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MMT Polishing

During 1995 the mirror blank's back surface was ground and polished mechanically flat (to ± 100 microns) to relieve stresses and to provide a flat surface to attach the loadspreaders for polishing, testing, and ultimately supporting the mirror in the telescope. The mirror was then inverted, placed on an array of specially designed hydraulic support actuators. The faceplate was then milled to within a few thousandths of an inch of its final shape. During 1996, the 6.5-m f/1.25 mirror was polished with the stressed lap shown here. The precise aspheric curve was already been diamond generated in the faceplate. When finished in mid-December 1996, the mirror had a figure of ± 23 nanometers (rms) with respect to the ideal parabolic surface. After testing in the telescope cell, a reflective aluminum coating was added in June 1999 using a specially built vacuum chamber in the telescope enclosure on Mount Hopkins in southern Arizona.