Optical telescope

The Large Binocular Telescope uses two curved mirrors to gather light
Schematic of a Keplerian refracting telescope. The arrow at (4) is a (notional) representation of the original image; the arrow at (5) is the inverted image at the focal plane; the arrow at (6) is the virtual image that forms in the viewer's visual sphere. The red rays produce the midpoint of the arrow; two other sets of rays (each black) produce its head and tail.
Eight-inch refracting telescope at Chabot Space and Science Center
The Keck II telescope gathers light by using 36 segmented hexagonal mirrors to create a 10 m (33 ft) aperture primary mirror
These eyes represent a scaled figure of the human eye where 15 px = 1 mm, they have a pupil diameter of 7 mm. Figure A has an exit pupil diameter of 14 mm, which for astronomy purposes results in a 75% loss of light. Figure B has an exit pupil of 6.4 mm, which allows the full 100% of observable light to be perceived by the observer.
Two of the four Unit Telescopes that make up the ESO's VLT, on a remote mountaintop, 2600 metres above sea level in the Chilean Atacama Desert.
Comparison of nominal sizes of primary mirrors of some notable optical telescopes
Harlan J. Smith Telescope reflecting telescope at McDonald Observatory, Texas

Telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electronic image sensors.

- Optical telescope

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Indian Astronomical Observatory

High Energy Gamma Ray Telescope (HAGAR), Hanle

The Indian Astronomical Observatory (IAO), located in Hanle near Leh in Ladakh, India, has one of the world's highest located sites for optical, infrared and gamma-ray telescopes.

Extremely Large Telescope

Astronomical observatory currently under construction.

ESO Council meets at ESO headquarters in Garching, 2012.
ELT construction status, January 2022.
The optical system of the ELT showing the location of the mirrors.
Cast of the first ELT main mirror segments.
ELT's M2 Mirror Blank.
ELT concept.
Size comparison between the ELT and other telescope domes.
The First ELT Instruments.
ELT compared to the VLT and the Colosseum
Artist's rendering of the ELT in operation.<ref>{{cite web|title=Artist's rendering of the ELT in operation|url=https://www.eso.org/public/images/eso1716a/|website=www.eso.org|access-date=29 May 2017}}</ref>
Diagram of the 40m-class ELT primary mirror.
ELT compared with one of the four existing VLT Unit Telescopes at Cerro Paranal, Chile
Rendering of ELT during the day.
Model of the gigantic and intricate structure inside the enclosure of the ELT.
Close-up of ELT primary mirror (artist's impression). <ref>{{cite web |title=Milestones Reached in Incredible Journey of ELT Main Mirror Segments. |url=https://www.eso.org/public/announcements/ann20009/ |access-date=11 February 2020 |language=en}}</ref>
Rendering of the MICADO instrument. <ref>{{cite web |title=ELT MICADO Instrument Passes Preliminary Design Review |url=https://www.eso.org/public/announcements/ann20002/ |website=www.eso.org |access-date=23 January 2020 |language=en}}</ref>
thumb|The night sky over the construction site for the Extremely Large Telescope.<ref>{{cite news|title=The Milky Way above the ELT sitey|url=https://www.eso.org/public/images/potw2011a/|access-date=16 March 2020}}</ref>

When completed, it is planned to be the world's largest optical/near-infrared extremely large telescope.

Zoom lens

Mechanical assembly of lens elements for which the focal length can be varied, as opposed to a fixed-focal-length (FFL) lens (see prime lens).

Nikkor 28–200 mm zoom lens, extended to 200 mm at left and collapsed to 28 mm focal length at right
TV camera and Canon DIGI SUPER 86 II zoom lens with 86× magnification
A photograph taken with a zoom lens, whose focal length was varied during the course of the exposure
Unusual trailed-zoom view of a VLT telescope building
The Voigtländer Zoomar, 36–82 mm 2.8
Cross section of Fujinon XF100-400mm zoom lens
Canon AE-1, a 35mm camera with a zoom lens. The advantage of a zoom lens is the flexibility, but the disadvantage is the optical quality. Prime lenses have a greater image quality in comparison.
A simple zoom lens system. The three lenses of the afocal system are L1, L2, L3 (from left). L1 and L2 can move to the left and right, changing the overall focal length of the system (see image below).
Movement of lenses in an afocal zoom system

In addition, the afocal part of a zoom lens can be used as a telescope of variable magnification to make an adjustable beam expander.

