Refracting telescope

A 200 mm refracting telescope at the Poznań Observatory
Optical diagram of Galilean telescope
y – Distant object; y′ – Real image from objective; y″ – Magnified virtual image from eyepiece;
D – Entrance pupil diameter; d – Virtual exit pupil diameter;  L1 – Objective lens;  L2 – Eyepiece lens e – Virtual exit pupil – Telescope equals
Engraved illustration of a 150 ft focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.
Alvan Clark polishes the big Yerkes achromatic objective lens, over 1 meter across, in 1896.
This 12 inch refractor is mounted in dome and a mount the rotates with the turn of the Earth
The Greenwich 28-inch refractor is a popular tourist attraction in 21st century London
The Apochromatic lens usually comprises three elements that bring light of three different frequencies to a common focus
The 102 cm refractor, at Yerkes Observatory, the largest achromatic refractor ever put into astronomical use (photo taken on 6 May 1921, as Einstein was visiting)
The "Große Refraktor" a double telescope with a 80cm (31.5") and 50 cm (19.5") lenses, was used to discover calcium as an interstellar medium in 1904.
Astronaut trains with camera with large lens
Touristic telescope pointed to Matterhorn in Switzerland
The Yerkes Great refractor mounted at the 1893 World's Fair in Chicago; the tallest, longest, and biggest aperture refactor up to that time.
The 68 cm refractor at the Vienna University Observatory

Type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope).

- Refracting telescope

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Optical telescope

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.

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

Refracting telescopes, which use lenses and less commonly also prisms (dioptrics)


Branch of optics dealing with refraction, similarly the branch dealing with mirrors is known as catoptrics.


Telescopes that create their image with an objective that is a convex lens (refractors) are said to be "dioptric" telescopes.

Objective (optics)

Optical element that gathers light from the object being observed and focuses the light rays to produce a real image.

Several objective lenses on a microscope.
Objective lenses of binoculars
Two Leica oil immersion microscope objective lenses; left 100×, right 40×.
Camera photographic objective, focal length 50 mm, aperture 1:1.4
The segmented hexagonal objective mirror of the Keck 2 Telescope

In a telescope the objective is the lens at the front end of a refracting telescope (such as binoculars or telescopic sights) or the image-forming primary mirror of a reflecting or catadioptric telescope.


Transmissive optical device which focuses or disperses a light beam by means of refraction.

A biconvex lens
Lenses can be used to focus light
Light being refracted by a spherical glass container full of water. Roger Bacon, 13th century
Lens for LSST, a planned sky surveying telescope
Types of lenses
The position of the focus of a spherical lens depends on the radii of curvature of the two facets.
A camera lens forms a real image of a distant object.
Virtual image formation using a positive lens as a magnifying glass.
Images of black letters in a thin convex lens of focal length f are shown in red. Selected rays are shown for letters E, I and K in blue, green and orange, respectively. Note that E (at 2f) has an equal-size, real and inverted image; I (at f) has its image at infinity; and K (at f/2) has a double-size, virtual and upright image.
An aspheric biconvex lens.
Close-up view of a flat Fresnel lens.

The practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, and the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands.

Johannes Kepler

German astronomer, mathematician, astrologer, natural philosopher and writer on music.

Portrait of Kepler by an unknown artist in 1620.
Kepler's birthplace, in Weil der Stadt
As a child, Kepler witnessed the Great Comet of 1577, which attracted the attention of astronomers across Europe.
Tycho Brahe
Remnant of Kepler's Supernova SN 1604
Karlová street in Old Town, Prague – house where Kepler lived. Now a museum
A statue of Kepler in Linz
Kepler's Platonic solid model of the Solar System, from Mysterium Cosmographicum (1596)
Diagram of the geocentric trajectory of Mars through several periods of apparent retrograde motion in Astronomia Nova (1609)
Two pages from Kepler's Rudolphine Tables showing eclipses of the Sun and Moon
Kepler's horoscope for General Wallenstein
Geometrical harmonies from Harmonice Mundi (1619)
A plate from Astronomiae Pars Optica, illustrating the structure of eyes of various species.
A diagram illustrating the Kepler conjecture from Strena Seu de Nive Sexangula (1611)
Monument to Tycho Brahe and Kepler in Prague, Czech Republic
The GDR stamp featuring Kepler
The Kepler crater as photographed by Apollo 12 in 1969
Epitome astronomiae copernicanae, 1618

Additionally, he did fundamental work in the field of optics, invented an improved version of the refracting (or Keplerian) telescope, and was mentioned in the telescopic discoveries of his contemporary Galileo Galilei.


Optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects, or various devices used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation.

