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INFO 15-1997: XMM telescope goes on show for the first time

INFO 15-1997: XMM telescope goes on show for the first time

13 May 1997

An assembly of 58 mirrors, carefully sized, formed and nested one inside another, makes the most sensitive X-ray telescope ever built. ESA's new satellite called XMM, for X-ray Multi-Mirror, will carry three identical telescopes of this kind when it goes into orbit in 1999. With its gold- coated reflecting surfaces totalling 300 square metres, XMM will revolutionize X-ray astronomy. Observations of X-rays from cosmic sources that previously took hours to accomplish will be done by XMM in a matter of seconds.

After years of little-publicized effort in various parts of Europe, one of the finished XMM telescopes is presented for the first time to the press and other visitors during a press briefing on 22 May at the Centre Spatial de Liège (CSL) in Belgium.

As the telescope is a flight model due to go into space, the inspection is hands-off, through a clean-room window. But visitors can see the multi-mirror module looking like a log of wood 70 centimetres wide and 60 centimetres long. In cross-section the nesting mirrors make concentric rings resembling the rings of annual growth in a tree - except that there are narrow gaps between the mirrors, to let in the X-rays.

Galileo and Newton would not recognize the XMM module as a telescope, but they never had to focus X-rays, which ignore lenses and ordinary mirrors. The German physicist Hans Wolter invented a near-cylindrical mirror that narrows in a geometric fashion, first along a parabola and then a hyperbola. Some of the X-rays entering through the wider end are twice reflected from the interior wall at grazing angles, and go to a focus.

When Wolter's concept went into telescopes for X-ray astronomy, most of the X-rays simply missed the mirror, so that any signal was weak. The remedy was to fill the interior with smaller concentric mirrors, to catch more of the X-rays. In practice, that meant either a few mirrors, fashioned and aligned with care to achieve a high resolving power, or many cruder mirrors which sacrificed optical sharpness for the sake of sensitivity. The XMM telescopes are the first to achieve both sharpness and sensitivity, thanks to innovations by European industry made possible by XMM's status as a Cornerstone Mission in ESA's science programme.

Carl Zeiss in Germany provided mandrels (moulds) of 58 sizes, all carefully polished to the required paraboloidal-hyperboloidal shape. Media Lario in Italy made the reflective surface by depositing gold on the mandrel under vacuum, and then backed it with nickel formed in an electrochemical bath. As each finished mirror was only about a millimetre thick, the Medio Lario team had to handle it with great care to avoid flexing, until it was glued into position among all the other mirrors, between wheel-like "spiders" fabricated by APCO in Switzerland.

If the telescope is correctly made, all X-rays coming from a certain direction, and entering any part of any of the mirrors, should go to the same focus. The specification requires that any spread at the focus should be less than a millimetre. The optical quality is tested first in a special apparatus at Liège called FOCAL X, and then at the Panter X-ray facility at Neuried in Germany.

"We take pride and satisfaction in helping to develop such remarkable telescopes," says Claude Jamar, director of CSL. "While others pioneered the methods of fabrication, here in Liège we had to invent novel ways of checking the performance. We use a wide beam of very short ultraviolet wavelengths to simulate X-rays, and verify the focus of each part of each XMM telescope."

About the Centre Spatial de Liège

As a laboratory unique in western Europe, CSL is run by the Universit de Liège as one of ESA's coordinated test facilities.Optical instruments for space missions can be checked with high accuracy, under a high vacuum that simulates the airless conditions in space.

ESA relies upon CSL for testing important optical components for many spacecraft. The long list includes the radiometer of the Meteosat weather satellite, the camera for Giotto which obtained unique pictures of Halley's Comet, and the telescopes of the Hipparcos star-fixing mission and the Infrared Space Observatory ISO.

CSL was an early recruit to the Europe-wide teams of scientists and engineers who are creating the XMM spacecraft and its instruments. Other optical devices currently under evaluation by CSL include the experimental laser system SILEX for communication between satellites, and the ozone-monitoring GOMOS instrument for ESA's environmental satellite Envisat.

Last Update: 1 September 2019
30-Nov-2021 18:24 UT

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