Using specextract to Extract ACIS Spectra and Response Files
OverviewLast Update: 14 Jun 2006 - corrected link in "Calibration Updates"; clarified information on GRADED mode data Synopsis:
specextract should be used for the analysis of extended sources only. Users working with point sources should instead use the psextract script. Analysis of point sources that was done with mkwarf/specextract should be redone with mkarf/psextract for the most accurate results, e.g. taking bad columns into account. specextract, a new script for creating ACIS spectra for extended sources, was released in CIAO 3.3 (21 November 2005). It allows the user to create source and background PHA or PI spectra and the associated ARF and RMF files. specextract can take a stack of input files and generate many spectra in one run of the script. Read the help file for full details on how the script works. This script is designed to replace the acisspec script currently distributed in the CIAO Scripts package. The major improvement over its predecessor is the ability to determine when mkacisrmf should be used in place of mkrmf (see the "Creating RMFs: mkrmf vs. mkacisrmf" section of this thread). Users are encouraged to start using specextract in place of acisspec for new extended source analyses. Note that specextract does not currently offer the option to coadd spectra as acisspec does. Purpose: To generate source and background PI (PHA) spectra of an extended ACIS source and build the proper RMFs and ARFs. Read this thread if: you are working with an ACIS observation of an extended source, whether imaging or grating data.
|
![[New]](../../imgs/new.gif)
(30 Mar 2006) The recommended use for the specextract
script has been changed. It has been determined
that the mkwarf tool, which is used for ARF
generation by specextract, may not produce
accurate results for point sources. The mkarf
tool, used by psextract, is preferred for point
source extraction.
Contents
- How specextract Works
- Get Started
- Single Spectrum
- Extracting Multiple Spectra
- More Information on the Output Files
- Running mkacisrmf Independently
- Fitting
- Caveats
- Parameter files:
- History
- Images
How specextract Works
specextract runs the dmextract, mkwarf, mkrmf or mkacisrmf (see the "Creating RMFs: mkrmf vs. mkacisrmf" section), dmgroup and dmhedit tools.
- dmextract: to extract source and (optionally) background spectra. This tool also creates the WMAP used as input to mkwarf and mkacisrmf; see ahelp mkwarf for details on why a WMAP is necessary.
- mkwarf: to create weighted ARF(s).
- mkrmf or mkacisrmf: to build the RMF(s); also see the "Creating RMFs: mkrmf vs. mkacisrmf" section.
- dmgroup: to group the source spectrum and/or background spectrum.
- dmhedit: to update the BACKFILE, RESPFILE and ANCRFILE keys in the source and background spectrum files.
Creating RMFs: mkrmf vs mkacisrmf
The tool mkacisrmf was introduced in CIAO 3.2 and represents an entirely new method for creating ACIS response matrices; details on why mkacisrmf is more accurate than mkrmf are available in the the mkacisrmf help file. mkacisrmf can be used to create RMFs for all -120 ACIS observations taken in any mode except for GRADED mode. For GRADED mode, it is possible to create responses for the back-illuminated chips (ACIS-S1 and S3) only. For more information, refer to the ACIS GRADED Mode Data section of the mkacisrmf thread. The Creating ACIS RMFs with mkacisrmf why topic contains details about the tool and its calibration.
specextract reads the source and background file header keywords to determine when the observation was taken and what calibration has been applied. The script then does a CALDB lookup to compare the calibration applied to the event file and the most recent file available in the CALDB. This means that the data needs to have been processed with very newest calibration, not just a "good enough" version; that is, even if the new calibration will have a minimal effect on your data, specextract requires that it be applied for the "use mkacisrmf" switch to be triggered.
If all the requirements are met, the script uses mkacisrmf to generate the source and background RMF(s) for the data. If the observation was not done at -120 or has not been reprocessed with the newest calibration available, specextract creates the RMF(s) with mkrmf. At higher verbosities, a message of this form will be printed to the screen:
Cannot use mkacisrmf because last applied gainfile does not match current CALDB gainfile: acisD2000-01-29gain_ctiN0003.fits != acisD2000-01-29gain_ctiN0005.fits Please re-run <your_filename> with acis_process_events if you wish to use mkacisrmf
Users who don't wish to reprocess can still use specextract, and run mkacisrmf afterwards to create new RMF files.