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.
17th century telescope
The 60-inch Hale (debuted in 1908) considered to be the first modern large research reflecting telescope.
The primary mirror assembly of James Webb Space Telescope under construction. This is a segmented mirror and its coated with Gold to reflect (orange-red) visible light, through near-infrared to the mid-infrared
Modern telescopes typically use CCDs instead of film for recording images. This is the sensor array in the Kepler spacecraft.
A 1.2-meter (47 in) reflecting telescope
The Very Large Array at Socorro, New Mexico, United States.
Einstein Observatory was a space-based focusing optical X-ray telescope from 1978.
The Compton Gamma Ray Observatory is released into orbit by the Space Shutte in 1991, and it would operate until the year 2000
The reflectors of HEGRA detect flashes of light in the atmosphere, thus detecting high energy particles
Equatorial-mounted Keplerian telescope
A diagram of the electromagnetic spectrum with the Earth's atmospheric transmittance (or opacity) and the types of telescopes used to image parts of the spectrum.
Six views of the Crab nebula supernova remnant, viewed at different wavelengths of light by various telescopes
The Five-hundred-meter Aperture Spherical radio Telescope in Guizhou, China, is the world's largest filled-aperture radio telescope

The first known practical telescopes were refracting telescopes with glass lenses and were invented in the Netherlands at the beginning of the 17th century.

Chromatic aberration

Failure of a lens to focus all colors to the same point.

Photographic example showing high quality lens (top) compared to lower quality model exhibiting transverse chromatic aberration (seen as a blur and a rainbow edge in areas of contrast.)
Chromatic correction of visible and near infrared wavelengths. Horizontal axis shows degree of aberration, 0 is no aberration. Lenses: 1: simple, 2: achromatic doublet, 3: apochromatic and 4: superachromat.
Color shifting through corner of eyeglasses.
Severe purple fringing can be seen at the edges of the horse's forelock, mane, and ear.
This photo taken with the lens aperture wide open resulting in a narrow depth-of-field and strong axial CA. The pendant has purple fringing in the near out-of-focus area and green fringing in the distance. Taken with a Nikon D7000 camera and an AF-S Nikkor 50mm f/1.8G lens.
Severe chromatic aberration

For example, this could result in extremely long telescopes such as the very long aerial telescopes of the 17th century.


Type of lens that is attached to a variety of optical devices such as telescopes and microscopes.

A collection of different types of eyepieces.
A 25 mm Kellner eyepiece
Simulation of views through a telescope using different eyepieces. The center image uses an eyepiece of the same focal length as the one on the left, but has a wider apparent field of view giving a larger image that shows more area. The image on the right has the same apparent field of view as the center eyepiece but has a shorter focal length, giving the same true field of view as the left image but at higher magnification.
The Plössl, an eyepiece with a large apparent field of view
Examples (from left to right) of 2" (51 mm), 1.25" (32 mm), and 0.965" (24.5 mm) eyepieces.
The eye relief. 1 Real image 2 - Field diaphragm 3 - Eye relief 4 - Exit pupil
Negative lens
Convex lens
Huygens eyepiece diagram
Ramsden eyepiece diagram
Kellner eyepiece diagram
Plössl eyepiece diagram
Orthoscopic eyepiece diagram
Monocentric eyepiece diagram
Erfle eyepiece diagram
König eyepiece diagram
RKE eyepiece diagram
Nagler type 2 eyepiece diagram
Nagler type eyepieces

These eyepieces work well with the very long focal length telescopes (in Huygens day they were used with single element long focal length non-achromatic refracting telescopes, including very long focal length aerial telescopes).


8×42 roof prism binoculars
A typical Porro prism binoculars design
Galilean binoculars
Cross-section of a relay lens aprismatic binocular design
Double Porro prism design
Porro prism binoculars
Schmidt–Pechan "roof" prism design
Abbe–Koenig "roof" prism design
Roof prism binoculars with the eyepieces in line with the objectives
Parameters listed on the prism cover plate describing 7 power magnification binoculars with a 50 mm objective diameter and a 372 foot field of view at 1000 yards
The small exit pupil of a 25×30 telescope and large exit pupils of 9×63 binoculars suitable for use in low light
Central-focusing binoculars with adjustable interpupillary distance
People using binoculars
Binoculars with red-colored multicoatings
Special reflective coatings on large naval ship 20×120 binoculars
Tower Optical coin-operated binoculars
Vector series laser rangefinder 7×42 binoculars can measure distance and angles and also features a 360° digital compass and class 1 eye safe filters
German U.D.F. 7×50 blc U-boat binoculars (1939–1945)
7×50 marine binoculars with dampened compass
US Naval ship 'Big eyes' 20×120 binoculars in fixed mounting
25 × 150 binoculars adapted for astronomical use
A simulated view of how the Andromeda Galaxy (Messier 31) would appear in a pair of binoculars
Beam path at the roof edge (cross-section); the P-coating layer is on both roof surfaces

Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects.

Reflecting telescope

Telescope that uses a single or a combination of curved mirrors that reflect light and form an image.

Stratospheric Observatory for Infrared Astronomy
24-inch convertible Newtonian/Cassegrain reflecting telescope on display at the Franklin Institute
A replica of Newton's second reflecting telescope that he presented to the Royal Society in 1672
The great telescope of Birr, the Leviathan of Parsonstown. Modern day remnants of the mirror and support structure.
Gran Telescopio Canarias
An image of Sirius A and Sirius B by the Hubble Space Telescope, showing diffraction spikes and concentric diffraction rings.
Main mirror of James Webb Space Telescope assembled at Goddard Space Flight Center, May 2016.
Light path in a Gregorian telescope.
Light path in a Newtonian telescope.
Light path in a Cassegrain telescope.
A prime focus telescope design. The observer/camera is at the focal point (shown as a red X).
Cassegrain design
Nasmyth/coudé light path.

The reflecting telescope was invented in the 17th century by Isaac Newton as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration.