no prefix.

Parameter=bands (string default=default)

What energy or PI band, or bands, should be used?

The script will create an exposure-corrected image - along with supporting data products - for each band. A band can be given using a name (which will use the appropriate definitions from the Chandra Source Catalog), by explicit limits, or by giving the name of a file containing the spectral weights (see ahelp make_instmap_weights).

For the first two options, the names for the output files are created using the minimum and maximum limits for each band. When using a weights file, the output files are labelled using "band1" to "bandn", where n is the number of weights file given to the parameter.

The default setting

The value of 'default' is converted to 'broad' for ACIS data and 'wide' for HRC data.

Band names

The following names - based on the definitions from the Chandra Source Catalog - can be used; energies are given in keV and the effective energy is the monochromatic energy used to calculate the exposure map for the band:

Band name Minimum Energy Maximum Energy Effective Energy
broad 0.5 7.0 2.3
soft 0.5 1.2 0.92
medium 1.2 2.0 1.56
hard 2.0 7.0 3.8
ultrasoft 0.2 0.5 0.4
wide n/a n/a 1.5

The value "csc" can be used as a short form for the combination of "soft,medium,hard".

Explicit ranges

If you want to use different values to those of the CSC then you can use the format lo:hi:eff where

Value Description
lo The minimum value to use: for ACIS this value is in keV and must be given, for HRC it is in PI channels and can be left blank.
hi The maximum value to use: for ACIS this value is in keV and must be given, for HRC it is in PI channels and can be left blank.
eff The effective energy for the instrument map and exposure map, in keV. This is required for both ACIS and HRC observations.
Spectral weights

If a filename is given then it is assumed to be an ASCII file containing spectral weights; two colums giving the energy in keV and the weight value for that energy. This file can be created using the make_instmap_weights script, the save_instmap_weights() routine from the sherpa_contrib.utils module, or manually; see the description of the spectrumfile parameter of mkinstmap for more information on the format of this file.

The energy limits to use for the images are calculated from this file. If the file contains two lines (before the data) with the format

# ENERG_LO = elo
# ENERG_HI = ehi

then the elo and ehi values are taken for the limits (they are assumed to be in keV). This format is created by the make_instmap_weights script and the save_instmap_weights() routine. If these lines do not exist, or the elo/ehi values are not valid numbers, then the limits will be calculated from the first column using the formulae (x1 is the first value, xn is the last, and xm the last-but-one value):

elo = (3 * x1 - x2) / 2

ehi = (3 * xn - xm) / 2

Multiple bands

Multiple bands can be given either by using the name "csc" or by using a comma-separated list. The different schemes can be combined, so the following are all valid (where wgt1.dat and wgt2.dat are weights files):


At present multiple bands can not be used with HRC data.

Parameter=xygrid (string default=)

xygrid for output or filename

When left at the default (empty) value, the script calculates the xygrid for the output image based on the observational data; the skyfov tool is used to determine the maximum area of the sky that is covered by the input data. In this mode the bin size is determined by the binsize parameter.

When set, it can either be:

  • a filename,
  • a grid specification of the form "xlo:xhi:#nx or dx,ylo:yhi:#ny or dy".

If a filename is given then its sky range is used to determine the output; this is useful if you want to create an exposure-corrected image that matches an existing image. Otherwise the grid range is given explicitly; examples include "3824.5:5112.5:#161,4008.5:5296.5:#161" and "3824.5:5112.5:8,4008.5:5296.5:8".

Parameter=binsize (real default=INDEF min=0)

Image binning factor

This parameter is only used when xygrid is set to "" (which is the default setting).

When set to INDEF a default value is used: 8 for ACIS data and 32 for HRC data. The binning factor has units of the SKY pixel (0.492" for ACIS and 0.1318" for HRC), so binsize=4 for an ACIS observation will create images with 1.968 arcsecond pixels. The value need not be an integer, and can be less than 1 if you want to look at small-scale structure near the aim point of the observation; in this case it is strongly suggested that a spatial filter is applied to ensure manageable file sizes.

Parameter=asolfile (file stacks=yes)

Aspect solution for the observation.