Get Started
Sample ObsIDs used: 869 (ACIS-S, ARP 220); 1842 (ACIS-I, G21.5-09); 1843 (ACIS-I, G21.5-09)
File types needed: evt2
Please ensure that you have set up ardlib to use the bad pixel file for your observation before following this thread.
One of the event files used in this thread - acis_869_evt2.fits - has been reprocessed with the newest calibration available by following the Create a New Level=2 Event File thread.
Choosing a background file
There are several options when choosing the background file for use with specextract:
- define the background region from a source-free region of the same event file
- use a different event file when defining the background
- omit the background completely (i.e. leave the bkgfile blank)
If you plan on using one of the "blank-sky" background files from the CALDB with specextract, read the Using the ACIS "Blank-Sky" Background Files caveats before continuing.
Single Spectrum
A simple example: extracting spectra with one source region and one background region.
Build Source and Background Regions
We need to define two regions, one for the source and another for the background. To do this, first display the file:
unix% ds9 acis_869_evt2.fits &
Refer to the Using CIAO Regions thread for information on creating region files. The files for this example look like:
unix% cat simple.reg # Region file format: CIAO version 1.0 ellipse(4026,4138.9,50,40,0) unix% cat bg_simple.reg # Region file format: CIAO version 1.0 circle(3975,4233,20)
The regions are shown displayed on the event file in
Figure 1 ![[Link to Image 1]](../../imgs/imageicon.gif){kind=small}
; the source is white
and the background is green.
Make sure that you save the regions in CIAO format (Regions -> File Format -> CIAO) so that they are fully compatible with the analysis tools.
Run specextract
Input the event file with the appropriate region file for the source and background:
unix% punlearn specextract unix% pset specextract infile="acis_869_evt2.fits[sky=region(simple.reg)]" unix% pset specextract outroot=simple unix% pset specextract bkgfile="acis_869_evt2.fits[sky=region(bg_simple.reg)]" unix% pset specextract grouptype=NUM_CTS binspec=15
We choose to use the default grouping values: the source spectrum will be grouped to a minimum number of 15 counts per new channel and the background spectrum will be ungrouped. The grouptype and binspec parameters are used to specify the source spectrum grouping, and the bkg_grouptype and bkg_binspec parameters specify the background spectrum grouping.
Running the tool with verbose=1 shows what it is doing:
unix% specextract verbose=1
Source events file(s) (acis_869_evt2.fits[sky=region(simple.reg)]):
Output directory path + root name for output files (simple):
Running: specextract
Checking initial status and initializing variables...
Extracting src spectra for item 1 of 1 in input list
----------------------------------------------------------------------
Input Parameters
----------------------------------------------------------------------
VERBOSE: infile = acis_869_evt2.fits[sky=region(simple.reg)][bin PI]
VERBOSE: outfile = simple_src1.pi
VERBOSE: Creating output file simple_src1.pi
VERBOSE: Write WMAP using acis_869_evt2.fits[sky=region(simple.reg)][energy=300:2000][bin det=8]
Creating src ARF for item 1 of 1 in input list
Parameters for mkwarf:
infile = simple_src1.pi[WMAP]
outfile = simple_src1.warf
weightfile = simple_src1.wfef
feffile = CALDB
egridspec = 0.3:11.0:0.01
spectrumfile =
threshold = 0
mirror = HRMA
ardlibpar = ardlib
clobber = no
verbose = 1
Creating src RMF for item 1 of 1 in input list
Using mkacisrmf...
*** mkacisrmf parameter inputs ***
input file: CALDB
output file: simple_src1.wrmf
wmap file: simple_src1.pi[WMAP]
energy: 0.3:11.0:0.01
channel: 1:1024:1
chantype: PI
ccd_id:
chipx:
chipy:
contour lvl: 100
log file:
gain file: CALDB
pixlib param. file: geom
threshold: 1.00e-06
clobber(1=yes, 0=no): 0
verbose level: 1
CALDB -> /soft/ciao/CALDB/data/chandra/acis/cpf/p2_resp/acisD2000-01-29p2_respN0005.fits
CALDB -> /soft/ciao/CALDB/data/chandra/acis/bcf/gain/acisD2000-01-29gain_ctiN0005.fits
Total 16 regions to be processed:
1> reg# 2287 processed
2> reg# 2288 processed
3> reg# 2289 processed
4> reg# 2290 processed
5> reg# 2319 processed
6> reg# 2320 processed
7> reg# 2321 processed
8> reg# 2322 processed
9> reg# 2351 processed
10> reg# 2352 processed
11> reg# 2353 processed
12> reg# 2354 processed
13> reg# 2383 processed
14> reg# 2384 processed
15> reg# 2385 processed
16> reg# 2386 processed
Grouping src spectrum for item 1 of 1 in input list
Parameters for dmgroup:
infile = simple_src1.pi[SPECTRUM]
outfile = simple_src1_grp.pi
grouptype= NUM_CTS
xcolumn = channel
binspec =
ycolumn = counts
tabspec =
tabcolumn=
stopspec=
stopcolumn=
errcolumn=
grouptypeval= 15.000000
maxlength= 0.000000
clobber = no
verbose = 1
Updating header of simple_src1.pi with RESPFILE and ANCRFILE keywords.
