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
The Large Binocular Telescope uses two curved mirrors to gather light

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Daedalus spacecraft concept

Project Daedalus

Study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible uncrewed interstellar probe.

Study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible uncrewed interstellar probe.

Daedalus spacecraft concept

The second stage would have two 5-metre optical telescopes and two 20-metre radio telescopes.

Pāṇini's Aṣṭādhyāyī, an early Indian grammatical treatise that constructs a formal system for the purpose of describing Sanskrit grammar.

Timeline of scientific discoveries

The timeline below shows the date of publication of possible major scientific breakthroughs, theories and discoveries, along with the discoverer.

The timeline below shows the date of publication of possible major scientific breakthroughs, theories and discoveries, along with the discoverer.

Pāṇini's Aṣṭādhyāyī, an early Indian grammatical treatise that constructs a formal system for the purpose of describing Sanskrit grammar.
Example of the early Greek symbol for zero (lower right corner) from a 2nd-century papyrus
Diophantus' Arithmetica (pictured: a Latin translation from 1621) contained the first known use of symbolic mathematical notation. Despite the relative decline in the importance of the sciences during the Roman era, several Greek mathematicians continued to flourish in Alexandria.
The age of Imperial Karnataka was a period of significant advancement in Indian mathematics.

1608 – Earliest record of an optical telescope

The 100-inch (2.54 m) Hooker reflecting telescope at Mount Wilson Observatory near Los Angeles, USA, used by Edwin Hubble to measure galaxy redshifts and discover the general expansion of the universe.

List of telescope types

The following are lists of devices categorized as types of telescopes or devices associated with telescopes.

The following are lists of devices categorized as types of telescopes or devices associated with telescopes.

The 100-inch (2.54 m) Hooker reflecting telescope at Mount Wilson Observatory near Los Angeles, USA, used by Edwin Hubble to measure galaxy redshifts and discover the general expansion of the universe.

Optical telescopes can be classified by three primary optical designs (refractor, reflector, or catadioptric), by sub-designs of these types, by how they are constructed, or by the task they perform.

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light.

Optical window

Range of wavelengths that are not blocked by the earth's atmosphere.

Range of wavelengths that are not blocked by the earth's atmosphere.

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light.
Solar irradiance spectrum above atmosphere and at surface. Extreme UV and X-rays are produced (at left of wavelength range shown) but comprise very small amounts of the Sun's total output power.

The first great astronomical discoveries such as the ones made by the famous Italian polymath Galileo Galilei were made using optical telescopes that received light reaching the ground through the optical window.

For the largest reflecting telescopes on the planet, the horizontal indicates the year built and the vertical direction indicates the size of the mirror measured in meters. Countries which contain several of these telescopes are color-coded for identification.

List of largest optical reflecting telescopes

For the largest reflecting telescopes on the planet, the horizontal indicates the year built and the vertical direction indicates the size of the mirror measured in meters. Countries which contain several of these telescopes are color-coded for identification.

This list of the largest optical reflecting telescopes with objective diameters of 3.0 m or greater is sorted by aperture, which is a measure of the light-gathering power and resolution of a reflecting telescope.

Diagram showing displacement of the Sun's image at sunrise and sunset

Atmospheric refraction

Deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height.

Deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height.

Diagram showing displacement of the Sun's image at sunrise and sunset
The atmosphere refracts the image of a waxing crescent Moon as it sets into the horizon.
Atmospheric refraction distorting the Sun’s disk into an uneven shape as it sets in the lower horizon.
Plot of refraction vs. altitude using Bennett's 1982 formula
The animated image of the Moon's surface shows the effects of atmospheric turbulence on the view.

If observations of objects near the horizon cannot be avoided, it is possible to equip an optical telescope with control systems to compensate for the shift caused by the refraction.

Light path in a Schmidt–Cassegrain

Schmidt–Cassegrain telescope

Catadioptric telescope that combines a Cassegrain reflector's optical path with a Schmidt corrector plate to make a compact astronomical instrument that uses simple spherical surfaces.

Catadioptric telescope that combines a Cassegrain reflector's optical path with a Schmidt corrector plate to make a compact astronomical instrument that uses simple spherical surfaces.

Light path in a Schmidt–Cassegrain
View of the corrector and primary mirror of a Schmidt–Cassegrain.
People demonstrating a Schmidt–Cassegrain telescope at a sidewalk gathering

Their high f-ratio means they are not a wide-field telescope like their Schmidt camera predecessor, but they are good for more narrow-field deep sky and planetary viewing.

Belgrano II Base

Permanent, all year-round Argentine Antarctic base and scientific research station named after General Manuel Belgrano, one of the Libertadores and the creator of the Argentine Flag.

Permanent, all year-round Argentine Antarctic base and scientific research station named after General Manuel Belgrano, one of the Libertadores and the creator of the Argentine Flag.

Astrolabel, through optical telescope, an agreement with the Astronomical Observatory of Córdoba)

The naked eye

Naked eye

The naked eye
A photographic approximation of a naked eye view of the night sky from a small rural town (top) and a metropolitan area (bottom). Light pollution dramatically reduces the visibility of stars.
The Milky Way is visible over the Very Large Telescope, demonstrating clear atmosphere above Paranal Observatory.

Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection.

Llano del Hato National Astronomical Observatory

Astronomical observatory in Venezuela.

Astronomical observatory in Venezuela.

There are four large optical telescopes, each in its own cupola or dome: a 1-m Askania Schmidt camera (one of the largest telescopes of this type in the world), a 65-cm Zeiss refractor, a 1-m Zeiss reflector and a 50-cm Askania double astrograph.