If the value is empty then the ASOLFILE keyword from the events file will be used to find the file(s) to use. Unlike the other ancillary products, an aspect solution is required for the script to run.

The aspect solution files have names like pcadf*_asol1.fits and are included in the output directory of the chandra_repro script. If multiple asol files exist for the observation and you want to explicitly select them - rather than use the file names given in the ASOLFILE keyword - then use the stack syntax (e.g. a comma, or space, separated string as described in "ahelp stack" for more information). So to use asol1.fits and asol2.fits you could say



asolfile="asol1.fits asol2.fits"

The files do not have to be given in time order.

Aspect histograms - the output of the asphist tool - can not be used here.

Parameter=badpixfile (file)

Input bad pixel file

If the parameter is empty then the BPIXFILE keyword in the events file will be used.

A value of "CALDB" will use the CalDB bad-pixel file and "NONE" will use no bad-pixel file (the value is case insensitive). The bad-pixel file is not required to run the script, but care should be taken when analyzing the output if it is not used.

The bad-pixel file has a name like acisf*bpix1.fits and is included in the output directory of the chandra_repro script. If you have used or created a custom bad-pixel file for the observation then this should be used.

Parameter=maskfile (file)

Mask file for the observation.

If the parameter is empty then the MASKFILE keyword in the events file will be used. A value of "NONE" will use no mask file in the analysis; in this case care should be taken when analyzing the results since the exposure map will not be correct if the data was taken using a sub array (i.e. not all the chips or ccds were read out).

The mask file has a name like acisf*msk1.fits and is included in the output directory of the chandra_repro script.

Parameter=dtffile (file)

Input dtf file for HRC observations

This parameter is only used with HRC observations.

If the parameter is empty then the DTFFILE keyword in the events file will be used. A value of "NONE" will mean that only a mean DTF value will be used.

The parameter block has a name like hrcf*dtf1.fits and is included in the output directory of the chandra_repro script.

Parameter=units (string default=default)

Units for the exposure map

This parameter determines the units of the instrument and exposure maps.

units Exposure map Exposure-corrected image
default cm^2 s count / photon photon / cm^2 / s
area cm^2 count / photon photon / cm^2
time s count / s

This parameter combines the normalize parameter of mkexpmap and the modifiers for the detsubsys and mirror parameters of mkinstmap (see 'ahelp ardlib').

Converting from the normalize parameter

To replicate the old normalize=no setting, use units=default and for normalize=yes use units=area.

What is units=time equivalent to?

When units=time is used, instrument maps are created with the additional settings:

  • ";IDEAL" is appended to the detsubsys parameter to remove any correction for the detector quantum efficiency (QE), quantum efficiency uniformities (QEU), or the ACIS contamination model;
  • the mirror parameter is set to "HRMA;area=1" rather than "HRMA" to ignore the effective area of the Chandra mirrors;
  • and the dafile parameter is set to "NONE" to remove the ACIS "dead area" correction, which accounts for the (small) decrease in detector efficiency due to the background cosmic ray flux.

The result is that the instrument map pixels have values of 0 or 1.

Parameter=expmapthresh (string default=1.5%)

Remove low-exposure regions?

If set to "" then no clipping of the counts image or exposure map is performed before creating the exposure-corrected image.

When set, the parameter determines how the counts image and exposure map are clipped before being combined together to create the exposure-corrected image. This can be used to exclude those pixels at the edge of a chip which, due to the strong gradient in the exposure map, end up dominating the final image.

For a full list of supported options, see the description of the cut parameter of dmimgthresh. Here we discuss the two most common options:

  • expmapthresh="2%", which excludes those pixels where the exposure map falls below 2% of its peak value,
  • and expmapthresh="100", which excludes those pixels where exposure map value is less than 100 (in this case the value of the units parameter is important).

Note that both the counts image and the exposure map are thresholded before being combined.

Parameter=background (string default=default)

Method for background removal (HRC-I)

This parameter is only used for HRC-I data. The reason for subtracting the particle background component is to help remove the "rings" seen in exposure-corrected HRC-I data, but does significantly increase the time taken to run the script.

If set to "none", or no HRC-I background CALDB file can not be found, then the data is processed with no background subtraction.


