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1 August 1990 Test and theoretical comparisons for bending and springing of the Keck segmented ten meter telescope
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Proceedings Volume 1271, Adaptive Optics and Optical Structures; (1990)
Event: The International Congress on Optical Science and Engineering, 1990, The Hague, Netherlands
The ten meter diameter Reck telescope, currently the largest of the telescopes under construction in the world today, consists of thirty-six 1.8 meter diameter off-axis hyperbolic hexagonal mirror segments. These are comprised of near zero coefficient of expansion glass ceramic substrates manufactured by Schott Glaswerke of Germany under the trade name of Zerodur. The blanks are approximately 1.9 meters in diameter and 7.5 centimeters thick. To produce the aspheric segments (consisting of six different configurations) in a timely fashion, scientists at the University of California have developed the technique of stressed mirror polishing. This method employs the introduction of shears and moments about the segment periphery, in its circular shape, to bend the mirror into the reverse of the desired shape . A true sphere is then ground and subsequently polished into the segment, after which the loads are removed and the desired optical prescription obtained. Next, the segment is cut to the hexagonal shape and a central hole, 0.25 meters in diameter, is core drilled partially through its hack. The segment is then mounted to its final support structure, Mirrors constructed in this fashion have many advantages, which include avoidance of the large capital facilities needed to cast large monolithic blanks, and logistical problems of transportation or risk in fabrication or repair. Conventional polishing techniques for segments, however, which have no symmetrical axis, are very expensive and time-consuming, unlike the stressed mirror process which involves the fabrication of spheres using a single, large tool. The process involves theoretical prediction of the loads required to bend the surface, and iterative solutions based on test measurements to fine tune the desired shape. The iteration requires the use of the theoretical math model, which consists of a closed form solution to a flat plate, as well as a detailed finite element model of the segment, which includes the effects of shear and curvature. This paper discusses some of the theoretical and test correlations achieved to produce the required configurations. Very good agreement is found, such that the deformed surface can be achieved after as little as one iteration accurate to the tenth of a micron level. After the segments are cut, their shape may warp slightly due to residual stress levels in the blank itself. Using the detailed finite element model, a first order analysis of the springing effect, including the cutting of the central hole, is presented, and further correlation to test data made. Stress birefringence data has been successfully utilized in determining the magnitude and shape of the expected change. Again, results show a reasonable correlation can be made, allowing anticipation of the presumed spring prior to polish of the segment. Prediction allows for adequate range of adjustment and minimized force application for the corrective devices used to warp the segments to their final shape.
© (1990) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
John W. Pepi "Test and theoretical comparisons for bending and springing of the Keck segmented ten meter telescope", Proc. SPIE 1271, Adaptive Optics and Optical Structures, (1 August 1990);

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