Second Gemini Mirror Blank Fused

January 30, 1996

Fifty-five blocks of low-expansion ULE™ glass were successfully fused together at Corning's Canton, New York facility today to form the second Gemini primary mirror blank. The resulting blank is over 8.1 meters in diameter, 27 cm thick, and weighs approximately 32,000 kg. It is the second of two that Corning is fabricating for the Gemini 8-M Telescopes Project, and comes only three months after the delivery of the first Gemini mirror blank.

Prior to fusing, the thermal expansion coefficient of each block, although nearly zero, was carefully measured and the position of each block within the blank was carefully optimized to result in minimum mirror surface figure deformation over temperature changes. Then the component blocks were laid on the surface of a turntable which served as the base of a furnace, and fused together at temperatures exceeding 1700 degrees C.

The blank will now be rough generated on all surfaces in preparation for slumping, which will form the meniscus shape with the desired radius of curvature. Then the blank will be diamond generated to net shape with a final thickness of 20cm, and transported to REOSC Optique in Paris, France for polishing of the optical surface. Completion of the second blank at Corning is scheduled for May 1997.