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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
Field of View at Instrument Focal Plane: 10 arcmin diameter. Limiting factors on FOV are the diameter of the central openings of the WFS mounting rings in the WFS, the portholes in the ISS, and the Science Fold Mirror diameter. PWFS can also vignette the beam - the PWFS can reach an outer radius = 7 arcmin before one edge of their field starts to be vignetted (the 14 arcmin diameter is slightly larger than the instrument FOV because the PWFS are upstream of the Science Fold and ISS ports).
Mirror Coatings: For optimal infrared and visual performance and durability, the primary and secondary mirrors and the science fold mirrors now have protected silver coatings. Mirrors are recoated every 3-5 years to maintain performance. This compilation of reflectivity data describes coating performance across a broad range of wavelengths and over time.
Throughput: Throughput may be calculated as the product of the reflectivities of the Primary, Secondary and Science Fold mirrors. The bottom port does not use the Science Fold mirror but Gemini cannot guarantee instrument port assignment.
Baffles: Gemini has permanently installed baffles that are deployable with two positions - IR and Optical. They do not allow use of the full 7.5 arcmin field without vignetting. The Optical mode (extended) works at 12.0 arcmin, with extended diameter 2003mm. The IR mode (retracted) works at 3.5 arcmin with retracted diameter 1124.6mm. There is also a sky periscope at the center of the secondary mirror for IR instruments to see sky rather than the primary's central obstruction. This periscope is shuttered for optical instruments.
Primary mirror during inspection at Gemini North, 1999
Gemini North's 1.0 m secondary mirror