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Refracting telescope
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A 200 mm diameter refracting telescope at the Poznań Observatory

A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and astronomical telescopes but is also used for long-focus camera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the reflecting telescope, which allows larger apertures. A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece.[1]

Refracting telescopes typically have a lens at the front, then a long tube, then an eyepiece or instrumentation at the rear, where the telescope view comes to focus. Originally, telescopes had an objective of one element, but a century later, two and even three element lenses were made.

Refracting telescopes use technology that has often been applied to other optical devices, such as binoculars and zoom lenses/telephoto lens/long-focus lens.

Invention

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Main article: History of the telescope

Refractors were the earliest type of optical telescope. The first record of a refracting telescope appeared in the Netherlands about 1608, when a spectacle maker from Middelburg named Hans Lippershey unsuccessfully tried to patent one.[2] News of the patent spread fast and Galileo Galilei, happening to be in Venice in the month of May 1609, heard of the invention, constructed a version of his own, and applied it to making astronomical discoveries.[3]

Refracting telescope designs

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All refracting telescopes use the same principles. The combination of an objective lens 1 and some type of eyepiece 2 is used to gather more light than the human eye is able to collect on its own, focus it 5, and present the viewer with a brighter, clearer, and magnified virtual image 6.

The objective in a refracting telescope refracts or bends light. This refraction causes parallel light rays to converge at a focal point; while those not parallel converge upon a focal plane. The telescope converts a bundle of parallel rays to make an angle α, with the optical axis to a second parallel bundle with angle β. The ratio β/α is called the angular magnification. It equals the ratio between the retinal image sizes obtained with and without the telescope.[4]

Refracting telescopes can come in many different configurations to correct for image orientation and types of aberration. Because the image was formed by the bending of light, or refraction, these telescopes are called refracting telescopes or refractors.

Galilean telescope

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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

The design Galileo Galilei used c. 1609 is commonly called a Galilean telescope.[5] It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610).[6] A Galilean telescope, because the design has no intermediary focus, results in a non-inverted (i.e., upright) image.[7]

Galileo's most powerful telescope, with a total length of just under 1 meter (39 in),[5] magnified objects about 30 times.[7] Galileo had to work with the poor lens technology of the time, and found he had to use aperture stops to reduce the diameter of the objective lens (increase its focal ratio) to limit aberrations, so his telescope produced blurry and distorted images with a narrow field of view.[7] Despite these flaws, the telescope was still good enough for Galileo to explore the sky. He used it to view craters on the Moon,[8] the four largest moons of Jupiter,[9] and the phases of Venus.[10]

Parallel rays of light from a distant object (y) would be brought to a focus in the focal plane of the objective lens (F′ L1 / y′). The (diverging) eyepiece (L2) lens intercepts these rays and renders them parallel once more. Non-parallel rays of light from the object traveling at an angle α1 to the optical axis travel at a larger angle (α2 > α1) after they passed through the eyepiece. This leads to an increase in the apparent angular size and is responsible for the perceived magnification.[citation needed]

The final image (y″) is a virtual image, located at infinity and is the same way up (i.e., non-inverted or upright) as the object.[citation needed]

Keplerian telescope

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Engraved illustration of a 46 m (150 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.[11]

The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design.[12] It uses a convex lens as the eyepiece instead of Galileo's concave one. The advantage of this arrangement is that the rays of light emerging from the eyepiece[dubious – discuss] are converging. This allows for a much wider field of view and greater eye relief, but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design, but, like the Galilean telescope, it still uses a simple single element objective lens so it needs to have a very high focal ratio to reduce aberrations[13] (Johannes Hevelius built an unwieldy f/225 telescope with a 200-millimetre (8 in) objective and a 46-metre (150 ft) focal length,[14][page needed] and even longer tubeless "aerial telescopes" were constructed). The design also allows for use of a micrometer at the focal plane (to determine the angular size and/or distance between objects observed).

Huygens built an aerial telescope for Royal Society of London with a 19 cm (7.5″) single-element lens.[15]

Achromatic refractors

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Main article: Achromatic telescope
Alvan Clark polishes the big Yerkes achromatic objective lens, over 1 meter (100 cm) across (1896).
This 12-inch (30 cm) refractor is mounted in a dome on a mount that matches the Earth's rotation.

