Over the past year, much work has been done on the shape of the pulse height response for the ACIS devices, most notably S3. This led to the release of new Fits Encoded Function (FEF) files, which encode the information necessary to create response matrix files (RMFs), in August 2001. These files pertain to the S3 chip at a focal plane temperature of -120 C, and are applicable to the large majority of S3 data. We are working toward release of S3 response matrix products at a focal plane temperature of -110 C (used operationally up to January 2000), and S1 response products (mostly of interest for HETG purposes).
There are still some residual deficiencies in these response matrix products. Notably, the gain seems to be acceptable down to an energy of about 0.7 keV, with remaining problems at softer energies. It was also determined that the gain of the S3 device is time-dependent, as it degrades at the expected rate due to radiation damage. Other small problems are being reported by users, and we are investigating them as they come in.
The CXC, in collaboration with the Penn State (PSU) and MIT instrument teams, has developed software (to be released as an optional part of the CIAO acis_process_events tool) to correct some of the effects of Charge Transfer Inefficiency (CTI) in the front-illuminated devices (currently the ACIS-I array and the S2 chip). The use of this corrector will require new response matrix products, and work has begun on their development. The MIT event simulator is run to produce of order 108simulated events, which are sorted by energy and chip location, and then generic functions are fit to the pulse-height distributions. The coefficients of these fits are distributed in the FEF files. The results are then compared to on-board calibration source observations (which we make once or twice per orbit, on entry into and exit from the radiation belts), and observations of other sources such as the soft, oxygen-rich, line-dominated supernova remnant 1E0102-72.3. We anticipate a release of these products in the summer of 2002. In the mean time, Leisa Townsley of PSU has kindly released her version of the CTI corrector and response matrices on the CXC Shared Software page:
http://cxc.harvard.edu/cgi-gen/cont-soft/soft-list.cgi
In collaboration with members of the XMM-Newton Guest Observer Facility (notably Steve Snowden), we have compared the response of the CCD cameras on the two observatories, for three sources with different types of spectra. These sources are the supernova remnant 1E0102-72.3, a soft, line-dominated thermal spectrum; the Crab-like supernova remnant G21.5-0.9, a hard, absorbed powerlaw spectrum; and the cluster MS1054.4-0321, which has a high-temperature absorbed thermal spectrum. Analysis to date shows that the fluxes derived by the two observatories match at the 5% level, though in some cases error ellipses for various fitted parameters do not overlap. Work continues on the relative calibration of these two important observatories. A presentation on the relative calibration, given by Steve Snowden at an XMM meeting at ESTEC in November 2001, is available in the Current Issues section of the CXC ACIS Calibration Web Page:
http://cxc.harvard.edu/cal/Acis/Presentations/cross_cal/wa2-snowden/index.html
The observatory successfully completed two challenging observations of the dark moon in 2001, one on the ACIS S-array (chips S2 and S3), and the other on the ACIS I-array (chips I2 and I3). A preliminary analysis of these data shows remarkable consistency with histogram-mode data taken when the HRC-I is in the focal position. This low bandwidth ACIS mode produces pulse-height spectra with no true sky-looking X-ray events, giving us an excellent source of background data. Both of these types of background spectra show somewhat lower rates than the sky-looking, point-source-removed, background event lists provided by the CXC. The differences are believed to be due to the diffuse x-ray background, which is thought to arise from thermal emission of hot gas in the Galaxy, and from unresolved point sources. Memos on backgrounds are posted on the CXC web pages.
http://cxc.harvard.edu/cal/Acis/Cal_prods/bkgrnd/current/background.html
We have made an effort to release progress reports to the community. In particular, copies of calibration group reports to the Chandra Users' Committee, and the calibration review held in October 2001, are available on the CXC Calibration Group web pages:
http://cxc.harvard.edu/cal/Acis/Cal_projects/index.html
ACIS continues to operate smoothly. We continue to obtain a great deal
of on-board calibration source data, obtained when the observatory is
in the near-earth radiation belts, used to monitor the performance of
the ACIS devices. The observatory has been shut down a number of times
by solar storms, which produce periods of high radiation at the
observatory. The Parallel CTI degradation continues at the expected
rate, which is 4.4 
0.1 x 10-6 yr-1 (3.5%)
for I-array, and 1.45
0.02 x 10-6 yr-1 (10.3%) for S3.
- Richard Edgar, on behalf of the extended ACIS Instrument, Operations, and Calibration Teams