Optical Technology for the Giant Magellan Telescope

The optical design of the Giant Magellan Telescope (GMT) is based on a Gregorian optical prescription with segmented primary and secondary mirrors where each primary segment is conjugated with a matching secondary segment. The total aperture of the telescope is 25.4 m, with an f/0.7 primary focal ratio and an f/8.2 final focal ratio. The GMT will provide a 10-arcmin-diameter uncorrected field of view and a 20-arcmin field of view with a wide field corrector. The primary mirror is composed of seven 8.4-m segments, and the secondary is composed of seven 1.05-m segments, where two different sets of secondary mirror systems – the ASM (adaptive secondary mirror) or FSM (fast-steering secondary mirror) can be used. The ASM, with deformable front surfaces, is the core of the GMT adaptive optics (AO) system and will support four different observing modes: natural seeing, ground-layer adaptive optics (GLAO), natural guidestar adaptive optics (NGSAO), and laser tomography adaptive optics (LTAO). The FSM is optically identical to the ASM and will consist of seven rigid, light-weight mirrors. The Korea Astornomy and Space Science Institute developed a prototype of the FSM from 2009 to 2014 in preparation for the manufacture of the real FSM segments, which is expected to begin in 2016.