The next major step in the evolution of refracting telescopes was the invention of the achromatic lens, a lens with multiple elements that helped solve problems with chromatic aberration and allowed shorter focal lengths. It was invented in 1733 by an English barrister named Chester Moore Hall, although it was independently invented and patented by John Dollond around 1758. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion, 'crown' and 'flint glass', to reduce chromatic and spherical aberration. Each side of each piece is ground and polished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.[citation needed]

Chester More Hall is noted as having made the first twin color corrected lens in 1730.[16]

Dollond achromats were quite popular in the 18th century.[17][18] A major appeal was they could be made shorter.[18] However, problems with glass making meant that the glass objectives were not made more than about four inches (10 cm) in diameter.[18]

In the late 19th century, the Swiss optician Pierre-Louis Guinand[19] developed a way to make higher quality glass blanks of greater than four inches (10 cm).[18] He passed this technology to his apprentice Joseph von Fraunhofer, who further developed this technology and also developed the Fraunhofer doublet lens design.[18] The breakthrough in glass making techniques led to the great refractors of the 19th century, that became progressively larger through the decade, eventually reaching over 1 meter by the end of that century before being superseded by silvered-glass reflecting telescopes in astronomy.[citation needed]

Noted lens makers of the 19th century include:[20]

The Greenwich 28-inch (71 cm) refractor is a popular tourist attraction in 21st century London.

Some famous 19th century doublet refractors are the James Lick telescope (91 cm/36 in) and the Greenwich 28 inch refractor (71 cm). An example of an older refractor is the Shuckburgh telescope (dating to the late 1700s). A famous refractor was the "Trophy Telescope", presented at the 1851 Great Exhibition in London. The era of the 'great refractors' in the 19th century saw large achromatic lenses, culminating with the largest achromatic refractor ever built, the Great Paris Exhibition Telescope of 1900.[citation needed]

In the Royal Observatory, Greenwich an 1838 instrument named the Sheepshanks telescope includes an objective by Cauchoix.[26] The Sheepshanks had a 6.7-inch (17 cm) wide lens, and was the biggest telescope at Greenwich for about twenty years.[27]

An 1840 report from the Observatory noted of the then-new Sheepshanks telescope with the Cauchoix doublet:[28]

The power and general goodness of this telescope make it a most welcome addition to the instruments of the observatory

In the 1900s a noted optics maker was Zeiss.[29] An example of prime achievements of refractors, over 7 million people have been able to view through the 12-inch Zeiss refractor at Griffith Observatory since its opening in 1935; this is the most people to have viewed through any telescope.[29]

Achromats were popular in astronomy for making star catalogs, and they required less maintenance than metal mirrors. Some famous discoveries using achromats are the planet Neptune and the Moons of Mars.[citation needed]

The long achromats, despite having smaller aperture than the larger reflectors, were often favored for "prestige" observatories. In the late 18th century, every few years, a larger and longer refractor would debut.[citation needed]

For example, the Nice Observatory debuted with 77-centimeter (30.31 in) refractor, the largest at the time, but was surpassed within only a couple of years.[30]

Apochromatic refractors

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Main article: Apochromat
Apochromat lens.svg
The Apochromatic lens usually comprises three elements that bring light of three different frequencies to a common focus

Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (tertiary spectrum) can be an order of magnitude less than that of an achromatic lens.[citation needed] Such telescopes contain elements of fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration.[31] Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture.

In the 18th century, Dollond, a popular maker of doublet telescopes, also made a triplet, although they were not really as popular as the two element telescopes.[18]

One of the famous triplet objectives is the Cooke triplet, noted for being able to correct the Seidal aberrations.[32] It is recognized as one of the most important objective designs in the field of photography.[33][34] The Cooke triplet can correct, with only three elements, for one wavelength, spherical aberration, coma, astigmatism, field curvature, and distortion.[34]

Technical considerations

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The 102 centimetres (40 in) refractor, at Yerkes Observatory, the largest achromatic refractor ever put into astronomical use (photo taken on 6 May 1921, as Einstein was visiting)

Refractors suffer from residual chromatic and spherical aberration. This affects shorter focal ratios more than longer ones. An f/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects); an f/16 achromat has much less color fringing.

