The AXAF CCD imaging spectrometer

The Advanced X–ray Astrophysics Facility (AXAF) CCD Imaging Spectrometer (ACIS) is being developed as a core focal plane instrument for AXAF, the third of NASA’s Great Observatories which is scheduled for launch late in this decade. The instrument consists of an image recording array of charge coupled devices (CCD’s) and a linear arrangement of CCD’s to record the spectra formed by the objective grating spectrometer. Both arrays employ CCD’s with pixel dimensions which correspond to about 0.5 arc second samples of the image. The CCD’s provide moderate spectral resolution (E/ΔE ≈ 45(E(keV/5.9))^1/2) and good detection efficiency over the energy range 0.5 to 10 keV. Spectral resolution of 200 or more is achievable using the objective grating with the grating array. Special clocking modes are planned for the CCD operation which will permit timing of temporal variations for point sources intensities down to 60 µ sec. The detection sensitivity for point sources exceeds 10^-15 ergs/cm²/sec for exposures of 10⁵ seconds. Radiation damage effects have been shown to degrade the energy resolution of CCD’s. Specially designed CCD pixel architecture has been employed together with shielding and low temperature operation to slow the effects of radiation damage. A heater has been included in the focal plane system design to anneal the CCD’s after radiation damage exceeds an acceptable level. The annealing will regain most of the lost performance.