Previous Article
Next Article
Table of Contents

Gemini Primary Mirror Blank Fused (1Dec94)
(from NOAO HIGHLIGHTS!, NOAO Newsletter No. 40, 1 December 1994)

Fifty-five blocks of low expansion ULE=81 glass were successfully fused
together at Corning's Canton, New York facility on 20 September to form
the first Gemini primary mirror blank. The resulting blank is over 8.1 meters
in diameter, 27 cm thick and weighs approximately 32,000 kg. Gemini
Project members inspected the fused blank in mid-October and were
delighted with the outcome. To their surprise, they were invited to walk on
the mirror (but dancing was strictly forbidden). The mirror is surpassing
performance specifications at this stage and appears outstanding.

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 because of temperature changes. The component blocks were
then laid on the surface of a turntable, which served as the base of a
furnace, and fused together at temperatures exceeding 1700øC.

Generation of the edge and plano surfaces has now begun in preparation for
slumping of the blank, scheduled for March 1995, which will form the
meniscus shape with the desired radius of curvature. The blank will then be
diamond-generated to net shape with a final thickness of 20-cm, and
transported next October to REOSC Optique in Paris, France for polishing
of the optical surface. Fabrication of the second primary mirror blank slated
for Gemini South is also well under way, with fusion of that blank
scheduled for December 1995.

                          [Figure not included]

Previous Article
Next Article
Table of Contents