In very large apertures, there is also a problem of lens sagging, a result of gravity deforming glass. Since a lens can only be held in place by its edge, the center of a large lens sags due to gravity, distorting the images it produces. The largest practical lens size in a refracting telescope is around 1 meter (39 in).[35]

There is a further problem of glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished in reflecting telescopes, which can be made in far larger apertures and which have all but replaced refractors for astronomical research.

The ISS-WAC on the Voyager 1/2 used a 6 centimetres (2.4 in) lens, launched into space in the late 1970s, an example of the use of refractors in space.[36]

Applications and achievements

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The "Große Refraktor", a double telescope with a 80cm (31.5") and 50 cm (19.5") lens, was used to discover calcium as an interstellar medium in 1904.
Astronaut trains with camera with large lens

Refracting telescopes were noted for their use in astronomy as well as for terrestrial viewing. Many early discoveries of the Solar System were made with singlet refractors.

The use of refracting telescopic optics are ubiquitous in photography, and are also used in Earth orbit.

One of the more famous applications of the refracting telescope was when Galileo used it to discover the four largest moons of Jupiter in 1609.[contradictory] Early refractors were also used several decades later to discover Titan, the largest moon of Saturn, along with three more of Saturn's moons.

In the 19th century, refracting telescopes were used for pioneering work on astrophotography and spectroscopy, and the related instrument, the heliometer, was used to calculate the distance to another star for the first time. Their modest apertures did not lead to as many discoveries and typically so small in aperture that many astronomical objects were simply not observable until the advent of long-exposure photography, by which time the reputation and quirks of reflecting telescopes were beginning to exceed those of the refractors. Despite this, some discoveries include the moons of Mars, a fifth moon of Jupiter, and many double star discoveries including Sirius (the Dog star). Refractors were often used for positional astronomy, besides from the other uses in photography and terrestrial viewing.

Touristic telescope pointed to Matterhorn in Switzerland
Singlets

The Galilean moons and many other moons of the solar system, were discovered with single-element objectives and aerial telescopes.

Galileo Galilei's discovered the Galilean satellites of Jupiter in 1610[contradictory] with a refracting telescope.[37]

The planet Saturn's moon, Titan, was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens.[38][39]

Doublets

In 1861, the brightest star in the night sky, Sirius, was found to have smaller stellar companion using the 18 and half-inch Dearborn refracting telescope.

By the 18th century refractors began to have major competition from reflectors, which could be made quite large and did not normally suffer from the same inherent problem with chromatic aberration. Nevertheless, the astronomical community continued to use doublet refractors of modest aperture in comparison to modern instruments. Noted discoveries include the moons of Mars and a fifth moon of Jupiter, Amalthea.

Asaph Hall discovered Deimos on 12 August 1877 at about 07:48 UTC and Phobos on 18 August 1877, at the US Naval Observatory in Washington, D.C., at about 09:14 GMT (contemporary sources, using the pre-1925 astronomical convention that began the day at noon,[40] give the time of discovery as 11 August 14:40 and 17 August 16:06 Washington mean time respectively).[41][42][43]

The telescope used for the discovery was the 26-inch (66 cm) refractor (telescope with a lens) then located at Foggy Bottom.[44] In 1893 the lens was remounted and put in a new dome, where it remains into the 21st century.[45]

Jupiter's moon Amalthea was discovered on 9 September 1892, by Edward Emerson Barnard using the 36-inch (91 cm) refractor telescope at Lick Observatory.[46][47] It was discovered by direct visual observation with the doublet-lens refractor.[37]

In 1904, one of the discoveries made using Great Refractor of Potsdam (a double telescope with two doublets) was of the interstellar medium.[48] The astronomer Professor Hartmann determined from observations of the binary star Mintaka in Orion, that there was the element calcium in the intervening space.[48]

Triplets

Planet Pluto was discovered by looking at photographs (i.e. 'plates' in astronomy vernacular) in a blink comparator taken with a refracting telescope, an astrograph with a 3 element 13-inch lens.[49][50]

For a chronological guide, see Timeline of discovery of Solar System planets and their moons.

