ACIS QE Contamination

Introduction | Technical Details | Applying the Correction | Turning Off the Correction

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 should be 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 were updated in CALDB 3.0.0 (15 December 2004) and CALDB 4.2.0 (15 December 2009).

Technical Details

The version N0005 upgrade to the ACIS QE contamination model, released in CALDB 4.2.0 (15 December 2009) contains separate components for the correction on ACIS-S and ACIS-I.

The changes in the vN0005 file will primarily affect low-energy absorption components between C-K edge (0.283 keV) and 1.2 keV. In general, hard spectra will not be affected much by the calibration change.

The following is a summary of the changes for ACIS-S and ACIS-I. For complete technical details and additional plots, refer to the "How CIAO 4.2 and CALDB 4.2.0 Affect Your Analysis" section of the release notes and the memo on the update to the contamination model from the CXC Calibration team.

ACIS-S

All ACIS-S observations taken after 2005 should use the vN0005 file to create the response files.

There was an acceleration in the QE degredation, illustrated in the following figure. The plot shows the difference in the measured optical depth between the top and center of ACIS-S3 (red data points) and between the bottom and center of ACIS-S3 (blue data points). The solid line is the model used for version N0005 of the ACIS-S contamination model.

Model used for the vN0005 of the ACIS-S contamination model.

The major changes in this new ACIS-S model are:

an adjustment in the time-dependence to account for the accelerated build-up of contaminant on ACIS-S3 relative to extrapolations from the vN0004 file, and
a new empirical determination of the extra absorption component, derived from two ACIS-S3 spectra of the Coma cluster (one taken in 1999, and the other in 2009).

ACIS-I

The ACIS-I model change will affect analyses done below 1 keV. ACIS-I does not experience the accelerated build-up of contamination in 2005 as with ACIS-S. However, the new file has adjusted C-O and C-F ratios for all mission times, and hence has a minor affect for all ACIS-I observations relative to the N0004 file.

The best-applicable model to ACIS-I uses separate analytic functions to model the time-dependence of the elemental and two-level components. The elemental ratios in this new model have been modified from the N0004 file, and are based on the average ratios measured over the course of the mission from gratings data.

ACIS-I3 aimpoint effective area: the dashed curves are with the old N0004 CONTAM file and the solid ones are derived using the new N0005 CONTAM file. The black curve is the EA without any CONTAM effect. The variations between the two CONTAM files are rather subtle, giving at most several percent differences between any two corresponding curves.

Additional technical information is available from:

The ACIS QE contamination memos
The ACIS Spatial Contamination Effects memo (PS)
The Spatial structure in the ACIS OBF contamination memo (PDF)
Composition of the Chandra ACIS contaminant (PS, 12 pages)
H. L. Marshall, A. Tennant, C. E. Grant, A. P. Hitchcock, S. O'Dell, P. P. Plucinsky
astro-ph/0308332

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)
psextract (calls mkarf)
acisspec (calls mkwarf)
fullgarf (calls mkgarf)
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"