dmhedit: verbose set to 1
dmhedit: Input file = simple_src1.pi
dmhedit: file list = none
dmhedit: key RESPFILE will be added.
dmhedit: verbose set to 1
dmhedit: Input file = simple_src1.pi
dmhedit: file list = none
dmhedit: key ANCRFILE will be added.
Updating header of simple_src1_grp.pi with RESPFILE and ANCRFILE keywords.
dmhedit: verbose set to 1
dmhedit: Input file = simple_src1_grp.pi
dmhedit: file list = none
dmhedit: key RESPFILE will be added.
dmhedit: verbose set to 1
dmhedit: Input file = simple_src1_grp.pi
dmhedit: file list = none
dmhedit: key ANCRFILE will be added.
Extracting bkg spectra for item 1 of 1 in input list
----------------------------------------------------------------------
Input Parameters
----------------------------------------------------------------------
VERBOSE: infile = acis_869_evt2.fits[sky=region(bg_simple.reg)][bin PI]
VERBOSE: outfile = simple_bkg1.pi
VERBOSE: Creating output file simple_bkg1.pi
VERBOSE: Write WMAP using acis_869_evt2.fits[sky=region(bg_simple.reg)][energy=300:2000][bin det=8]
Creating bkg ARF for item 1 of 1 in input list
Parameters for mkwarf:
infile = simple_bkg1.pi[WMAP]
outfile = simple_bkg1.warf
weightfile = simple_bkg1.wfef
feffile = CALDB
egridspec = 0.3:11.0:0.01
spectrumfile =
threshold = 0
mirror = HRMA
ardlibpar = ardlib
clobber = no
verbose = 1
Creating bkg RMF for item 1 of 1 in input list
Using mkacisrmf...
*** mkacisrmf parameter inputs ***
input file: CALDB
output file: simple_bkg1.wrmf
wmap file: simple_bkg1.pi[WMAP]
energy: 0.3:11.0:0.01
channel: 1:1024:1
chantype: PI
ccd_id:
chipx:
chipy:
contour lvl: 100
log file:
gain file: CALDB
pixlib param. file: geom
threshold: 1.00e-06
clobber(1=yes, 0=no): 0
verbose level: 1
CALDB -> /soft/ciao/CALDB/data/chandra/acis/cpf/p2_resp/acisD2000-01-29p2_respN0005.fits
CALDB -> /soft/ciao/CALDB/data/chandra/acis/bcf/gain/acisD2000-01-29gain_ctiN0005.fits
Total 5 regions to be processed:
1> reg# 2417 processed
2> reg# 2418 processed
3> reg# 2449 processed
4> reg# 2450 processed
5> reg# 2481 processed
Updating header of simple_bkg1.pi with RESPFILE and ANCRFILE keywords.
dmhedit: verbose set to 1
dmhedit: Input file = simple_bkg1.pi
dmhedit: file list = none
dmhedit: key RESPFILE will be added.
dmhedit: verbose set to 1
dmhedit: Input file = simple_bkg1.pi
dmhedit: file list = none
dmhedit: key ANCRFILE will be added.
Updating header of simple_src1.pi with BACKFILE keyword.
dmhedit: verbose set to 1
dmhedit: Input file = simple_src1.pi
dmhedit: file list = none
dmhedit: key BACKFILE will be added.
Updating header of simple_src1_grp.pi with BACKFILE keyword.
dmhedit: verbose set to 1
dmhedit: Input file = simple_src1_grp.pi
dmhedit: file list = none
dmhedit: key BACKFILE will be added.
