The pointing and alignment of XMM.

XMM’s optical system The exploded view of the XMM spacecraft shown in Figure 1 identifies the main elements of the optical system*. Three Mirror Modules (MM-1, 2 and 3), equipped with two Reflecting Grating Assemblies (RGA-1 and 2), and the self-contained Optical Monitor (OM) are mounted at the heart of the spacecraft. The optical system is completed by five Charge Coupled Device (CCD) detector cameras, of which three (EPCH and EMCH-2 and 3) are placed at the ‘primary focus’ of the respective Mirror Modules. The remaining two cameras (RFC-1 and 2) are suitably positioned at a ‘secondary focus’ on the imaginary ‘Rowland circle’, where the spectrum created by the two grating assemblies can be collected. Requirements and goals Pointing The pointing requirements originate from the scientists and have been thoroughly defined and negotiated between the Project and the instrument Principal Investigators early in XMM’s design phase (Phase-B). Since a stringent pointing requirement is a direct cost driver, it was decided to split the scientists’ demands into: