One of the twin Keck’s on Mauna Kea on the Big Island of Hawaii became the world’s largest working optical (and Infrared) telescope in 1993. Since then the Gran Telescopio Canarias, in many ways a slightly scaled Keck-clone, became the largest at 10.4m diameter. Each uses 36 hexagonal mirror segments to create a single optical surface 10m across. A combination of optical calibration with electronic edge sensing keeps the surface self-aligned as it points. The Keck primary mirror structure has become the template for most proposed larger telescopes, except for the Giant Magellan Telescope.
Giant Magellan Telescope
The Giant Magellan Telescope (GMT) uses 7 circular mirror segments that combine the light through a hole in the central 8.4m mirror. Six identical 8.4m segments are arranged in a hexagonal pattern around the central subaperture. The mirrors are held in relative alignment using a combination of mechanical/optical (edge sensing) and a system for optically phasing and aligning the subapertures. Unlike Keck-like designs the GMT secondary mirror optics work in a scalable way with each primary mirror subaperture. Four of the large mirror segments are polished or in-process. Ground breaking for GMT occurred in Chile with commissioning due to begin in the early 2020’s.
Thirty Meter Telescope
The Thirty Meter Telescope (TMT) is a “Keck-Era” telescope with hexagonal mirror segments slightly smaller than Keck (about 1.4m across). The TMT will use about 500 segments on a complex active mechanical structure that creates a single optical primary surface. The TMT mirrors are a major world effort with major contributions from Japanese, American, Canadian, Indian, and Chinese partners. A final site is under review with the preference for a northern hemisphere location like Mauna Kea.
European Extremely Large Telescope
The European Extremely Large Telescope (EELT) project is also a Keck-like design with hexagonal mirror segments very similar to TMT. With a diameter of 39m, it will use about 800 segments. Its optical design is complex , using 4m-class secondary and tertiary mirrors. Its optomechanical primary support structure creates a single optical surface to reflect the light up and through its tertiary optics. It is due to begin commissioning in the late 2020’s.
The PLANETS Foundation Telescopes
The off-axis parabolic building blocks of PLANETS, ELF, and Colossus are the key ingredients of telescopes that abandon the Keck primary mirror design philosophy. To minimize diffracted light, each of these is optimized to have the largest subaperture size, and to create a scalable optical system whereby each primary segment illuminates a single (small) secondary mirror.
These optics no longer depend on a stiff mechanical structure, since the phasing of each subaperture is controlled by the individual secondary optics. This means that these telescopes, unlike Keck-era designs, are optimized for narrow-field-of-view observations, where there is a relatively bright source in the field-of-view. This is precisely the observing condition for exoplanet studies. PLANETS is a small aperture 1.9m off-axis parabolic telescope that will use one of the telescope world’s thinnest and lightest weight primaries. ELF will be the first thin-mirror telescope with the phasing control envisioned for the 70-100m Colossus telescope, to be constructed from 8m thin glass mirror segments.
By decreasing the mass of the mirror segments and using starlight to create a coherent optical imaging system over very large apertures the Colossus group expects to decrease the cost per square meter of telescope primary aperture area by as much as a factor of 5 over current telescope designs.