XMM is the second of ESA's four ``cornerstone'' missions defined in the Horizon 2000 Programme. Its scheduled launch date is 2000 January 21. It will carry two distinct types of telescopes: three Wolter type-1 X-ray telescopes, with different X-ray detectors in their foci (which are described in this chapter), and a 30-cm optical/UV telescope with a microchannel-plate pre-amplified CCD detector in its focal plane (see also below). Thus, XMM offers simultaneous access to two windows of the electromagnetic spectrum: X-ray and optical/UV.
XMM provides the following three types of science instruments:
3 CCD cameras for X-ray imaging, moderate resolution spectroscopy, and X-ray photometry; the two different types of EPIC cameras, MOS and pn, are described in § 3.3. XMM will carry 2 MOS cameras and one pn.
2 essentially identical spectrometers for high-resolution X-ray spectroscopy and spectro-photometry (§ 3.4).
for optical/UV imaging and grism spectroscopy; § 3.5 provides an overview.
The three EPIC cameras and the two detectors of the RGS spectrometers reside in the focal planes of the X-ray telescopes, while the OM has its own telescope. A sketch of the XMM payload is displayed in Fig. 1. There are in total six science instruments onboard XMM, which are operated simultaneously (unless prohibited by constraints, like e.g., excessive target brightness). The instruments can be operated independently and each in different modes of data acquisition. Observers will receive data from all science instruments.