List of largest optical reflecting telescopes
This list of the largest optical reflecting telescopes with objective diameters of 3.0 metres (120 in) or greater is sorted by aperture, which is one limit on the light-gathering power and resolution of a reflecting telescope's optical assembly. The mirrors themselves can be larger than the aperture, and telescopes may use aperture synthesis achieved by interferometry. Telescopes designed to be used as optical astronomical interferometers such as the Keck I and II used together as the Keck Interferometer (up to 85 meters) can reach very high resolutions, although at a narrower range of observations. When the two mirrors are on one mount, the combined mirror spacing of the Large Binocular Telescope (22.8 meters) allows fuller use of the aperture synthesis.
Largest does not always equate to being the best telescopes, and overall light gathering power of the optical system can be poor measure of a telescope's performance. Space-based telescopes, such as the Hubble Space Telescope, take advantage of being above the Earth's atmosphere to reach higher resolution and greater light gathering through longer exposure time. Location in the northern or southern hemisphere of the Earth can also limit what part of the sky can be observed.
Table of reflecting telescopes
This list is ordered by optical aperture, which has historically been a useful gauge of limiting resolution, optical area, physical size, and cost. Multiple mirror telescopes that are on the same mount, may have a working beam combiner, and can form an image may be ranked by an equivalent aperture to this reported by sources. HET-style or fixed telescopes are ranked by an equivalent aperture also.
There are only a few sites capable of polishing the mirrors for these telescopes. SAGEM in France polished the four VLT mirrors, the two Gemini mirrors, and the 36 segments for GTC. The Steward Observatory Mirror Lab cast and polished the two LBT mirrors, the two Magellan mirrors and the MMT replacement mirror. It is currently making the LSST primary mirror and the mirrors for the Giant Magellan Telescope. The Keck segments were made by Schott AG. The SALT and LAMOST segments were cast and polished by LZOS. The mirror for Subaru was cast by Corning and polished at Contraves Brashear Systems in Pennsylvania.
This table does not include all the largest mirrors manufactured. The Steward Observatory Mirror Lab produced the 6.5-metre f/1.25 collimator used in the Large Optical Test and Integration Site of Lockheed Martin, used for vacuum optical testing of other telescopes.
Segmented mirrors are also referred to as mosaic mirrors. Single mirrors are also referred to monolithic mirrors, and can be sub-categorized in types, such as solid or honeycomb.
Top telescopes of 2010
The largest telescopes are multi-telescope interferometers, and may have longer baselines. However, these astronomical interferometers are less flexible in use, and many interferometers cannot form images or see faint objects despite their high peak angular resolution.
The largest telescope during the first decade of the 21st century could be either the Gran Telescopio Canarias (one 10.4 diameter mirror), the Large Binocular Telescope (two 8.4 diameter mirrors on a binocular mount), or the Very Large Telescope (with four 8.2 m telescopes and four 1.8 m auxiliary telescopes). However, as these were still coming online in the period, the two 10 meter Keck Telescopes (with 85 m aperture synthesis) were possibly the largest in full scientific operation.
Note other wavelengths of telescopes can be much larger physically, such as the 305 m aperture Arecibo radio telescope, but since radio wavelengths are also larger, this does not necessarily translate into brighter or higher resolution images.
|Name||Out||In||aperture (m)||equiv. Area (m)||Area (m²)||Mirrors||Note||Altitude|
|Gran Telescopio Canarias (GTC)||10.4||10.4||74 m²||36 x 1.9 m hexagonal segments for M1||Commissioned 2009; Largest single mirror||2267 m|
|Southern African Large Telescope (SALT)||11 (max.)||9.2||66–45 m²||92 x 1 m hexagon; 11 x 9.8 m mirror||HET twin but larger optically. Spherical M1 w/ fixed mirror; spectroscopy (see here )||1783 m|
|Keck 1 & 2||10 m each||10 m||76 m2  each||36 x 1.8 m hexagons M1 mirrors each||largest twin telescopes
152 m² total optical area
|VLT 1,2,3, & 4||8.2 m each||8.2||53 m² each
||1 x 8.2 m M1 each||Largest quadruplet telescope
210 m² total optical area
|Large Binocular Telescope (LBT)||8.4 m each||11.7||111 m² (both sides)||2 x 8.4 m M1 mirrors; 1 mount||Largest Binocular; largest non-segmented mirrors; 22.8 m aperture planned ||3221 m|
Largest telescopes with interferometer mode, and two image-forming interferometer arrays.
