With the cancellation of AXAF-S, some may have the impression that AXAF has lost all of its spectroscopic capabilities. This is not the case. AXAF-I will carry two sets of gratings; the High Energy Transmission Gratings (HETG, Claude Canizares, Principal Investigator), normally read out with the AXAF CCD Imaging Spectrometer (ACIS, Gordon Garmire, Principal Investigator), and the Low Energy Transmission Gratings (LETG, Albert Brinkman, Principal Investigator), normally read out with the High Resolution Camera (HRC, Stephen Murray, Principal Investigator). In addition, ACIS itself is a spectrometer, albeit of lower resolving power, and is the spectrometer of choice for most extended or faint sources.
The HRC is primarily an imaging instrument and will have limited intrinsic spectroscopic capability. The microchannel plates which make up its sensing elements have energy resolution poorer than that of a proportional counter. However, even with this limited resolution, two-color photometry similar to that performed with the High Resolution Imager on ROSAT should be possible. The HRC, however, does serve a significant role in AXAF spectroscopy by acting as the primary readout for the LETG and as the secondary readout for the HETG. The HRC-S, one of two detector assemblies within the HRC, is designed specifically as a spectroscopy readout. The HRC-S is comprised of three microchannel plate and crossed-grid charge-detector segments which roughly match the Rowland circle of the transmission gratings. The filters that lie in front of the microchannel plates and shield them from ultraviolet light and ions are designed to provide useful sensitivity to X-rays with energies less than 0.1 keV (wavelengths longer than 124 ). The photocathode coating of the microchannel plates, KBr, was selected to boost the detection efficiency at low energies.
The spectroscopic capabilities of the gratings and ACIS are described in more detail below -- see Spectroscopy with the AXAF Gratings and Spectroscopy with ACIS.