This list of the largest optical reflecting telescopes with
objective diameters of or greater is sorted by
aperture
In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...
, which is a measure of the
light-gathering power and resolution of a
reflecting telescope
A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternati ...
. The mirrors themselves can be larger than the aperture, and some telescopes may use
aperture synthesis
Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection. At each separation and ...
through
interferometry
Interferometry is a technique which uses the ''interference (wave propagation), interference'' of Superposition principle, superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important inves ...
. Telescopes designed to be used as optical
astronomical interferometer
An astronomical interferometer or telescope array is a set of separate telescopes, mirror segments, or radio telescope antennas that work together as a single telescope to provide higher resolution images of astronomical objects such as stars, n ...
s such as the Keck I and II used together as the
Keck Interferometer
The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and, when c ...
(up to 85 m) can reach higher 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 m) 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 a poor measure of a telescope's performance.
Space-based telescopes, such as the
Hubble Space Telescope
The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
, take advantage of being above the Earth's
atmosphere
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
to reach higher resolution and greater light gathering through longer exposure times. Location in the northern or southern hemisphere of the Earth can also limit what part of the sky can be observed, and climate conditions at the observatory site affect how often the telescope can be used each year.
The combination of large mirrors, locations selected for stable atmosphere and favorable climate conditions, and
active optics and
adaptive optics
Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used in Astronomy, astronomical telescopes and laser communication systems to remove the effects of Astronomical seeing, atmo ...
to correct for much of atmospheric turbulence allow the largest Earth based telescopes to reach higher resolution than the Hubble Space Telescope. Another advantage of Earth based telescopes is the comparatively low cost of upgrading and replacing instruments.
Table of reflecting telescopes
Telescopes in this list are ordered by effective optical
aperture
In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...
, given as the diameter of a circle with equivalent collecting area. Aperture has historically been a useful gauge of telescopes' limiting resolution, optical area, physical size, and cost. Multiple mirror or
segmented mirror telescopes that are on the same mount and usually form a single combined image are ranked by their equivalent combined aperture. Telescopes that cannot use their entire primary mirror at once (e.g. HET or LAMOST) are listed by their maximum effective aperture. Telescopes that are sometimes used for
optical interferometry are listed individually, not as a combined instrument. All telescopes with an effective aperture of at least at
visible or
near-infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...
wavelengths are included; selected smaller telescopes
are listed elsewhere.
Chronological list of largest telescopes
These telescopes were the largest in the world at the time of their construction, by the same aperture criterion as above.
Future telescopes
Under construction
These telescopes are under construction and will meet the list inclusion criteria once completed:
*
Extremely Large Telescope, Chile – . Construction began in 2018,
first light planned in 2028.
*
Thirty Meter Telescope, Hawaii, USA – . Construction began in 2014 but halted in 2015; it has not resumed.
*
Giant Magellan Telescope, Chile – seven 8.4 m mirrors on a single mount. This provides an effective aperture equivalent to a 21.4 m mirror and the resolving power equivalent to a 24.5 m mirror. First light planned in 2029.
*
Vera C. Rubin Observatory, Chile – . First light planned in 2025.
*
San Pedro Martir Telescope, Baja California, Mexico – . First light planned in 2023.
*
Magdalena Ridge Observatory Interferometer, New Mexico, USA – An optical interferometer array with ten telescopes. The light gathering power is equivalent to a single aperture. The first telescope was installed in 2016; construction was paused in 2019 due to insufficient funding and has not resumed.
*
Timau National Observatory, Indonesia – . Construction expected to be completed by early 2025.
Proposed

Selected large telescopes which are in detailed design or pre-construction phases:
*
Large UV Optical Infrared Surveyor (LUVOIR), a proposed space telescope for launch in the mid 2030s.
*
MUltiplexed Survey Telescope (MUST), a 6.5 m spectroscopic survey telescope.
*
Chinese Giant Solar Telescope (CGST), an infrared and optical solar telescope, with light-gathering power equivalent to a 5 m diameter aperture.
See also
*
List of largest infrared telescopes
*
List of telescope types
*
Lists of telescopes
This is a list of lists of telescopes.
*List of astronomical interferometers at visible and infrared wavelengths
*List of astronomical observatories
*List of highest astronomical observatories
*List of large optical telescopes
*List of largest i ...
References
Further reading
*
External links
List of large reflecting telescopesSelected largest telescopes(date 1884)
{{Portal bar, Astronomy, Stars, Outer space, Solar System
Optical telescopes
Largest optical telescopes
Telescope
A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...