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7 July 2000 Optical fabrication of the MMT adaptive secondary mirror
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We describe the optical fabrication of the adaptive secondary mirror for the MMT. The 640 mm f/15 secondary consists of a flexible glass shell, 1.8 mm thick, whose shape is controlled by 336 electromagnetic actuators. It is designed to give diffraction-limited images at a wavelength of 1 micron. For generating and polishing, the shell was supported by attaching it to a rigid glass blocking body with a thin layer of pitch. It could then be figured and measured using techniques developed for rigid secondaries. The highly aspheric surface was polished with a 30 cm stressed lap and small passive tools, and measured using a swing-arm profilometer and a holographic test plate. The goal for fabrication was to produce diffraction-limited images in the visible, after simulated adaptive correction using only a small fraction of the typical actuator forces. This translates into a surface accuracy of less than 19 nm rms with correction forces of less than 0.05 N rms. We achieved a surface accuracy of 8 nm rms after simulated correction with forces of 0.02 N rms.
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Hubert M. Martin, James H. Burge, Ciro Del Vecchio, Lee R. Dettmann, Stephen M. Miller III, Bryan Keener Smith, and Francois P. Wildi "Optical fabrication of the MMT adaptive secondary mirror", Proc. SPIE 4007, Adaptive Optical Systems Technology, (7 July 2000);

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