XMM's payload consists of three X-ray telescopes, an optical/UV telescope and two star trackers. Here, the X-ray telescopes can be seen (bottom left)

The name XMM refers to the large number of mirrors - more than any previous X-ray observatory - the satellite will use to capture and focus incoming X-rays. These gold-coated mirrors are shaped like barrels, one inside the other. The smallest mirror is 30.6 cm across and the largest measures 70 cm.

XMM mirrors during reassembly

Each of XMM's three telescopes uses 58 of these 'nested' mirrors. This complex system will make XMM the most sensitive X-ray observatory in the world. It will be able to detect galaxies which lie only a few thousandths of a degree apart in the sky (a full Moon is half a degree across) - equivalent to distinguishing two footballs lying side by side at a distance of 40 km.

XMM's curved nested mirrors will enable it to capture 60 per cent of incoming X-rays

Incoming X-rays will glance off the nested mirrors and be focussed at the far end of a 10- metre-long tube where the detectors and scientific instruments are located. The UK is involved in all three of XMM's science instruments.

The European Photon Imaging Camera (EPIC) will take X-ray pictures of the sky and study the energy of the incoming X-rays. The University of Leicester is leading the team responsible for building the telescope, which also includes the University of Birmingham.

Mullard Space Science Laboratory (MSSL) in Surrey, part of University College London, is helping to build a spectrometer to study the energy and type of X-rays in great detail. It works by spreading the X-rays across an array of detectors. Just as a glass prism bends and separates light into the colours of the rainbow producing a spectrum, so the spectrometer will bend and separate the beam of X-rays according to their wavelength. Each chemical element gives out its own unique set of X-ray wavelengths, so XMM will give astronomers information about which elements are present.

MSSL is also leading the construction of a sensitive conventional telescope called an optical monitor (OM). This will look at the same region of sky as the X-ray telescopes, providing images in visible and ultraviolet light and enabling astronomers to rapidly identify the X-ray sources.

The EPIC camera. The CCDs are in the centre, surrounded by the detector electronics board

The Reflection Grating Spectrometer (RGS) Focal Camera and its black passive radiator (photons come in from the bottom).

The Optical Monitor (OM), with its baffle mounted in front, undergoing tests.

Astronomers at Leicester, MSSL and Cambridge will also play a key role in the XMM Survey Science Centre which is located in Leicester, following up XMM's discoveries. They will compile a list of all the new X-ray sources so that scientists can learn more about them with ground-based optical and radio telescopes.

XMM, which has to be taken apart for transportation, is reassembled at the ESTEC testing site

Since XMM will follow a highly elliptical (oval) orbit, its distance from the Earth will vary from 7,000 km to 114,000 km. Most of its operations will take place near the furthest point of each orbit. This elongated path also means that XMM can stay in touch with ground stations for 40 hours out of every 48-hour circuit.