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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A Rake's Progress

Series of eight paintings by 18th-century English artist William Hogarth.

William Hogarth, The Painter and his Pug, 1745. Self-portrait with his pug, Trump, in Tate Britain, London.

Tom begins to go mad, as indicated by both a telescope for celestial observation poking out of the barred window (an apparent reference to the longitude rewards offered by the British government) and an alchemy experiment in the background.

Phases of Venus

The phases of Venus are the variations of lighting seen on the planet's surface, similar to lunar phases.

The phases of Venus and evolution of its apparent diameter
Diagram of the orbit of Venus in relationship to the Earth, observed by Galileo Galilei in 1610

The phases of Venus result from the planet's orbit around the Sun inside the Earth's orbit giving the telescopic observer a sequence of progressive lighting similar in appearance to the Moon's phases.

Siding Spring Observatory

Siding Spring Observatory near Coonabarabran, New South Wales, Australia, part of the Research School of Astronomy & Astrophysics (RSAA) at the Australian National University (ANU), incorporates the Anglo-Australian Telescope along with a collection of other telescopes owned by the Australian National University, the University of New South Wales, and other institutions.

Siding Spring Mountain with Anglo-Australian Telescope dome visible near centre of image.

Between 1953 and 1974, the 74 in reflecting telescope at Mount Stromlo was the largest optical telescope in Australia.

Iron sights

Sight picture through iron sights of an H&K MP5 submachine gun. The annular shroud around the front post sight is aligned with the rear peep sight to ensure the firearm is properly trained.
A center hold sight picture with focus on the front sight; the out-of-focus gray dot represents the target.
A 6 o'clock sight picture with focus on the front sight; the out-of-focus gray dot represents the target.
A selection of open sights, and one aperture sight suitable for use with long eye relief: A) U-notch and post, B) Patridge, C) V-notch and post, D) Express, E) U-notch and bead, F) V-notch and bead, G) trapezoid, H) ghost ring. The gray dot represents the target
Rear, rotating diopter drum sight of a SIG SG 550 assault rifle. The viewing aperture above the "3" (denoting the 300 m setting) can be seen
Pictures taken under identical conditions through large (left) and small (right) diameter aperture sights, with camera focused on front sight
Example of ghost ring on Stevens Model 350 shotgun.
A 10 metre air rifle target diopter and globe aperture sight picture
Military M1917 ladder aperture sight calibrated out to 1600 yd
Various methods of open sight contrast enhancement. Left to right: Three dot, white outline, Straight-eight, red insert, dot and bar, gold bead
Steyr triangular pistol sights
Tritium-illuminated handgun night sights on a FN Five-seven
Green fiber optic contrast enhancement rods used in an adjustable open sight rear element

Iron sights are a system of physical alignment markers (usually made of metallic material) used as a sighting device to assist the accurate aiming of ranged weapons (such as a firearm, airgun, crossbow or even compound bow), or less commonly as a primitive finder sight for optical telescopes.

Telescopic sight

Optical sighting device based on a refracting telescope.

View through a 4× telescopic sight
Leupold and Stevens Mark 6 scope with variable magnification X3-X18, mounted on an M24 SWS
German military sniper rifle with a mounted telescopic sight and dismounted NSV80 clip on optoelectronic image intensifier
Telescopic sight (German made ZF Ajack 4×90 (4×38 in modern terminology) for the World War II pattern Swedish sniper rifle m/1941.
Russian Model 1891/30 sniper rifle with PU 3.5×21 sight
The Zielgerät ZG 1229 Vampir displayed by a British soldier (ca 1945)
A Swift model 687M variable power rifle telescopic sight with parallax compensation (the ring around the objective lens is used for making parallax adjustments).
The adjustment controls of a telescopic sight with an elevation adjustment knob featuring a zero-stop and second revolution indicator.
Various reticles.
Rangefinder reticle.
TA31RCO-M150CPO 4×32 ACOG sight using a combination of fiber optics (visible on top) and self-luminous tritium for reticle illumination
Simple animation demonstrating the extent of noticeable parallax shift with eye movements in telescopic sights with and without parallax compensation.
Austrian military issued Steyr SSG 69 sniper rifle with Kahles ZF 69 6×42 mm telescopic sight adjusted to be parallax free at 300 m
Scrome LTE J10 F1 with a lens hood mounted at the ocular and a flip-open cover at the objective mounted on a PGM Hécate II
Two Diarange telescopic sights with integrated laser rangefinders
Colt Python Silhouette, with 8-inch barrel, factory scope, and case – 500 made in 1981 by the Colt Custom Gun Shop.
A mount with three scope rings for telescopic sight interface and Picatinny rail for receiver interface.
Drawing of Zeiss rail compatible telescopic sight and mount (left) and a traditional ring mount (right). Both feature a picatinny rail receiver interface.
Telescopic sight fitted with scope rings on a Picatinny/MIL-STD-1913 rail mounted above the receiver of a sniper rifle.
Design difference in grabber interfaces between the Picatinny rail and the new NATO Accessory Rail.
The scope mount itself can be used as the interface for attaching other accessories.
A telescopic sight mounting set featuring three rings on a heavy-recoiling .338 Lapua Magnum chambered TRG-42 sniper rifle
Looking through a USMC sniper rifle's scope
PSO-1 reticle, the bottom-left corner can be used to determine the distance from a 170-cm-tall target (expected average height of an enemy combatant).
Swedish Ak4OR (H&K G3 variant) with Hensoldt 4×24 M1 telescopic sight.
Dual combat sighting system: ZF 3×4° optical sight topped with red dot sight as used on German G36A1 assault/sniper rifles.
Picatinny rail on a rifle receiver for mounting sights.
A dovetail rail on a rifle receiver for mounting sights with a drilling on top for an additional shape connection.
Side mounting rail on a PKP Pecheneg machine gun.
"STANAG" claw mount (receiver interface) on an FN FAL. This type of mount has also been used on several previous models by Heckler & Koch, such as for example MP5 and G3.

