What are the Gemini mirrors made of?
The Gemini telescopes collect light with mirrors, not lenses. Each telescope has an 8.1-m diameter “primary” mirror to collect and focus the light as it comes in from space, and a smaller “secondary” mirror to further focus the collected light. A Gemini primary mirror collects as much light as 2.5 million human eyes! Each primary mirror starts out as a “blank” consisting of 42 hexagonal blocks of ultra-low-expansion glass, fused into one large, thin disk. The blank is polished to extreme smoothness. If the mirror were the diameter of the earth, the largest bump on it would be less than 30 cm high! The glass mirror blank is coated with a layer of metal only 1/1,000th the thickness of a human hair. The mirror must be recoated every few months. The coating can be aluminium (standard for most telescope mirrors) or silver. The unique silver coating gives a much better performance at infrared wavelengths. The Gemini primary mirrors are only about 20 cm thick.
