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Figure 1.

excellent treatment of the Gemini “mirror controversy” can be found

The Gemini South mirror as it emerges from the coating chamber with a fresh coat of protected silver. The coating technology developed at Gemini has resulted in highly durable coatings that have exceeded expectations in performance and is being adopted by other projects like the Thirty Meter Telescope currently under development.

the Patrick McCray’s book Giant Telescopes published by the Harvard University Press.) As the heart of any telescope, the mirror reflects more than just starlight. In Gemini’s case, the mirror is a reflection of innovation and a mindset to experiment with new ideas. Nowhere is this more apparent than in the decision to coat Gemini’s mirrors using protected silver in order to meet the strict infrared performance goals set by Fred Gillett and recommended in the Bahcall Report Decadal Survey. Keith Raybould, who spearheaded much of the early work on selecting a sputtering-based coating chamber, has the air of a proud parent over the news of successful silver coatings now lasting three to four years at Gemini. “That’s remarkable,” he said. “I would never have thought it would have lasted that long. It was very difficult to get the protective layers that would provide Jim Oschmann: After spending 10 years at Gemini, Jim (Jacobus) Oschmann left Gemini in 2002 as Gemini’s project manager and associate director of engineering. Since then he

a coating that we thought was as durable as aluminum which are the typical coatings of a primary and secondary [telescope mirrors]. That’s tremendous!” Larry Stepp (currently at the Thirty Meter Telescope) pointed out that other telescopes will profit by this success: “We are planning to have similar coating capabilities for the TMT mirrors, using a

has worked as the project manager

protected silver coating process that’s very similar to that developed

at the National Solar Observatory

by Gemini,” he said. “I think that has been a true contribution to the

and is currently at Ball Aerospace & Technologies Corporation as vice president & general manager for

field of astronomy and telescope making.” Today, thanks in large part to the silver coating technology developed at Gemini, the total optical system’s thermal emissivity is about 3%;

Antenna and Video Technologies. Jim’s

significantly better than any other ground-based telescope and below

background in optics and systems

the value specified in the early Gemini plans.

engineering began before Gemini at TRW, where he spent some time

A Secondary Issue

working under Dick Kurz who is also

Of course, bold success is sometimes tempered, and Gemini’s

featured in this article.

secondary mirrors were less successful. Larry Stepp summarized the



Issue 38 - June 2009  


Issue 38 - June 2009