CXO has two focal plane instruments - the High-Resolution Camera (HRC) and the Advanced CCD Imaging Spectrometer (ACIS). Each of these instruments, in turn, has two detectors, one optimized for direct imaging of x rays that pass through the optics and the other optimized for imaging x rays that are dispersed by the objective transmission gratings when the latter are commanded into place directly behind the HRMA. Each focal-plane detector operates in essentially photon counting mode and has low internal background. A slide mechanism is utilized to place the appropriate instrument at the focus of the telescope. Provision for focus adjustment is also present.