List of the largest refracting telescopes

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The Yerkes Great refractor mounted at the 1893 World's Fair in Chicago; the tallest, longest, and biggest aperture refractor up to that time.
The 68 cm (27 in) refractor at the Vienna University Observatory

Examples of some of the largest achromatic refracting telescopes, over 60 cm (24 in) diameter.

See also

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Refracting telescope

View on Grokipedia
from Grokipedia
A refracting telescope, also known as a refractor, is an that utilizes one or more lenses to gather and focus incoming light rays, thereby magnifying distant objects and forming a viewable image. The primary component, called the objective lens, is typically convex and positioned at the front of the tube, where it refracts parallel light rays from a distant source to converge at a focal point inside the telescope. An lens, placed near the observer's eye, then intercepts this focused image and magnifies it for detailed viewing, often achieving magnifications from a few times up to 30 times or more in early designs. The refracting telescope's origins trace back to 1608 in the , where spectacle makers Hans Lippershey, , and Jacob Metius independently developed the first practical instruments, initially for terrestrial applications like surveying and military observation. In 1609, Italian constructed his own version after learning of the Dutch "perspective glass," refining it into a tool for astronomical use that revealed Jupiter's moons, the , and the rugged surface of the , as detailed in his 1610 publication . Early refractors followed two main configurations: the design, featuring a concave for an upright image but limited , and the later Keplerian design, introduced by , which used a convex for inverted images and wider fields, though it required additional to correct orientation. Refractors operate on the fundamental optical principle of , where bends as it passes from air into glass due to a change in speed, allowing the lens to concentrate faint celestial into a brighter, sharper image. Their advantages include a sealed tube that protects the optics from dust and weather, producing high-contrast images without central obstructions, making them ideal for observing planets, the , and double stars. However, limitations such as —where different wavelengths of focus at slightly different points, causing color fringing—restrict their use for large-scale astronomy, as do challenges in large, flawless lenses without sagging under . The largest operational refracting telescope is the 40-inch (1.02 m) instrument at , completed in 1897, which remains the maximum practical size for astronomical research due to these engineering constraints. Despite these drawbacks, refractors continue to play a vital role in education, , and specialized professional observations.

History

Invention and early development

The invention of the refracting telescope originated among Dutch spectacle makers in the early , amid a thriving trade in optical lenses for eyeglasses. On October 2, 1608, Hans Lippershey, a master lens grinder and spectacle maker based in Middelburg, , petitioned the States General for a 30-year on an optical device he termed a kijker (meaning "looker" or "spyglass"). This instrument consisted of a convex objective lens and a concave eyepiece lens housed in a tube, producing an upright, magnified image suitable for distant viewing, with an initial of approximately 3x. Although the was denied—due to independent similar inventions by others, such as in Middelburg and Jacob Metius, a fellow spectacle maker in —the device represented the first documented refracting telescope, crudely assembled from off-the-shelf spectacle lenses. Word of the Dutch kijker spread rapidly through European merchant and diplomatic channels, reaching by mid-1609. There, , a professor of mathematics in , learned of the invention and promptly built his own version without seeing an example, drawing on descriptions alone. Recognizing its potential, Galileo refined the design by personally grinding and polishing lenses to higher quality, constructing telescopes with magnifications ranging from 3x to 20x or more, far surpassing the originals in clarity and power. These improvements addressed the limitations of spectacle-grade glass, which often distorted images, enabling more precise observations. Galileo's enhanced telescopes facilitated his pioneering astronomical applications, transforming the spyglass from a novelty into a . In late 1609 and early 1610, he turned the device skyward, observing the rugged surface of the and, on January 7, 1610, discovering four satellites —now known as the —which demonstrated that not all celestial bodies revolved around . He published these findings in (Starry Messenger) in March 1610, crediting the Dutch origins while emphasizing his modifications. The Dutch spectacle-making community's expertise in convex and concave lenses was crucial to this breakthrough, fostering an environment of optical experimentation in Middelburg and nearby towns. Galileo's independent refinements and astronomical focus propelled the refracting telescope's adoption, paving the way for subsequent designs like the Keplerian variant.

