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New optics eye COSMOS
Europe's largest-ever science satellite will harness grazing-incidence telescope technology to gather deep-space x-rays
By Roy O'Connor -- Design News, September 7, 1998
Dietzenbach, Germany--Like sirens in the sky, black holes, quasars, and vampire stars beckon modern astronomers. These celestial objects, often undetectable in the visible spectrum, leave telltale x-ray signatures--clues to the origins of the universe.
Scientists will be sifting through the first of these clues sometime after August 1999, if plans at the European Space Agency (ESA) go according to schedule. That's the date the ESA will launch its x-ray Multi-Mirror (XMM) satellite. Prime contractor Dornier Satellitensysteme GmbH (Daimler-Benz Aerospace) and 35 other companies from 14 countries are participating in the project.
Expected to observe more than 1 million x-ray sources and to perform detailed spectroscopy on perhaps 30,000 of these over its 10-year lifespan, the XMM satellite features a unique optical, as well as structural, layout.
Three-in-one telescope. One of the fundamental problems with astronomical telescopes is that detection of distant objects demands a large-diameter opening to gather enough light, or x-radiation. Because focal length increases with diameter, a telescope can become too long for a satellite-borne application.
The XMM satellite, consequently, carries three grazing-incidence, x-ray telescope modules. Each consists of 58 concentric, co-focused mirrors with a focal length of 7.5m. Processing combines x-ray images from the separate mirror modules.
Because the x-rays to be detected exhibit wavelengths of less than 0.1 nanometer, mirror surfaces must be extremely precise. To save weight, the mirrors are also very thin and nest closely together. Spacing between 1 and 2 mm focuses the maximum amount of x-rays.
Mirror production calls for close cooperation between contracting companies Carl Zeiss (Oberkochen, Germany) and Media Lario (Bosisio Parini, Italy). Zeiss manufactures the aluminum mandrels on which the double-conical-shaped mirrors are deposited. These mandrels, or molds, are first coated with a 0.1-mm layer of "electroless" nickel, a form of nickel that can be plated by purely chemical means without electric current. The coating is hard, homogeneous, and resistive to oxidation.
After coating, mandrel lapping, and polishing occur. Wilhelm Egle, Zeiss's project manager, points to the fundamental challenges in this type of work: "The first problem is to produce surfaces as large as 1.5 sq m with a uniform roughness of less than 0.4 nanometer. The second problem is to measure it."
Zeiss therefore supplies a range of instruments to ESA/Media Lario for mandrel and mirror measurement, including a special microscope system with an amplitude accuracy better than 0.1 nanometer. "It's the ultra-precision finish on the mirrors that produces the high-contrast x-ray images needed by astronomers," Egle says.
Italy's Media Lario produces the actual mirrors. Managing Director Arnoldo Valenzuela explains the procedure: Each mandrel first receives a 0.25-micron-thick reflective coating of gold in a vapor-deposition chamber. Then an electroforming process applies a backing layer of nickel on top of the gold. Cooling with liquid nitrogen exploits the different thermal expansion properties of the aluminum mandrel and the nickel mirror to separate the tube-like mirror from its mandrel.
Strong but lightweight tube. To obtain high-quality x-ray images, mirror modules must be kept rock steady in relation to detectors in the focal plane. Stiffness and resonance characteristics of the telescope tube are critical. Unfortunately so is weight.
A carbon-fiber-reinforced plastic (CFRP) sandwich meets these conflicting demands, according to Patria Finavicomp (Halli, Finland). Responsible for telescope tube design and construction, the Finnish company uses an ultra-high-modulus M60J carbon fiber with a cynate ester resin to obtain the necessary combination of strength and weight. At 160 kg, the tube represents a small fraction of the satellite's 4-ton weight.
"One problem in using CFRP materials on spacecraft is moisture absorption," explains Juhani Hanka of Finavicomp's Space Structures Unit. "Outgassing in the vacuum of space stresses the material leading to changes in the telescope's dimensional stability." Hanka adds that the material selected for the XMM tube offers a particularly low moisture absorption in atmospheric conditions as well as excellent outgassing characteristics. Excel Fiberite supplies the pre-impregnated layers required for making the tube.
In addition to mirror design and tube construction, satellite guidance is critical to the mission's success. Traveling around the Earth once every 48 hours, the XMM will follow a highly elliptical orbit extending 120,000 km into space. In this "quiet" region the satellite can make scientific observations throughout a 40-hour period, undisturbed by the Earth's radiation belts.
The Attitude and Orbit Control System (AOCS) will keep the satellite on course and maintain its relative pointing error to within 0.25 arcseconds. Mike Backler, AOCS project manager at Matra Marconi Space in the UK, cites some of the critical technologies: attitude sensors, star trackers incorporating CCD detectors, and sun sensors.
A control computer takes in the attitude information, processes it, and drives the reaction wheels to maintain the required pointing attitude. The reaction wheels spin in either direction at speeds to 4,000 rpm. Bearing friction has been so reduced, says Bakler, that the wheels take about one and one half hours to come to a standstill after de-energizing.
Complex as the guidance system may be, the satellite's mirrors are the heart of the mission. When asked about XMM technology, Jan van Casteren, the XMM spacecraft manager for the ESA, replies: "The spacecraft embodies lots of advanced engineering, but the x-ray mirrors are the key to the whole project."
What this means to you
- Computer processing can combine images from several sources, such as separate mirror modules, to increase a telescope's--or other instrument's--range
- Microscope systems are available that feature amplitude accuracy better than 0.1 nanometer
- Carbon-fiber-reinforced plastic yields stiff yet lightweight structures
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