The contents of the parameter file may be checked with plist specextract.
Examining the Output Files
The number of files created depends on if a background event file was provided and if source and/or background grouping was specified. In this case, the output files are:
Source: simple_src1.pi ungrouped spectrum simple_src1.warf weighted ARF simple_src1.wfef FEF weight file simple_src1.wrmf weighted RMF simple_src1_grp.pi grouped spectrum Background: simple_bkg1.pi ungrouped spectrum simple_bkg1.warf weighted ARF simple_bkg1.wfef FEF weight file simple_bkg1.wrmf weighted RMF
The FEF weight files (.wfef) are required as input to mkrmf; they are created by mkwarf before the scripts determines whether to use mkrmf or mkacisrmf. After the RMFs are created, these files are no longer needed.
Extracting Multiple Spectra
In this example, we show how specextract can create multiple output spectra from a single run of the script.
Build Source Regions and Background Regions
This example uses the two observations of G21.5-09. Both observations will be processed by specextract at the same time, producing two sets of output files; this is explained further in the Run specextract section.
We define have defined a source and background region for each observation:
unix% cat 1842_src.reg # Region file format: CIAO version 1.0 circle(2249.5,4221.5,102.0092) unix% cat 1842_bg.reg # Region file format: CIAO version 1.0 circle(2565.5,4129.5,40) unix% cat 1843_src.reg # Region file format: CIAO version 1.0 circle(1635.5,4113.5,135.11408) unix% cat 1843_bg.reg # Region file format: CIAO version 1.0 circle(2129.5,4007.5,40)
The regions are shown displayed on the event files in
Figure 2 ; ObsID 1842 is on the
left and ObsID 1843 is on the right. The
source region is in white and the background region is in
green in each frame.
Run specextract
The event files are input to the script as a stack; this syntax means that a separate spectrum will be created for each of the file/region pairs:
unix% cat multi_src.lis 1842_evt2.fits[sky=region(1842_src.reg)] 1843_evt2.fits[sky=region(1843_src.reg)]
When working with stack inputs to specextract, the source and background stacks must contain the same number of items, unless you are not extracting background spectra. Make sure that the background file definitions are in the same order as the source files:
unix% cat multi_bg.lis 1842_evt2.fits[sky=region(1842_bg.reg)] 1843_evt2.fits[sky=region(1843_bg.reg)]
Set the parameters:
unix% punlearn specextract unix% pset specextract infile="@multi_src.lis" unix% pset specextract outroot=multi unix% pset specextract bkgfile="@multi_bg.lis" unix% pset specextract grouptype=BIN binspec=10
The source spectra will be grouped into bins of 10 counts each.
Note that this method allows you to input as many event file/region file pairs as you like, but the same outroot and grouping will be applied to all of them.
unix% specextract Source events file(s) (@multi_src.lis): Output directory path + root name for output files (multi): CALDB->/soft/ciao/CALDB/data/chandra/acis/ Atten: grid input,"energy=0.3:11.0:0.01", Total 54 regions to be processed: --- Region #1 processed --- Region #2 processed ...
The rest of the screen output for this run has been omitted. specextract prints status messages to the screen even when verbose=0, as seen here. This is a bug which will be fixed in the future.
The contents of the parameter file may be checked with plist specextract.
Examining the Output Files
The number of files created depends on if a background event file was provided and if source and/or background grouping was specified. In this case, the output files are:
Source 1 (1842_src.reg): multi_src1.pi ungrouped spectrum multi_src1.warf weighted ARF multi_src1.wfef FEF weight file multi_src1.wrmf weighted RMF multi_src1_grp.pi grouped spectrum Source 2 (1843_src.reg): multi_src2.pi multi_src2.warf multi_src2.wfef multi_src2.wrmf multi_src2_grp.pi Background 1 (1842_bg.reg): multi_bkg1.pi multi_bkg1.warf multi_bkg1.wfef multi_bkg1.wrmf Background 2 (1843_bg.reg): multi_bkg2.pi multi_bkg2.warf multi_bkg2.wfef multi_bkg2.wrmf
The script simply numbers the output to match the order in which the files were input: src1 = multi_pt.reg and src2 = multi_jet.reg. The bkg1 and bkg2 files will be identical in this case, as we used the same region files for both, except for the header keyword values.
The filetypes listed for Source 1 apply similarly to Source 2 and both sets of background files.