Pisgah Astronomical Research Institute

Non-profit astronomical observatory located in the Pisgah National Forest near Balsam Grove, North Carolina.

PARI Central Campus with 26 West and 26 East
Redstone rocket engine on display
Knightia fossils from the Green River Formation
Carboniferous Ferns
Petrified Wood
Brazilian Amethyst
Toluca meteorite
Seymchan meteorite
Canyon Diablo Meteorite
Fragment of D'Orbigny meteorite
Agates collected between 1920 - 1950

PARI operates multiple radio telescopes and optical telescopes for research and teaching purposes.

Probing Lensing Anomalies Network

The Probing Lensing Anomalies NETwork (PLANET) collaboration coordinates a network of telescopes to rapidly sample photometric measurements of the magnification of stars in the galactic bulge undergoing gravitational microlensing by intervening foreground stars (or other compact massive objects).

PLANET logo depicting the locations of the five telescopes used
The 40 inch (1 m) Elizabeth Telescope at the South African Astronomical Observatory

of five 1m-class optical telescopes distributed in longitude around the southern hemisphere in order to perform quasi-continuous round-the-clock precision monitoring.

Dennis Walsh

English astronomer.

He used a wide variety of optical telescopes, having learnt about optical observing using the Isaac Newton Telescope, before being one of the first western astronomers to use the Soviet BTA-6 telescope.

Sloan Digital Sky Survey

SDSS map shown as a rainbow of colors, located within the observable Universe (the outer sphere, showing fluctuations in the Cosmic Microwave Background). As we look out in distance, we look back in time. So, the location of these signals reveals the expansion rate of the Universe at different times in cosmic history. (2020)
Quasars acting as gravitational lenses. To find these cases of galaxy–quasar combinations acting as lenses, astronomers selected 23,000 quasar spectra from the SDSS.
Light from distant galaxies has been smeared and twisted into odd shapes, arcs, and streaks.
A simplified graphical representation of a 7-fibre bundle. MaNGA measures 17 galaxies at a time, using bundles of 19, 37, 61, 91, and 127 fibres.
LRG-4-606 is a Luminous Red Galaxy. LRG is the acronym given to a catalog of bright red galaxies found in the SDSS.

The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States.

Star hopping

Technique that amateur astronomers often use to locate astronomical objects in the night sky.

Star chart showing the constellation Hercules

Telescopes or binoculars collect much more light, making faint objects visible, but have a smaller field of view, thus complicating orientation on the sky.


Accessory sighting device used in astronomy and stargazing, typically a small auxiliary refracting telescope/monocular mounted parallelly on a larger astronomical telescope along the same line of sight.

A 50mm right-angle finderscope mounted on a 150mm telescope.
Tour guide points out the double Finderscope on the 24.5 inch Cassegrain Telescope at the Goldendale Observatory State Park.
Reflex sights such as the Telrad (pictured) are popular alternatives to traditional finderscopes, and are often used in conjunction with them.

Since the sight uses a beam splitter "window", instead of an optical telescope with the ability to gather light, objects dimmer than the naked eye limit can not be seen through it.

Deep-sky object

Any astronomical object that is not an individual star or Solar System object (such as Sun, Moon, planet, comet, etc.).

Several nebulae in the constellation Orion commonly called deep-sky objects
Map of the constellation Cygnus marking the location of bright and more challenging deep-sky objects

The classification is used for the most part by amateur astronomers to denote visually observed faint naked eye and telescopic objects such as star clusters, nebulae and galaxies.