urn:nasa:pds:context:telescope:gemini-south.8m1 1.0 1.11.0.0 Product_Context 2018-10-19 1.0 Created from information from Gemini Website: http://www.gemini.edu urn:nasa:pds:context:facility:observatory.gemini-south telescope_to_facility urn:nasa:pds:context:instrument:gemini-south.8m1.t-recs telescope_to_instrument 8.1 109.26331 -30.24075 2722 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). 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 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. The effective focal lengths of the telescopes at Cassegrain focus are 128.12 meters (f/16) with a plate scale of 1.610 arcsec/mm. Maximum 10 arcmin diameter on the bottom port, and 7 arcmin diameter on the side ports due to science fold mirror size. 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). 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 2003 mm. The IR mode (retracted) works at 3.5 arcmin with retracted diameter 1124.6 mm. 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. Information from the Gemini telescope website: http://www.gemini.edu/sciops/telescopes-and-sites/optics