ACIS Calibration Issues in Progress

                Richard J. Edgar
                edgar@head-cfa.harvard.edu

• XMM/Chandra cross calibration presentation presented by Steve Snowden

• Calibration Review, Oct 30-31, 2001
  

The response of the Back Illuminated (BI) chips is being reworked.

• We expect to release new BI FITS Encoded Function files (FEFs, from which the mkrmf tool makes response matrices) in July 2001. These include fits to the monoenergetic pulse-height distributions including up to ten Gaussian components (vice 3 in the CALDB 2.0 release), which allows us to model the low pulse-height tails of the distributions. Fits to astrophysical data are much improved at low energies, and fit parameters such as temperatures, absorbing column densities, etc. are more physical with the new products.
• S3 products for -120 C will be the highest priority, followed closely by S3 products for -110 C, and then S1 matrices at both temperatures.
• Several bugs in the process are being investigated as of this writing.

The January 2000 (CALDB 2.0) release of the BI matrices has systematic problems with its energy scale and line widths. See discussions here for the energy scale, and here for the line widths.

Quantum Efficiency. There is some evidence (ref: Herman Marshall's page http://space.mit.edu/ASC/calib/letg_acis/ck_cal.html) that the S3/S2 sensitivity ratio is larger than we have advertised by 10-20% in the energy regime below 1 or 1.2 keV. We are investigating this by several methods, including flight grating spectra of multiple AGN and QSO targets, re-analysis of low-energy ground calibration data, and fitting of spectra from other flight targets (notably hot clusters of galaxies). It's difficult to disentangle this effect from the (to date poorly modelled) asymmetries in the line shape function at low energies. Re-analysis of ground calibration data will be required to decide which (if either) of the QE curves is the correct one. This is an ongoing topic of debate within the calibration group and the PI team, and may not be resolved until late 2001 (or later).

Front Illuminated (FI) chip response, and CTI correction

• A Charge-Transfer-Inefficiency (CTI) corrector is being developed by the CXC data system. This will allow the user to correct, on the ground, data taken with ACIS, to recover a portion of the pre-flight performance of the instrument. New response products will be needed to use with these corrected data.
• Starting July 2001, an intensive effort will be underway to produce response products for both CTI-corrected and uncorrected data from the FI chips of ACIS. We expect to release these products in late 2001.
• The released (CALDB 2.0) response products model lines in ACIS as Gaussians which are substantially narrower than the data in some cases. We have posted a caveat message about this here: http://asc.harvard.edu/ciao2.1/caveats/specres.html

Also in work:

• Make a Bad Pixel list for -110 C
• Rework web page to make it easier to find descriptions of CALDB products and files.

Here are the viewgraphs presented to the Chandra Users' Committee on 2001-JUN-29. http:CUC_2001_06_29.ps

Last revised: Thu Aug 2 10:29:02 EDT 2001