This is the same as method=time.


This setting scales the background file by the ratio of the observation to background exposure times. It is the method used in the thread discussed above.


This option scales the background by the ratio of the counts in the range determined by the bkgparams parameter, rather than by exposure time.

Header keywords

The full path to the selected background event file is added to the header file of the background-subtracted image and fluxed image using the BEVTFILE keyword. The scaling method and factor are added to the relevant files using the BKGMETH and BKGSCALE keywords, respectively.

Parameter=bkgparams (string default=[pi=300:500])

Optional argument for background subtraction

This is only used when background subtraction occurs and the method being used is set to "particle". The parameter is taken to be a Data Model filter to apply to both the observation and the particle background file, and is expected to be of the form "[pi=lo:hi]". It is used to select a subset of the observation that represents the particle background - i.e. it is not strongly "contaminated" by cosmic emission - but is close enough (in PI) to the source emission to minimise differences in the spectral shape of the particle emission between the observation and that seen in the background observations.

Parameter=random (integer default=0 min=0)

Random seed for randomization.

The random parameter is sent to reproject_events when processing the HRC-I background files to match the observation. A value of 0 uses the system time to seed the random number generator.

Parameter=parallel (boolean default=yes)

Run processes in parallel?

When run on a multi-processor system, many of the tasks can be run in parallel to reduce the execution time of the script. Since the tasks are likely to be memory or I/O-bound, the reduction in run time will be less than the number of cores on a machine. When parallel=yes, the default behaviour is to use all the CPU processors, but this can be changed with the nproc parameter.

This option can be ignored when run on a single-processor system.

Parameter=nproc (integer default=INDEF)

Number of processors to use

This parameter is only used when parallel=yes. It determines the number of processors to use. If maxproc is the actual number of processors on your machine, then a value of INDEF - the default value - means that all maxproc processors will be used. A positive value means to use that number of processors (any value larger than maxproc will be set to maxproc). A negative value is added to maxproc (and any value less than one is set to one).

Parameter=tmpdir (string default=${ASCDS_WORK_PATH})

Directory for temporary files.

Directory for storing temporary files that require further processing before becoming useful. If the directory does not exist then it will be created for use by the script, and then deleted.

Parameter=cleanup (boolean default=yes)

Cleanup intermediary files on exit.

If set to "yes", the intermediate data products are deleted when the script ends, leaving only the counts image, clipped counts image, combined exposure map, and exposure-corrected image.

Parameter=clobber (boolean default=no)

Overwrite existing files?

Parameter=verbose (integer default=1 min=0 max=5)

Output verbosity.

The default verbosity value of 1 prints status messages as the script runs. Higher verbosity settings print the commands that are being run. Setting verbose=0 turns off most of the screen output (some output is currently unavoidable).

Energy bands

The energy bands - when given by name - are taken from the Chandra Source Catalog definitions. The default bands setting (i.e. "default") means to use the "broad" band setting for ACIS data and the "wide" band setting for HRC data. Another common choice (for ACIS data) is "csc", which creates the "soft", "medium", and "hard" bands.

Examples of specifying the bands are:

unix% fluximage evt2.fits out/
unix% fluximage evt2.fits out/ bands=soft
unix% fluximage evt2.fits out/ bands=csc
unix% fluximage evt2.fits out/ bands=csc,broad
unix% fluximage evt2.fits out/ bands=wgt.dat

which use

  • the default band (so broad for ACIS, wide for HRC),
  • the soft band,
  • the CSC bands (i.e. soft, medium, and hard),
  • and four bands (soft, medium, hard, and broad),
  • and a spectral-weights file (wgt.dat), created either manually or with the make_instmap_weights script.

The examples and description of the bands parameter describe how either

  • explicit energy or PI ranges and effective energies,
  • or a spectral weights file

can be specified.

Parallel processing

When analyzing an observation with multiple chips (ACIS) or HRC-S plates, or if using multiple energy bands, the execution time can be reduced by running some steps in parallel. This is controlled by the parallel flag, which defaults to yes. The nproc parameter controls the number of processors that will be used; the default is to use all of then. If run on a single-processor machine then both parameters are ignored.

Output files

The primary output files are named using the following scheme:

Flag Type Filename
E Counts image outroot_blabel.img
E Exposure map outroot_blabel.expmap
C Clipped counts image outroot_blabel_thresh.img
C Clipped exposure map outroot_blabel_thresh.expmap
A Exposure-corrected image outroot_blabel_flux.img

where blabel is either the band name (e.g. soft, broad, ...), "lo-hi" where lo and hi are the limits of the range (keV for ACIS, PI for HRC), or band1 to bandn (when using spectral weight files). The Flag column indicates when the file is created (when cleanup=yes): A indicates always, T is for when the expmapthresh parameter is set, and E is for when the the expmapthresh parameter is empty.

The above is when outroot does not end in a "/"; if it does then remove "outroot_" and the files will be created in the directory named by outroot. Examples of the exposure-corrected image name for a combination of bands and outroot settings are given in the following table:

Names of the exposure-corrected images

outroot bands=broad bands=0.5:2:1.5 (ACIS) bands=30:200:1.1 (HRC) bands=wgt.dat
fimg fimg_broad_flux.img fimg_0.5-2_flux.img fimg_30-200_flux.img fimg_band1_flux.img
odir/ odir/broad_flux.img odir/0.5-2_flux.img odir/30-200_flux.img odir/band1_flux.img
odir/fimg odir/fimg_broad_flux.img odir/fimg_0.5-2_flux.img odir/fimg_30-200_flux.img odir/fimg_band1_flux.img

For HRC data, using bands=::eval results in a label of "all" being used.

The intermediary files follow the same rule for outroot and have the form given below, where ccd refers to the plate number for HRC data:

Type Filename
Aspect histograms outroot_ccd.asphist
Instrument maps outroot_ccd_blabel.instmap
Per chip/plate exposure maps outroot_ccd_blabel.expmap
HRC-I image (no background subtraction) outroot_unsubtracted.img
HRC-I image of the particle background outroot_particle_bgnd.img
HRC-I image after background subtraction outroot.img

By default, the intermediary per-chip data products are removed after the script has completed running. To save these (potentially numerous) files, set the cleanup parameter to 'no'.

Output image area and size

The default behavior - when xygrid="" - is to calculate the output image size so that it covers all the data: a FOV file is created for the observation and the limits of this file are used to determine the X and Y range of the image. In this mode the binning is set by the binzise parameter, is in units of the native pixel size (i.e. 0.492" for ACIS and 0.1318" for HRC), and can be set to sub-pixel values (e.g. 0.5).

The tool can also be given an explicit grid for the output image size using the xygrid parameter (in this mode the binsize parameter is ignored). The grid can either be:

  • a filename,
  • or a grid specification, of the form "xlo:xhi:#nx or dx,ylo:yhi:#ny or dy".

When given a file, the sky range of the file is used for the output (e.g. the get_sky_limits tool is run on the file and its xygrid parameter value is used). Otherwise, the grid is given using the same format as the xygrid parameter of mkexpmap; examples include:




If the requested grid results in the last row or column being partially filled - in other words the width or height divided by the bin size is not a whole number - then the exposure map values in the column or row will be over estimated (since they are calculated assuming that the pixel is "full").

The default binsize parameter value (of default) is equivalent to 8 (ACIS) or 32 (HRC) pixels.

Handling filter or grids that do not overlap the observation

The tool will error out when there is no overlap between the observation and the xygrid (if given) or any spatial filter applied to the input file.

Required data files

For observations that has been reprocessed by chandra_repro, then all that is needed is to give the events file since the required ancillary files can be found using the header of the events file (the chandra_repro script ensures that all the files needed for this script are copied to the same directory as the events file). These ancillary files can be located in one of the following directories, relative to the events file:

  • .
  • repro/
  • primary/
  • secondary/
  • ../repro/
  • ../primary/
  • ../secondary/

If the files are not in one of these directories, they have been renamed, or new versions have been created during your analysis (e.g. a new bad-pixel file or a reprojected aspect solution created by reproject_aspect) then they can be explicitly set using the following parameters:

Ancillary data products

Parameter name Description
asolfile The aspect solution for the observation, which may be split over multiple files.
badpixfile The bad-pixel file for an observation. If set to CALDB then the bad-pixel file in the Chandra CalDB will be used rather than the observation-specific version. If set to NONE then the instrument map, and hence exposure map, will be created with no bad pixels.
maskfile The mask file for an observation. This determines the active area of each CCD or chip (e.g. if a sub-array was used).
dtffile The Dead-Time Factor correction file for HRC observations.