Key historical milestones

In 1611, proposed an improved telescope design in his treatise Dioptrice, suggesting the use of a convex lens in combination with a convex objective lens, which produced an inverted but allowed for higher and a wider compared to the earlier configuration with a concave . This Keplerian arrangement laid the foundation for subsequent astronomical refractors, enabling clearer views of celestial objects despite the image inversion. In the 1660s, astronomers such as and at used refractors for observations, contributing to the integration of telescopes into early scientific practices. A major advancement came in 1758 when English optician John Dollond developed the by combining convex crown glass with concave elements, significantly reducing that had plagued earlier refractors by causing colored fringes around images. Dollond's for this doublet design revolutionized refractor quality, enabling sharper planetary and stellar views and spurring widespread adoption in the . In the 1810s, German physicist and optician advanced technology further by producing high-quality objectives through precise measurements of refractive indices of various glass types, yielding exceptionally clear images for astronomical use. These innovations, refined through Fraunhofer's optical research, set new standards for refractor performance and influenced large-scale instrument production. The 19th century saw refractors grow in scale, exemplified by American lensmaker Alvan Clark's construction of an 18-inch aperture refractor installed at Dearborn Observatory in 1864, which became one of the largest operational instruments of its time and enabled detailed studies of double stars and faint objects. Clark's craftsmanship in grinding high-quality achromatic objectives propelled the era of "great refractors," with this telescope remaining a benchmark for optical excellence until surpassed by even larger models. Key discoveries underscored these technological strides; in 1671, used a 17-foot refracting telescope at the to resolve finer details in Saturn's rings, including early indications of gaps that he later confirmed as the prominent division in 1675. Similarly, William Herschel's planetary observations in the 1780s, though primarily conducted with his innovative reflectors, highlighted limitations in refractor designs and spurred improvements in lens quality and mounting stability for competing instruments.

Optical Principles

Basic ray optics and image formation

A refracting telescope is an optical instrument that employs a convex objective lens to refract incoming parallel light rays from a distant object, converging them to a focal point where a real, inverted image is formed; this image is then magnified and viewed through an eyepiece lens. The objective lens, typically the larger of the two, collects light over its aperture diameter DD, gathering more photons from faint celestial sources compared to the unaided eye, while the eyepiece acts as a simple magnifier to enlarge the intermediate image for comfortable viewing. In the basic configuration, parallel rays from an infinitely distant object, such as a star, enter the objective parallel to the optical axis and are bent by refraction toward the focal point on the axis, with off-axis rays forming the inverted image in the focal plane. The focal length ff of the objective lens, which determines the position of the image plane, is governed by the lensmaker's formula for a thin lens in air: 1f=(n−1)(1R1−1R2)\frac{1}{f} = (n - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) where nn is the refractive index of the lens material, and R1R_1 and R2R_2 are the radii of curvature of the lens surfaces (with the sign convention that R1R_1 is positive for a convex surface facing the incoming light and R2R_2 negative for the opposite surface). This formula arises from the geometry of refraction at spherical surfaces and highlights how the telescope's magnifying power depends on precise lens shaping and material selection to achieve a long focal length for the objective relative to the eyepiece. For ideal image formation, the eyepiece is positioned such that the intermediate real image lies at or just inside its focal plane, producing a virtual, magnified image at infinity for relaxed viewing. The angular magnification MM of a Keplerian refracting telescope, which uses two convex lenses, is given by M=−fo/feM = -f_o / f_e, where fof_o is the of and fef_e that of the ; the negative sign indicates an inverted image. This enhances the apparent angular of the object without altering its physical scale, allowing observers to discern fine details in extended astronomical features like planetary disks. The telescope's resolving power, limited by , is characterized by the Rayleigh criterion, which defines the minimum resolvable angular separation θ\theta as θ=1.22λ/D\theta = 1.22 \lambda / D, where λ\lambda is the of and DD the diameter—larger apertures thus yield sharper images by reducing this limit.

