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 have protected silver coatings.