ACIS QE Contamination
Introduction
The effective low-energy ACIS QE is lower now than it was at launch. This problem is thought to be associated with the deposition of one or more materials on the ACIS detectors or optical blocking filters. Since the depth of these contaminants is growing with time, the effective low-energy QE is becoming lower as time passes. A correction for this contamination is incorporated when creating ACIS response files.
The ACIS QE contamination model also accounts for spatial variations in the contamination on the ACIS optical blocking filters. The contamination is expressed as a function of time, energy, and ACIS chip coordinate. For imaging analysis of extended sources or point sources far off-axis, there is a significant change in instrument and exposure maps when the calibration is applied.
The response tools are designed to incorporate corrections for ACIS contamination via ARDLIB and a CALDB contamination file. The necessary calibration files have been available since CALDB 2.26 (2 February 2004) and was most recently updated in CALDB 4.4.10 (30 May 2012).
The version N0007 upgrade, released in CALDB 4.4.10, includes a ~15% change in the optical depth at 700 eV for observations in 2012 as compared with those in early 2009. To learn about previous updates to the contamination model, click here.
Technical Details
As the Chandra mission proceeds, the contamination on the ACIS Optical Blocking Filters for ACIS-I and ACIS-S continues to evolve, with a marked increase in the rate of accumulation during the year 2009. Now that enough time has passed since this change, a quantitative correction may be made to the ACIS contamination modeling file (CONTAM) in CalDB. In particular there are updates to the time-dependence of the contaminant at the center region of ACIS-I and ACIS-S (Fig. 1), as well as significant changes to the time-dependence of the spatial component, since 2009 (Fig. 2).
Fig. 1: Updated time-dependence in the center of both ACIS-I and S to include the latest A1795 measurements. Black points are the ECS L/K measurements, the blue points are ACIS-S and the red ones are ACIS-I. The solid line is the fit in the latest N0007 model, the dotted and dashed lines are the time dependencies with CONTAM version N0006 for ACIS-S and -I, respectively.
Fig. 2: Updated time-dependence of the contaminant spatial pattern. The same keys apply as in Fig. 1 above. Abell 1795 measurements show that the time evolution of the spatial pattern in I and S is consistent, and that there is a very quick rise since 2009.
The spatial pattern for ACIS-I has been updated from an azimuthally-symmetric pattern to a hyperbolic fit. For ACIS-S, the polynomial fit has been replaced by an exponential one.
Additional technical information is available from:
- The Update to the ACIS Contamination Model memo (8 January 2010)
- The Contamination on the ACIS OBF and Changes in the Low Energy QE page
- The ACIS Spatial Contamination Effects memo (19 January 2005) (PDF)
- The Spatial structure in the ACIS OBF contamination memo (20 April 2004) (PDF)
-
The Composition
of the Chandra ACIS contaminant paper (August 2003)
H. L. Marshall, A. Tennant, C. E. Grant, A. P. Hitchcock, S. O'Dell, P. P. Plucinsky
Applying the Correction
The following CIAO response tools automatically take the contamination into account:
As well as the scripts which use them:
- specextract (calls mkwarf and mkarf)
- fullgarf (calls mkgarf)
- fluximage (calls mkinstmap)
- merge_all (calls mkinstmap)
Each of the tools contains an ardlibparfile parameter with the value"ardlib.par." The location of the calibration file is specified in the ardlib.par file by a set of 10 parameters (one per CCD):
unix% plist ardlib | grep CONTAM AXAF_ACIS0_CONTAM_FILE = CALDB Enter ACIS Contamination File AXAF_ACIS1_CONTAM_FILE = CALDB Enter ACIS Contamination File AXAF_ACIS2_CONTAM_FILE = CALDB Enter ACIS Contamination File AXAF_ACIS3_CONTAM_FILE = CALDB Enter ACIS Contamination File AXAF_ACIS4_CONTAM_FILE = CALDB Enter ACIS Contamination File AXAF_ACIS5_CONTAM_FILE = CALDB Enter ACIS Contamination File AXAF_ACIS6_CONTAM_FILE = CALDB Enter ACIS Contamination File AXAF_ACIS7_CONTAM_FILE = CALDB Enter ACIS Contamination File AXAF_ACIS8_CONTAM_FILE = CALDB Enter ACIS Contamination File AXAF_ACIS9_CONTAM_FILE = CALDB Enter ACIS Contamination File
If anything other than "CALDB" is returned, issue the following command so that the tool will be able to find the correct file:
unix% foreach d ( 0 1 2 3 4 5 6 7 8 9 )
foreach? pset ardlib AXAF_ACIS${d}_CONTAM_FILE="CALDB"
foreach? end
You may also use "punlearn ardlib" to reset all the ardlib parameters to the default values. This will also clear out any other information that has been set, however, such as bad pixel filenames.
Turning Off the Correction
It is possible to "turn off" the contamination correction, e.g. if you would like to compare results with and without it applied. To do so, the ARDLIB qualifier "CONTAM=NO" must be specified in the appropriate parameter, as given in the following table:
| Tool | Parameter |
|---|---|
| mkarf | detsubsys |
| mkgarf | detsubsys |
| mkwarf | detsubsysmod |
| mkinstmap | detsubsys |
There are examples in the help files on how to use the qualifier with each tool. For example, when running mkarf on an ACIS-S3 observation:
unix% pset mkarf detsubsys="ACIS-S3;CONTAM=NO"
Examining the Effects of the Correction
Comparing ARFs using Old vs. New Contamination Model
It is useful to compare ARF responses created using an older (4.4.8 or earlier) versus the newest contamination model (4.4.10) to examine the effects of the correction. One could do this by following the procedure below, which uses ACIS-S imaging observation 11800, taken in July 2010, as an example. The CIAO tool mkwarf is used to create the old and new on-axis ARF responses, and the comparison of the two ARFs is done in ChIPS.
1) Find out how 'old.arf' was created using dmhistory, which will show that it was either mkarf or mkwarf (mkwarf in this example).
% dmhistory infile=old.arf tool= # dmhistory (CIAO 4.4): WARNING: Found "pixlib" library parameters # dmhistory (CIAO 4.4): WARNING: Found "ardlib" library parameters TOOL :mkwarf infile="11800_tdet.fits[wmap]" outfile="old.arf" weightfile="11800.wfef" spectrumfile="" egridspec="0.3:11.0:0.01" pbkfile="pbk0.fits" threshold="0" feffile="CALDB" mskfile="msk1.fits" asolfile="" mirror="HRMA" detsubsysmod="" dafile="CALDB" ardlibpar="ardlib" geompar="geom" clobber="no" verbose="1"
2) Re-run dmhistory, but this time, updating the mkwarf parameter file with the parameter settings returned in step 1. Check that the mkwarf parameter file was properly set by using the 'plist' command.
% dmhistory infile=old.arf tool=mkwarf action=pset
# dmhistory (CIAO 4.4): WARNING: Found "pixlib" library parameters
# dmhistory (CIAO 4.4): WARNING: Found "ardlib" library parameters
% plist mkwarf
Parameters for /home/user/cxcds_param4/mkwarf.par
infile = 11800_tdet.fits[wmap] Input detector WMAP
outfile = old.arf Output weighted ARF file
weightfile = 11800.wfef Output FEF weights
spectrumfile = Input Spectral weighting file (<filename>|NONE)
egridspec = 0.3:11.0:0.01 Output energy grid [kev]
pbkfile = pbk0.fits Parameter block file
(threshold = 0) Percent threshold cut for FEF regions
(feffile = CALDB) FEF file
(mskfile = msk1.fits) Mask file
(asolfile = ) Stack of aspect solution files
(mirror = HRMA) ARDLIB Mirror specification
(detsubsysmod = ) Detector sybsystem modifier
(dafile = CALDB) Dead area file
(ardlibpar = ardlib) Parameter file for ARDLIB files
(geompar = geom) Parameter file for Pixlib Geometry files
(clobber = no) Clobber existing outputs
(verbose = 1) Tool chatter level
(mode = ql)
3) After updating the CALDB to the latest version with the most recent contamination model, re-run mkwarf with these parameter settings, except for changing the outfile name to 'new.arf'. (Note that you may need to change directories or adjust the file paths depending on how things were run to create old.arf.)
% mkwarf outfile=new.arf
4) Compare the old and new ARFs by plotting them together in ChIPS. For this dataset, the old and new ARFs are shown in Fig. 3, which was created using the ChIPS commands shown below.
Fig. 3: An on-axis ARF response created using older CALDB version 4.4.8 (black curve) for an ACIS-S observation taken in July 2010, compared to that produced with the updated contamination model included in CALDB 4.4.10 (red curve).
% chips -n
chips-1> ocr = read_file("old.arf")
chips-2> elo = get_colvals(ocr, "energ_lo")
chips-3> ehi = get_colvals(ocr, "energ_hi")
chips-4> oy = get_colvals(ocr, "specresp")
chips-5> add_window(10, 8, "inches")
chips-6> add_histogram(elo, ehi, oy)
chips-7> set_plot_ylabel("Area (cm^2)")
chips-8> ncr = read_file("new.arf")
chips-9> ny = get_colvals(ncr, "specresp")
chips-10> add_histogram(elo, ehi, ny, ["*.color", "red"])
chips-11> split(2)
chips-12> adjust_grid_yrelsize(2, 0.5)
chips-13> add_histogram(elo, ehi, ny/oy)
chips-14> set_plot_xlabel("Energy (keV)")
chips-15> set_plot_ylabel("New / Old")
chips-16> limits(Y_AXIS, 0.6, 1.05)
chips-17> current_plot("plot1")
chips-18> obsdate = ocr.get_key_value('DATE-OBS')
chips-19> ptitle = "{}: CALDB 4.4.10 (red) CALDB 4.4.8 (black)".format(obsdate[:10])
chips-20> set_plot_title(ptitle)
chips-21> set_plot(['title.size', 24])
chips-22> bind_axes('plot1', 'ax1', 'plot2', 'ax1')
chips-23> log_scale()
chips-24> limits(Y_AXIS, 10, AUTO)
chips-25> limits(X_AXIS, 0.3, 3)
chips-26> set_xaxis('all', ['ticklabel.visible', False, 'minortick.visible', False])
chips-27> current_plot('plot2')
chips-28> x = [0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3]
chips-29> xl = ["0.3", "", "0.5", "", "", "", "", "1", "2", "3"]
chips-30> set_xaxis('all', ['minortick.visible', False])
chips-31> set_arbitrary_tick_positions('ax1', x, xl)
chips-32> current_plot('all')
chips-33> set_axis(['ticklabel.size', 16, 'label.size', 20])