|Name||Longest baseline (m)||Mirrors||Area||Equiv.||Note|
|VLTI||200||4 x 8.2 m (VLT 1,2,3, & 4)||210 m² ||16||Behind schedule in 2010, 4 units at 130 m success in 2012|
|Keck Interferometer||85||2 x 10 m (Keck 1 & 2)||152 m² ||Operations ceased in 2012.|
|LBT (LBTI)||22.8*||2 x 8.4||110 m² ||11.9||One telescope mount*; Fizeau & Amplitude Interferometry|
|CHARA array||330||6 x 1 m||2.45 m² |
|NPOI||437||6 siderostats (visible)||Sparse Aperture ||Optically resolved Mizar (star)|
*Baseline does not reduce with viewing angle
Top telescopes of 2000
|Name||Out||In||aperture (m)||Area (m²)||Mirrors||Note||Altitude|
|Keck 1 & 2||10 m each||76 m2  each||36 x 1.8 m hexagons M1 mirrors each||largest twin telescopes, largest aperture mirrors, 152 m² total optical area||4145 m|
|Hobby-Eberly Telescope (HET)||9.2 (max.)||66–45 m²||92 x 1 m hexagon; 11 x 9.8 m mirror||Spherical M1 w/ fixed mirror; spectroscopy||2030 m|
|Subaru Telescope||8.2||53 m²||1 x 8.3 m M1||Largest single piece mirror (tied with the 4 VLT)||4145 m|
|VLT 1,2,3, & 4||8.2 m each||53 m² each||1 x 8.2 m M1||Largest quadruplet telescope; largest single piece mirrors (tied with Subaru)||2635 m|
|Years Largest||Name||Out||In||aperture (m)||Area (m²)||M1 Mirror||Note||Altitude|
|2009–Present||Gran Telescopio Canarias||10.4 m||74 m²||36 x 1.9 m hexagons M1 mirror||Segmented mirror||2267 m|
|1993–2009||Keck 1||10 m||76 m2 ||36 x 1.8 m hexagons M1 mirror||Segmented mirror, M1 f/1.75||4145 m|
|1976–1993||BTA-6||6 m||26 m²||605 cm f/4 M1 mirror||Mirror replaced twice||2070 m|
|1948–1976||Hale (200 inch)||5.1 m||m²||508 cm f/3.3 M1 mirror||Art deco dome||1713 m|
|1917–1948||Hooker (100 inch)||2.54 m||m²||also used for 1st optical interferometer||1742 m|
For earlier top telescopes see List of largest optical telescopes historically
||This list has no precise inclusion criteria as described in the Manual of Style for standalone lists. (August 2013)|
Below are selected telescopes that are still in the conceptual/proposed stage or still under construction.
- Under construction or planned construction
- European Extremely Large Telescope 39.3 m
- Thirty Meter Telescope 30 m
- Giant Magellan Telescope 7×8.4 m mirrors = 24.5 m aperture (21.4 m equivalent area)
- Large Synoptic Survey Telescope 8.4 m (development approved as of 2012 for 2021 first light)
- James Webb Space Telescope 6.5 m (2018 launch planned)
- Pan-STARRS 4 x 1.8 m (1 of 4 complete)
- Magdalena Ridge Observatory Telescope Array 10 x 1.4 m
- Advanced Technology Solar Telescope 4 m (see also list of solar telescopes)
- International Liquid Mirror Telescope 4 m
- 3.6 m Devasthal Optical Telescope  (2013 first light planned)
- Proposed or planned
- HDRT 22m equiv.
- ALPACA telescope, 8 m liquid mirror 
- Advanced Technology Large-Aperture Space Telescope (ATLAST), 8-15m space telescope
- Chinese Future Giant Telescope (CFGT). Proposed optical telescope with a 30 meter diameter primary mirror.
- 30m Ring Interferometric Telescope (RIT). A 30m Ring Interferometric Telescope (RIT) has been proposed in China. 
- Chinese Giant Solar Telescope (CGST). China aim to build a very large infrared and optical solar telescope, with the spatial resolution expected to be equivalent to an 8m-diameter telescope and light-gathering power equivalent to a 5m-diameter full aperture telescope  
- The Colossus Telescope.
- Extremely large telescope
- List of largest optical refracting telescopes
- List of optical telescopes
- List of astronomical interferometers at visible and infrared wavelengths
- List of largest optical telescopes historically
- List of largest optical telescopes in the 20th century
- List of largest optical telescopes in the 19th century
- List of largest optical telescopes in the 18th century
- Strehl ratio
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- The Colossus
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- "The History of the Telescope", Henry C. King. (1955)
- "The Historical Growth of Telescope Aperture", René Racine, Publications of the Astronomical Society of the Pacific, 116
- JRASC (1929) vol 23, p351
- Sky&Telescope (April 1981) p303
- Sky&Telescope (July 1993) vol 86, p 27-32
- James H. Burge, 1993 Dissertation at UA, "Advanced Techniques for Measuring Primary Mirrors for Astronomical Telescopes"
- Bell, R.M. and Robins, G.C. and Eugeni, C. and Cuzner, G. and Hutchison, S.B. and Baily, S.H. and Ceurden, B. and Hagen, J. and Kenagy, K. and Martin, H.M. and Tuell, M. and Ward, M. and West, S.C. (July 2008). "LOTIS at completion of Collimator integration". Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series 7017. doi:10.1117/12.791889.