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Optics

Telescope Optical Configuration: Ritchey-Chretien Cassegrain

Primary Mirrors: Each mirror is an f/1.8, 8.1 m diameter, 20 cm thick meniscus, and weighs 22,200 kg. Each was made from 55 blocks of low expansion (ULE-581) glass fused together at 1700 degrees C and slumped at Corning's Canton, New York facility. Each mirror blank was then shipped to REOSC Optique in Paris, France, for polishing of its reflecting surface to that of a concave hyperboloid. The measured surface accuracy is 15.6 nm (rms).

Secondary Mirrors: Each secondary mirror is 1.0 m in diameter. Each was prefabricated by Schott from Zerodur and then polished by Zeiss to a convex hyperbolic shape and lightweighted by 85%. The weight of each secondary is 54 kg. The rms surface accuracies are 17 nm for the Gemini N mirror and 13 nm for the Gemini S mirror. Each secondary mirror is mounted on a rapid tip-tilt and chopping mechanism designed and built by Lockheed. Tip-tilt correction is possible up to 200 Hz. The chopping capabilities are: any position angle on the sky at frequencies up to 3 Hz; amplitudes up to 15 arcsec on the sky; and a duty cycle of 87% at the above values.

Science Fold Mirrors: These are plane mirrors at 45 degrees that reflect the converging beam near the Cassegrain focus of each telescope to an instrument mounted on one of four side-looking ports of the Instrument Support Structure (ISS) or may be retracted to allow the beam to reach the up-looking instrument on the bottom port of the ISS.

Effective Focal Lengths of Telescopes at Cassegrain Focus: 128.12 m (f/16)
Plate Scale at Cassegrain: 1.610 arcsec/mm

For optimal infrared and visual performance and durability, the primary and secondary mirrors and the science fold mirrors now have protected silver coatings.

Gemini North primary mirror
Primary mirror during inspection at Gemini North, 1999
Gemini North secondary mirror
Gemini North's 1.0 m secondary mirror