Large reflecting telescopes History of the telescope

the 200-inch (5.1 m) hale telescope @ mount palomar

in 1856–57, karl august von steinheil , léon foucault introduced process of depositing layer of silver on glass telescope mirrors. silver layer not more reflective , longer lasting finish on speculum mirrors, had advantage of being able removed , re-deposited without changing shape of glass substrate. towards end of 19th century large silver on glass mirror reflecting telescopes built.

the beginning of 20th century saw construction of first of modern large research reflectors, designed precision photographic imaging , located @ remote high altitude clear sky locations such 60-inch hale telescope of 1908, , 100-inch (2.5 m) hooker telescope in 1917, both located @ mount wilson observatory. these , other telescopes of size had have provisions allow removal of main mirrors re-silvering every few months. john donavan strong, young physicist @ california institute of technology, developed technique coating mirror longer lasting aluminum coating using thermal vacuum evaporation. in 1932, became first person aluminize mirror; 3 years later 60-inch (1,500 mm) , 100-inch (2,500 mm) telescopes became first large astronomical telescopes have mirrors aluminized. 1948 saw completion of 200-inch (510 cm) hale reflector @ mount palomar largest telescope in world until completion of massive 605 cm (238 in) bta-6 in russia twenty-seven years later. hale reflector introduced several technical innovations used in future telescopes, including hydrostatic bearings low friction, serrurier truss equal deflections of 2 mirrors tube sags under gravity, , use of pyrex low-expansion glass mirrors. arrival of substantially larger telescopes had await introduction of methods other rigidity of glass maintain proper shape of mirror.

active , adaptive optics

the 1980s saw introduction of 2 new technologies building larger telescopes , improving image quality, known active optics , adaptive optics. in active optics, image analyser senses aberrations of star image few times per minute, , computer adjusts many support forces on primary mirror , location of secondary mirror maintain optics in optimal shape , alignment. slow correct atmospheric blurring effects, enables use of thin single mirrors 8 m diameter, or larger segmented mirrors. method pioneered eso new technology telescope in late 1980s.

the 1990s saw new generation of giant telescopes appear using active optics, beginning construction of first of 2 10 m (390 in) keck telescopes in 1993. other giant telescopes built since include: 2 gemini telescopes, 4 separate telescopes of large telescope, , large binocular telescope.

eso s vlt boasts advanced adaptive optics systems, counteract blurring effects of earth s atmosphere.

adaptive optics uses similar principle, applying corrections several hundred times per second compensate effects of rapidly changing optical distortion due motion of turbulence in earth s atmosphere. adaptive optics works measuring distortions in wavefront , compensating them rapid changes of actuators applied small deformable mirror or liquid crystal array filter. ao first envisioned horace w. babcock in 1953, did not come common usage in astronomical telescopes until advances in computer , detector technology during 1990s made possible calculate compensation needed in real time. in adaptive optics, high-speed corrections needed mean bright star needed close target of interest (or artificial star created laser). also, single star or laser corrections effective on narrow field (tens of arcsec), , current systems operating on several 8-10m telescopes work in near-infrared wavelengths single-object observations.

developments of adaptive optics include systems multiple lasers on wider corrected field, and/or working above kilohertz rates correction @ visible wavelengths; these in progress not yet in routine operation of 2015.