ACIS QE Degradation

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 degradation should be incorporated when creating ACIS response files.

The ACIS QE degradation 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).

Technical Details

Technical information on the contamination is available from the ACIS QE degradation memo.

Additional information on spatial contamination may be found in the ACIS Spatial Contamination Effects memo (PS, 10 pages) and the Spatial structure in the ACIS OBF contamination memo (PDF).

Additional information is also available from:

• 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:

• mkarf
• mkinstmap
• mkgarf
• mkwarf
• mkinstmap

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:

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"