Within the final part of the Observatory, the science instrument module, there are four major components, the most fundamental of which is the structure and mechanical subsystem, which provides a secure mount for the positioning assembly that contains the focal plane science instruments, and which contains the attachments to the X-ray telescope. Complications here include the need for good thermal isolation between the telescope optical bench assembly and the science instrument module, and the need for exclusion of stray light from the X-ray focal plane.
The moving part of the module is the focal plane science instrument positioning assembly. Movement of this assembly in two dimensions places one of the science instruments at the focus of the telescope: the accuracy with which the science instruments can be placed is better than parallel to the telescope axis (i.e., in the direction of varying focus) and better than perpendicular to the telescope axis, along the line separating the focal plane science instruments.
A radiation detector is also contained in the science instrument module, to provide measurements of the local flux of high energy particles. These data back up the stored commands, which will normally be used to turn the focal plane science instruments off as the radiation belts are approached, and will provide an alarm that can be used to power the instruments off during periods of unexpectedly high radiation.
Finally, of course, the science instrument module houses a variety of components from other (spacecraft) subsystems: components that control the thermal environment of the focal plane science instruments (most obviously the ACIS radiator), components that pass commands into the instruments and data from the instruments, etc. One unusual set of components that form a part of the pointing control and aspect determination subsystem is the fiducial light assembly, which provides a set of reference lights, attached to the fronts of the science instruments. These lights are viewed by the aspect camera through a periscope system, so that the motions of the science instruments can be measured relative to the fundamental reference frame defined by guide stars.