Being optical telescopes, prism scopes can focally compensate for a user's astigmatism, including when used as a non-magnified (1×) model.

Non-achromatic objective

Objective lens which is not corrected for chromatic aberration.

Several objective lenses on a microscope.

In telescopes they can a be pre-18th century simple single element objective lenses which were used before the invention of doublet achromatic lenses.

Exploration of the Moon

Impact on the surface of the Moon on September 14, 1959.

Apollo 12 Lunar Module Intrepid prepares to descend towards the surface of the Moon. NASA photo by Richard F. Gordon Jr.
Galileo's sketches of the Moon from the groundbreaking Sidereus Nuncius
A study of the Moon from Robert Hooke's Micrographia, 1665
The earliest surviving daguerrotype of the Moon by John W. Draper (1840)
Photo of the Moon made by Lewis Rutherfurd in 1865
The first picture of another world from space and of the Moon's far side, photographed by Luna 3 in 1959.
Museum replica of Luna 1 and Luna 2
Scale model of Luna 3
First image of the Moon taken by a U.S. spacecraft, Ranger 7 in July 1964
Block III Ranger probe
First photo ever taken from the surface of the Moon, by Luna 9 in February 1966.
Luna 9 was the first spacecraft to achieve a landing on the Moon in Feb 1966.
Earthrise taken by William Anders of Apollo 8 in Dec 1968
1966 stamp with a drawing of the first soft landed probe Luna 9, next to the first view of the lunar surface photographed by the probe.
Apollo 17 astronaut Harrison Schmitt standing next to a boulder at Taurus-Littrow during the third EVA (extravehicular activity).
Luna 16 first lunar sample return for the USSR in Sep 1970.
Cassini–Huygens took this image during its lunar flyby, before it traveled to Saturn
Animation of Lunar Reconnaissance Orbiter trajectory from 23 June 2009 to 30 June 2009
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The invention of the optical telescope brought about the first leap in the quality of lunar observations.

Special Astrophysical Observatory of the Russian Academy of Science

Astronomical observatory, set up in 1966 in the USSR, and now operated by the Russian Academy of Sciences.

BTA-6 as seen from in front of the main entrance.
The 1-meter Zeiss-1000 and the 60-centimeter Zeiss behind it
The RATAN-600 Radio Telescope

Based in the Bolshoi Zelenchuk Valley of the Greater Caucasus near the village of Nizhny Arkhyz, the observatory houses the BTA-6 and RATAN-600, an optical and radio telescope, respectively.

BTA-6

Inside the main observatory

The BTA-6 (Большой Телескоп Альт-азимутальный) is a 6 m aperture optical telescope at the Special Astrophysical Observatory located in the Zelenchuksky District of Karachay-Cherkessia on the north side of the Caucasus Mountains in southern Russia.

Digiscoping

Spotting scope with a digital camera mounted afocally using an adapter.
Digiscoping waterfowl
Typical uncropped digiscope image; the spotting scope has 20x magnification. Camera focal length is 24 mm; distance to the subject is about 90 meters.

Digiscoping is a neologism for afocal photography, using a (digital) camera to record distant images through the eyepiece of an optical telescope.