XMM-Newton Users Handbook

3.2 X-ray Telescopes

XMM-Newton's three X-ray telescopes are co-aligned with a relative astrometry between the three EPIC cameras calibrated to better than 1-2" across the full FOV. One telescope has a light path as shown in Fig. 2; the two others have grating assemblies in their light paths, diffracting part of the incoming radiation onto their secondary focus.

Figure 2: The light path in XMM-Newton's open X-ray telescope with the pn camera in focus (not to scale).

Figure 3: The light path in the two XMM-Newton telescopes with grating assemblies (not to scale). Note that the actual fraction of the non-intercepted light that passes to the primary MOS focus is 44%, while 40% of the incident light is intercepted by grating plates of the RGA.

Fig. 3 shows the light path in the latter two X-ray telescopes on board XMM-Newton. Ca. 44% of the incoming light focused by the multi-shell grazing incidence mirrors is directed onto the camera at the prime focus, while 40% of the radiation is dispersed by a grating array onto a linear strip of CCDs. The remaining light is absorbed by the support structures of the RGAs.

The focal instruments are described in §§ 3.3, 3.4 and 3.5. First we explain the most important properties of the mirror assemblies.

The performance of each X-ray telescope is characterised by:

1. the image quality,

2. the effective area, and

3. the straylight rejection efficiency,

which will be described in the following sections.