More Information on the Output Files
Which RMF tool was used?
When specextract is run with verbose greater than zero, the status messages printed to the screen report which tool was used to create the RMF, mkacisrmf or mkrmf. This information is also recorded in the FITS file that is created. If you have verbosity set to zero, or simply don't want to parse all that screen output, use the dmhistory tool:
unix% dmhistory simple_src1.wrmf all mkacisrmf infile="CALDB" outfile="simple_src1.wrmf" wmap="simple_src1.pi[WMAP]" energy="0.3:11.0:0.01" channel="1:1024:1" chantype="PI" ccd_id="0" chipx="0" chipy="0" gain="CALDB" logfile="" contlvl="100" geompar="geom" thresh="1e-06" clobber="no" verbose="1" unix% dmhistory multi_src1.wrmf all mkrmf infile="CALDB" outfile="multi_src1.wrmf" axis1="energy=0.3:11.0:0.01" axis2="pi=1:1024:1" logfile="" weights="multi_src1.wfef" thresh="1e-05" outfmt="legacy" clobber="no" verbose="0" axis3="none" axis4="none" axis5="none"
The single spectrum example created the RMF with mkacisrmf, while the multiple spectra example used mkrmf. Refer back to the Creating RMFs section for information on how the script decides which tool to use.
If specextract used mkrmf, but you know the calibration is good enough for mkacisrmf, read the Running mkacisrmf Independently section.
Header keywords
The RESPFILE and ANCRFILE header keywords in the source and background spectra have been updated, as well as the BACKFILE in the source spectra only. For example, in the single source output files:
unix% dmlist simple_src1_grp.pi header | grep FILE 0109 BACKFILE simple_bkg1.pi String 0110 CORRFILE none String 0111 RESPFILE simple_src1.wrmf String 0112 ANCRFILE simple_src1.warf String unix% dmlist simple_bkg1.pi header | grep FILE 0111 BACKFILE none String 0112 CORRFILE none String 0113 RESPFILE simple_bkg1.wrmf String 0114 ANCRFILE simple_bkg1.warf String
If stack inputs were used, the source and background file header values are matched up appropriately:
unix% dmlist multi_src1_grp.pi header |grep FILE 0110 BACKFILE multi_bkg1.pi String 0111 CORRFILE none String 0112 RESPFILE multi_src1.wrmf String 0113 ANCRFILE multi_src1.warf String unix% dmlist multi_src2_grp.pi header | grep FILE 0110 BACKFILE multi_bkg2.pi String 0111 CORRFILE none String 0112 RESPFILE multi_src2.wrmf String 0113 ANCRFILE multi_src2.warf String
This means that when the spectra are read into Sherpa, all the supporting files will automatically be read as well; the background (if available) will be defined, as will the source and background response files.
Running mkacisrmf Independently
Users with "good enough" calibration who wish to run mkacisrmf to create new RMF response files should now follow the Using mkacisrmf with the specextract script section of the Creating ACIS RMFs with mkacisrmf thread.
This also applies to users with GRADED mode data on chips S1 or S3 (the back-illuminated chips), as explained in the ACIS GRADED Mode Data section of that thread.
Fitting
If you would like to fit the background-subtracted source spectrum using a common RMF and ARF for source and background, simply read the source spectrum FITS file into Sherpa, subtract the background, and fit it. See the Introduction to Fitting PHA Spectra thread for details.
To fit source and background spectra simultaneously with proper and distinct RMFs and ARFs, load the source and background as different datasets. This procedure is discussed in the Independent Background Responses thread.
Caveats
Using the ACIS "Blank-Sky" Background Files
Subtracting the Background
If you intend to subtract the background spectrum (i.e. not fit it), then you do not need to create a background RMF and ARF. You may simply use dmextract to create the spectrum. In this case, leave the bkgfile parameter blank so that specextract will only create the source spectrum and responses.
Header Keyword Issue
The older ACIS "blank-sky" background files in the CALDB do not work correctly when input to this script "as-is". Users will see a warning of this form:
# mkwarf (CIAO3.3): WARNING: No files found matching CALDB search:
tel=CHANDRA
inst=ACIS
det=-
filt=-
product=FEF_PI
start_date=1998-01-01T00:00:00
start_time=00:00:00
stop_date=1998-01-06T18:53:20
stop_time=18:53:20
query=CTI_CORR.eq.NO
# mkwarf (CIAO3.3): WARNING: No files found matching CALDB search:
tel=CHANDRA
inst=ACIS
det=-
filt=-
product=FEF_PHA
start_date=1998-01-01T00:00:00
start_time=00:00:00
stop_date=1998-01-06T18:53:20
stop_time=18:53:20
query=CTI_CORR.eq.NO
Follow the workaround in the specextract bugs page in order to use these files with the script.