The only required information is that provided by the aspect solution file(s); any other file can be ignored, (either explicitly, by setting the parameter to NONE, or implicitly because it can not be found), in which case the appropriate correction or information is ignored.

Processing steps

The script primarily makes use of the asphist, mkinstmap, and mkexpmap tools; generally following the outline of the "Step-by-Step Guide" sections of the "Exposure Maps" threads of the "Imaging" science analysis threads. The following steps are taken to create the data products:

  • skyfov and get_fov_limits: to determine the output image area
  • dmcopy: to create an events image with the specified binning factor
  • hrc_bkgrnd_lookup, reproject_events, and dmimgcalc: to subtract the particle background (HRC-I only)
  • asphist: to build the aspect histogram(s)
  • mkinstmap: to calculate the instrument map(s) for the effective energy given for each energy band
  • mkexpmap: to calculate the exposure map(s) in each energy band
  • dmimgcalc: to combine the exposure maps if there are multiple ACIS CCDs or HRC-S plates
  • dmimgthresh: to make a "threshold cut" before dividing the image by the exposure map, to reduce the noise at the edge of the chip (optional)
  • dmimgcalc: to normalize the image by the exposure map

Background subtraction of HRC-I data

HRC-I data will have an estimate of the particle background subtracted from the data before dividing by the exposure map if the background parameter is not set to none and the CALDB "HRC background event files" package is installed. This step is intended to reduce the ring artifacts that can be seen in HRC-I data. The raw files used to create the background-subtracted image are retained when the cleanup parameter is set to no. In this case, assuming the output fluxed image is called wide_flux.img, then

wide_flux.img = wide_thresh.img / wide_thresh.expmap

wide_thresh.img = wide_unsubtracted.img - s * wide_particle_bgnd.img

scale_time = observation exposure time / particle background exposure time

scale_particle = observation counts PI=300:500 / particle background counts PI=300:500

where the thresholding step has been ignored for simplicity and s=scale_time (method=default) or s=scale_particle (method=particle). The backgtround method and scaling factor are written to the header of the relevant files as the BKGMETH and BKGSCALE keywords. The name of the background event file used is stored in the BEVTFILE keyword.

STATUS filtering

The STATUS filter used to create the observation event file is used to filter the particle background file; this value is displayed on screen and written to the background files as the STATFILT keyword.

Changes in the scripts 4.8.2 (January 2016) release

The random parameter has been added. It is used to control the random parameter of reproject_events when background subtracting HRC-I data.

Changes in the scripts 4.8.1 (December 2015) release

The code has been updated to avoid warning messages from NumPy version 1.9. There is no difference to how the script behaves.

Changes in the scripts 4.6.6 (September 2014) release

Minor improvements

There have been minor improvements to error messages and better support for data with missing keywords, such as that produced by Marx.

Changes in the scripts 4.6.5 (June 2014) release

Filtering the aspect solution by the event file GTI

The skyfov is run in order to calculate the sky range covered by each observation; e.g. see ahelp get_fov_limits. The aspect solutions are now filtered by the good-time intervals (GTI) of the event file - that is, the script sets


(assuming the aspect solution is called asol1.fits and the event file evt.fits) when calling skyfov. This should not change most results but may produce slightly-smaller output images in cases where the GTI of the observation covers only a small fraction of the aspect solution.

HRC-I analysis

It is now possible to apply a PI filter range for the images by specifying PI limits in the bands parameter, that is bands=30:200:1.2 will use a PI range of 30 to 200 when creating all the imaging products. The monochromatic energy used for the instrument map is still given in keV, so 1.2 keV in this example.

