X-ray vision probes the outer limits

The XMM satellite, due for launch next week, will give astronomers a more detailed view of the universe than ever before. Mark Venables looks at the key role played by UK scientists in developing its X-ray and optical instruments

When the Ariane 5 launcher lifts off on 10 December from the European space centre in French Guiana it will be carrying an important new cargo – the X-ray Multi Mirror satellite (XMM).

XMM is the largest scientific satellite to be built in Europe. Its telescope mirrors are the most powerful ever developed, and its sensitive detectors will see more than any previous X-ray satellite.

By allowing scientists to view the Universe through X-rays, XMM will allow them to see further and in more detail, as X-rays are less prone to scattering than visible light. Scientists will also be able to study objects too hot for optical telescopes, and even to watch material being sucked into black holes.

As objects go beyond white-heat, the amount of visible light they emit reduces, explains Dr Martin Turner of Leicester University, who led the team developing the satellite’s X-ray camera. `They become so hot that they emit almost nothing in the visible region, so they are almost invisible to optical instruments. What can seem to be a faint star to an optical telescope can appear enormously bright to an X-ray telescope.’

The 3.9-tonne orbital observatory will follow an elliptical orbit, its distance from the earth varying from 7,000km to 114,000km. This elongated orbit will allow XMM to stay in contact with Earth for 40 hours out of each 48-hour orbit.

The Particle Physics and Astronomy Research Council, the UK’s strategic science investment agency, has put more than £20m into the project. British scientists led the development of two of the three instruments on board: the X-ray camera (the European photon imaging camera, or Epic); and the optical monitor. They were also heavily involved in the development of the telescope’s spectrometer.

XMM contains 120m2 of gold coated mirrors – enough to cover a tennis court. They are shaped like barrels and are nested one inside the other in sizes ranging from 30.6cm to 70cm. Incoming X-rays glance off the mirrors and are focused at the far end of three 10m tubes, where three cameras plus detectors and recording equipment are located.

The camera is fitted with a filter wheel which allows the operator to position one of six filters in the beam from the mirrors to filter out the optical wavelengths and let different X-ray frequency bands through.

The University of Leicester provided the team responsible for building the cameras. These are charge-coupled device cameras, based on the same technology used in camcorders and digital cameras. Special X-ray sensitive CCDs were developed by the university, working with electronic and optical equipment manufacturer EEV.

Turner, team leader for the Epic project, says: `For every photon that arrives we can measure the wavelength.’ From the X-rays the camera produces an image with the shape of the object, and translates different X-ray wavelengths into colours corresponding to different chemical elements.

To operate effectively the CCDs have to be cooled to a temperature of -100 degrees c. This is achieved by means of a three-stage passive radiator. The CCDs are connected via a `cold finger’ to a radiator on the dark side of the satellite. The cameras also have to be protected against radiation, says Turner, so are guarded by a 3cm aluminium shield.

Mullard Space Science Laboratory (MSSL) in Surrey, which is part of University College London, is helping build the spectrometer to study the energy and type of the 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 in the visible spectrum, so the spectrometer separates the beam of X-rays.

Each chemical element emits a unique set of X-ray wave lengths, so XMM will supply astronomers with information about the elements present in the objects being studied.

MSSL also led the construction of the optical monitor, a sensitive conventional telescope. It will be used to look at the same region of the sky as the X-ray telescopes, providing images in visible and ultraviolet light.

Professor Keith Mason of MSSL, principal investigator for the optical monitor project, says: `The optical monitor is a small UV telescope of fairly classical design which sits alongside the X-ray telescopes, extending their view into the optical and UV regimes.

`This enables us to look at sources in X-rays, optical and UV all at the same time. `It gives us a frame of reference so that any time we see something new we can relate it back to something we more readily understand – an optical image.’

Because the main instrumentation is the X-ray array, the optical monitor has been forced to perform in an environment that is far from ideal. `Optical satellites such as the Hubble Space Telescope are engineered to be rock solid,’ Mason says. `XMM is much cheaper. It doesn’t have to be as stable as that for the X-ray telescope to work. However, for the optical monitor we need an equivalent stability to that of the Hubble.’

To overcome this problem, an image correction system was developed. `We can’t afford to throw photons away – we don’t have many of them,’ says Mason. `So we count every one we detect, calculate how much the spacecraft has moved since the last time we counted and then electronically correct the image to compensate for the movement of the spacecraft.’

UK industry has also had a big involvement in the project. Matra Marconi Space at Stevenage designed and manufactured the attitude and orbit control subsystem and Wolverhampton-based Dowty Aerospace designed and manufactured an all-titanium propellant tank for the satellite.

Derek Hancock of Dowty Aerospace says: `We had to develop a sophisticated propellant management device which operated under the various operating mission scenarios.’

Scientists are extremely excited about XMM’s approaching launch. The extensive programme of astronomical research planned for the satellite will include studying exotic objects such as quasars, the most luminous objects in the universe, which are about the same size as our solar system.

`We have been working on this for the past decade,’ says Mason. `It is hard to believe that it is finally going to happen.’