Analysis at the edges of ACIS CCDs
Users should be cautious about analyzing the data for sources near the edges of the ACIS CCDs.
-
For X-rays passing through the mirrors, the very bottom of each CCD is obscured by the frame store. As a result, some of the events in rows with CHIPY <= 8 are not detected. (The set of rows affected varies from CCD to CCD.) Since the CIAO tools do not compensate for this effect, the ARFs and exposure maps for sources in these regions may be inaccurate.
-
For sources within about thirty-two pixels of any edge of a CCD, the source may be dithered off the CCD during part of an observation. The aspect histogram, which is used to create ARFs and exposure maps, is designed to compensate for this effect.
-
A contaminant has accumulated on the optical-blocking filters of the ACIS detectors, as described in the ACIS QE Degradation why topic. Since there is a gradient in the temperature across the filters (the edges are colder), there is a gradient in the amount of material on the filters. (The contaminant is thicker at the edges.) Within about 100 pixels of the outer edges of the ACIS-I and ACIS-S arrays, the gradient is relatively steep. Therefore, the effective low-energy (' 1 keV) detection efficiency may vary within the dither pattern in this region. The ARF and instrument map tools are designed to read a calibration file which describes this spatial dependence.
Parameters for /home/username/cxcds_param/specextract.par
infile = acis_869_evt2.fits[sky=region(simple.reg)] Source events file(s)
outroot = simple Output directory path + root name for output files
(bkgfile = acis_869_evt2.fits[sky=region(bg_simple.reg)]) Background events file(s)
(ptype = PI) PI or PHA
(grouptype = NUM_CTS) Spectrum grouping type (same as grouptype in dmgroup)
(binspec = 15) Spectrum grouping specification (NONE,1:1024:10,etc)
(bkg_grouptype = NONE) Background spectrum grouping type (NONE, BIN, SNR, NUM_BINS, NUM_CTS, or ADAPTIVE)
(bkg_binspec = ) Background spectrum grouping specification (NONE,10,etc)
(energy = 0.3:11.0:0.01) Energy grid
(channel = 1:1024:1) RMF binning attributes
(energy_wmap = 300:2000) Energy Range for WMAPs
(binwmap = det=8) Binning factor for WMAPs
(clobber = no) OK to overwrite existing output file?
(verbose = 0) Debug Level(0-5)
(mode = ql)
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Parameters for /home/username/cxcds_param/specextract.par
infile = @multi_input.lis Source events file(s)
outroot = multi Output directory path + root name for output files
(bkgfile = @multi_bkg.lis) Background events file(s)
(ptype = PI) PI or PHA
(grouptype = BIN) Spectrum grouping type (same as grouptype in dmgroup)
(binspec = 10) Spectrum grouping specification (NONE,1:1024:10,etc)
(bkg_grouptype = NONE) Background spectrum grouping type (NONE, BIN, SNR, NUM_BINS, NUM_CTS, or ADAPTIVE)
(bkg_binspec = ) Background spectrum grouping specification (NONE,10,etc)
(energy = 0.3:11.0:0.01) Energy grid
(channel = 1:1024:1) RMF binning attributes
(energy_wmap = 300:2000) Energy Range for WMAPs
(binwmap = det=8) Binning factor for WMAPs
(clobber = no) OK to overwrite existing output file?
(verbose = 0) Debug Level(0-5)
(mode = ql)
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History
| 01 Feb 2006 | new for CIAO 3.3 |
| 15 Feb 2006 | created Running mkacisrmf Independently section |
| 31 Mar 2006 | specextract use update added to Overview |
| 05 Apr 2006 | In light of the specextract usage change, the thread has been rewritten to use extended sources in the examples |
| 14 Apr 2006 | added Analysis at the edges of ACIS CCDs caveat |
| 24 May 2006 | added new information to Using the ACIS "Blank-Sky" Background Files caveat |
| 14 Jun 2006 | corrected link in "Calibration Updates"; clarified information on GRADED mode data |
