APPLICATION STORY


The biggest telescopes require sophisticated control
to keep them in focus, using high performance load cells.



 The Gemini telescopes have
 1.8 metre mirrors,
 weighing 23 tonnes.


Although there has been much publicity surrounding the success of the Hubble Space telescope. there are many advantages in building and operating large ground-based optical and infrared telescopes. Large telescopes are essential for spectroscopy of faint stars and distinct galaxies in order to derive physical properties such as mass, age and chemical composition but it is difficult to put a telescope much larger than 4m into orbit. Furthermore, when equipment fails in space it is very expensive to replace. Although ground based telescopes can suffer from variable weather and atmospheric effects, they can be built for about 3 per cent of the cost of the Hubble telescope and can be easily repaired or upgraded with the latest technology. However, building large ground telescopes presents problems of its own, especially those of a mechanical nature. Cooperation The Gemini project, which is an international cooperation between Argentina, Australia, Brazil, Canada, Chile, the UK and the US has involved the building of two massive 8.lm telescopes. These have now seen 'first light' and already scientists are amazed at the quality of the images being received. In order to provide the best year round atmospheric conditions and the widest possible view of the sky, one telescope is located in the Northern hemisphere and one in the Southern. The northern telescope is located at 4,200m on Mauna Kea on the island of Hawaii (40 per cent through the atmosphere) while the southern telescope is located at the peak of the 2,800m Cerro Pachon mountain in Chile. When combined, these two sites provide an unrestrained view of the whole sky, so that key astronomical objects, such as the centre of our Milky Way, small satellite galaxies, the Magellanic clouds and Andromeda galaxy are fully accessible to Gemini cameras and spectrographs. The Gemini telescopes are a magnificent achievement of modem technology and provide a powerful optical - infrared astronomical facility. In order to achieve good image quality, the shape of the primary mirror must be maintained over a range of telescope orientations and environmental conditions. The previous generation of large telescopes did this by using primary mirrors that were very thick and hence resisted distortion. This simple 'brute force' method for maintaining rigidity is not feasible for an 8 metre mirror. The mass increases approximately as the fourth power of the diameter and if the Gemini mirrors had been built using this approach they would weigh more than 230tonnes and retain heat for many days, affecting image quality. Not only would the logistics of transporting such mirrors be totally impractical (especially given the remote location) but any support and drive mechanism would be unmanageable.
Distortion To overcome this problem, the Gemini telescopes use thin meniscus mirrors mounted on an ingenious support system which can deform the mirror to compensate for gravitational and thermal distortion. With a mass of only 23 tonnes the primary mirrors are by comparison ultra-light. The mirrors were assembled, like a giant jigsaw puzzle, from a large number of hexagonal blocks of special ultra-low expansion titanium silicate glass less than 200mm thick These blocks, or boules, were fused together in a special furnace at the Corning Glass Company in Canton, New York and then cooled to form a solid flat disc. To obtain the necessary curvature to focus the light, the flat discs were reheated and 'slumped' over a mould, shaped to define the final curvature. From here the mirrors were shipped to REISC near Paris for final polishing. The precision of this operation is such that the maximum peak to valley deviation is only 141 billionths of a metre (141 nanometres). Incredibly this means that if the mirrors were the diameter of the earth, the largest bump would be less than 300mm high! Each mirror can collect more light than 2million human eyes and when combined with its adaptive optics, the telescopes can produce infrared images that are sharp enough to resolve the separation between a pair of car headlights at a distance of 3,000 miles, exceeding the resolution of the Hubble telescope. Support Despite the advantages offered scientists by modern computer techniques and electronics, much of the success of the project has depended on the design of the mechanism supporting the mirrors. The mirrors are made from hexagonal glass blocks, above, fused together and held in shape during use by a system of load cells and hydraulic and pneumatic actuators.
Surprisingly, at the heart of the structure are hundreds of strain gauge load cells - a technology first used 60 years ago to determine weight and balance data for military aircraft. The Royal Greenwich Observatory, based in Cambridge, was primarily responsible for the design and building of the support mechanisms. Each mirror is supported on 120 special nodes, equally spaced beneath its surface, in conjunction with a large inflatable rubber cushion like a giant air bag. The load cells have been designed and built by transducer specialists Applied Measurements Ltd., which has extensive experience in providing bespoke force measuring solutions for a diverse range of applications. Each node consists of two load cells, mounted one on top of each other. At the top, a 500N load cell is used for the fine control of the mirror, while immediately below is a 2,000N load cell. Although the two load cells are mounted in series, an ingenious force transfer mechanism ensures that the top load cells do not carry the full load of the mirror if the main support fails, the load being transferred to the lower cells.
All the support points are used in conjunction with pneumatic and hydraulic actuators which can pull and push parts of the mirror to maintain optimal shape and to compensate for distortions due to gravity and wind buffeting. To keep the mirror circular as the orientation of the telescope changes, 100 special 5,000N load cells are equally spaced around the outside of the rim of the mirror (and in the same plane). Traverse The complete mechanism is part of a closed loop which is part of a closed CAN bus computer system and allows the mirror to traverse through an arc of ±9odegrees without loss of image quality. The overall precise optical figure of the telescope is controlled and maintained by an 'active optics system' that senses distortions in the mirror surface using light from reference stars. Given the precision of the whole system, the design demands placed on the load cells were very exacting. It is vitally important that the load cells give the same 'output per unit load', for both positive (push) and negative (pull) loading to an accuracy of ± 0.02 per cent FS. Each load cell has a custom designed electronics module incorporating a microprocessor and amplifier which gives a fully balanced ± 5V output. This enables calibration adjustments to be carried out via a dedicated PC system. A further advantage of the approach is the ability to automatically locate and identify any problems with individual load cells. Peter Lewis, managing director of Applied Measurements, is understandably proud of the part played by his company in this prestigious international project. Although we have designed and built many special load cells over the past ten years, the Gemini units are for the most intriguing and complex application. Now that we can see the finished telescopes and the quality of the first pictures from outer space, we can only marvel at the ingenuity of the overall design and construction." _____________________________________________________ For more information, please contact :- Applied Measurements Ltd. Tel: +44(0) 118 981 7339 Fax: +44(0) 118 981 9121 Email: sales@appmeas.co.uk Web: www.appmeas.co.uk January 2001
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