A new subtraction method, which scales the background to match the data using the ratio of the counts in the band PI=300:500 (this can be changed with the new bkgparams parameter), can be selected by setting method=particle. The default method uses the exposure times of the observation and particle background files for the scaling. The scaling method and factor are added to the relevant files using the BKGMETH and BKGSCALE keywords, respectively.

The status filter used to filter the HRC-I particle background file is now included in the screen output at the default verbosity level and added to the particle background files (retained when cleanup=no) using the STATFILT keyword. The background-unsubtracted files retained when cleanup=no now match the documentation (rather than include the typo 'undsubtracted').

Changes in the scripts 4.6.3 (March 2014) release

Removing individual instrument-map files

The per-chip instrument map files are now deleted when cleanup=yes and multiple bands were supplied.

HRC-I background files

The original and background-subtracted images are now retained when cleanup=no; they are named outroot_unsubtracted.img and outroot_particle_bgnd.img respectively.

Changes in the scripts 4.6.2 (February 2014) release

There have been some internal changes to try and avoid incompatibilities between CIAO and FTOOLS parameter files.

Setting maskfile to NONE

The bug which meant that maskfile could not be set to "NONE" has been fixed.

The BEVTFILE keyword for HRC-I datasets

The BEVTFILE keyword is added to HRC-I datasets when background subtraction is used; it contains the name of the background event file used.

Changes in the scripts 4.6.1 (December 2013) release

Removal of the pbkfile parameter

The pbkfile parameter has been removed as it is no longer needed to run mkinstmap on ACIS observations. This means that it is no longer possible to turn off the ACIS dead-area correction for the exposure map; users who need this flexibility will have to generate the exposure maps manually (e.g. the ACIS single-chip thread).

For old data sets the removal of the pbkfile parameter can lead to error messages about missing keywords; in this case please download the latest archive version of your observation with download_chandra_obsid and then reprocess with chandra_repro.

mkinstmap obsfile parameter

The mkinstmap tool is now run with the obsfile parameter set to the observation event file rather than the aspect histogram for the chip or plate.

Changes in the scripts 4.5.4 (August 2013) release

The description of the bands parameter has been updated to point out that when using the explicit format - e.g. a:b:c - for an energy band then the a and b values are required for ACIS data but should not be set for HRC data.

The handling of files with a DataModel filter has been improved.

Changes in the scripts 4.5.2 (April 2013) release

Aspect solutions

The program now checks that the aspect-solution files are not aspect histograms.

Changes in the scripts 4.5.1 (December 2012) release

The script has been updated to work in CIAO 4.5.

About Contributed Software

This script is not an official part of the CIAO release but is made available as "contributed" software via the CIAO scripts page. Please see this page for installation instructions - such as how to ensure that the parameter file is available.


Size of image too large [New] (10 Apr 2017)

This script may sometimes create an image larger than expected. When a CCD_ID filter is used, the intent of the script is to create an image just as big as needed to enlose the chips listed. However, for some datasets, the image produced may be larger and include area covered by additional CCDs. This only affect the size of the image, not the image pixel values.


WARNING about creating a large image  (11 Jul 2011)

If the chosen binning factor is small enough (e.g. binsize=1 for an ACIS-I observation) then the following warning will be seen:

# DMCOPY (CIAO): WARNING: Creating large image: 84 MB. Current max set at 50 MB.
Increase maximum using [opt mem=n] or increase blocking to reduce size.

The output files are completely valid; the warning is just there to let you know that the files that are being created are large. At present there is no way to hide this message.

See Also

acis_bkgrnd_lookup, acis_fef_lookup, acis_set_ardlib, addresp, aprates, asphist, combine_grating_spectra, combine_spectra, dither_region, dmarfadd, dmextract, eff2evt, flux_obs, fullgarf, hrc_bkgrnd_lookup, make_instmap_weights, mean_energy_map, merge_obs, mkacisrmf, mkarf, mkexpmap, mkgarf, mkgrmf, mkinstmap, mkpsfmap, mkrmf, mkwarf, psextract, psf_project_ray, readout_bkg, reproject_obs, rmfimg, sky2tdet, specextract

Last modified: January 2016
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