  The flight calibration of the spectral response of the Advanced CCD Imaging
Spectrometer (ACIS) below 1.5 keV is difficult because of the lack of strong
lines in the calibration source on-board the Chandra X-ray Observatory. We have
been using 1E0102.2-7219 (the brightest supernova remnant in the SMC) to
evaluate the ACIS response matrices, since this remnant has strong lines of O,
Ne and Mg below 1.5 keV. The spectrum of 1E0102.2-7219 has been
well-characterized using the gratings on Chandra and XMM. We have used the
high-resolution spectral data from both gratings instruments to develop a
spectral model for the CCD spectra. Fits with this model are sensitive to any
problems with the gain calibration and the spectral redistribution model. We
have fit the data with the latest response matrix for the S3 device released in
August 2001 (available in CALDB 2.7 or higher). We have also applied the
charge-transfer inefficiency (CTI) correction software (version 1.37) developed
at Penn State to both the Backside-illuminated (BI) and Front-side illuminated
(FI) data and fit the data with the associated CTI-corrected response matrices.
All fits show a significant improvement in the low energy response with the new
matrices. The fits to the FI CCDs demonstrate the utility of the FI CCDs for
spectral analysis, in spite of the radiation damage the devices suffered early
in the mission.