Lens aberrations and corrections

In refracting telescopes, arises because the of glass varies with wavelength, causing different colors of light to focus at different points along the , resulting in colored fringes around images and reduced contrast. This axial separation of focal points, known as the secondary spectrum, persists even after primary correction, limiting resolution in single-lens objectives. Achromatic doublets address this by combining two lens elements, typically a convex crown glass lens (low dispersion) and a concave flint glass lens (high dispersion), cemented together to bring two wavelengths—often the red C-line (656 nm) and blue F-line (486 nm)—to a common focus. The effective of the achromat is given by the thin-lens approximation for the combined system: 1fach=(nc−1)(1R1−1R2)+(nf−1)(1R3−1R4),\frac{1}{f_\text{ach}} = (n_c - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) + (n_f - 1) \left( \frac{1}{R_3} - \frac{1}{R_4} \right), where ncn_c and nfn_f are the refractive indices of crown and flint glasses, and R1R_1 to R4R_4 are the radii of curvature. Dispersion is minimized when the ratio of Abbe numbers (a measure of dispersive power, ν=(nd−1)/(nF−nC)\nu = (n_d - 1)/(n_F - n_C)) satisfies νc/νf≈(nf−1)/(nc−1)\nu_c / \nu_f \approx (n_f - 1)/(n_c - 1), ensuring the chromatic dispersions cancel. This design reduces color fringing but leaves residual secondary spectrum for other wavelengths. Spherical aberration in refractors occurs due to the spherical shape of lens surfaces, where marginal rays from off-axis points focus closer to the lens than paraxial rays, blurring the across the field. This monochromatic defect affects the entire field uniformly and cannot be fully eliminated by refocusing, degrading sharpness even at . Correction typically involves multi-element lenses where individual elements introduce equal but opposite , or aspheric surfaces to match the ideal conic profile, though the latter increases manufacturing complexity. Off-axis aberrations such as coma, astigmatism, and field curvature further distort images away from the optical axis in simple refractors. Coma produces comet-like tails on point sources due to varying focal lengths across the aperture for oblique rays; astigmatism creates elliptical or crossed-line foci from differing curvatures in meridional and sagittal planes; and field curvature bends the image plane into a sphere, sharpening edges at the expense of the center on flat sensors. Apochromatic objectives, using three or more elements including low-dispersion materials like fluorite (CaF₂), correct chromatic aberration for three wavelengths (e.g., red, green, blue) while simultaneously reducing these monochromatic aberrations through optimized spacing and curvatures. For instance, fluorite triplets achieve near-diffraction-limited performance across 440–670 nm with Strehl ratios above 0.95, minimizing spherochromatism and off-axis blur. These corrections introduce trade-offs, as multi-element designs increase optical complexity, raise costs due to precision grinding and exotic glasses, and add weight that strains mountings in larger telescopes. The shift from single-element Huygenian objectives to compound systems marked a fundamental in refractor performance, balancing aberration control against practical constraints.

Designs and Types

Galilean and Keplerian telescopes

The Galilean telescope, developed by around 1609, consists of a convex objective lens and a concave eyepiece lens. The objective focuses incoming parallel rays toward a point before the eyepiece, which diverges them to produce an erect without forming an intermediate . Its angular magnification is given by M=fobj∣feye∣M = \frac{f_{\text{obj}}}{|f_{\text{eye}}|}, where fobjf_{\text{obj}} is the of the objective and feyef_{\text{eye}} is the negative of the eyepiece. This design yields a limited field of view, typically around 15 arcminutes, preventing the use of crosshairs or reticules since no plane exists for such attachments. In contrast, the Keplerian telescope, proposed by in his 1611 treatise Dioptrice, employs a convex objective lens paired with a convex lens. The objective forms a real, inverted intermediate image at its focal plane, which the then magnifies, allowing for the placement of reticules or measuring devices at that plane for precise astronomical observations. This configuration supports higher magnification potential and a wider compared to the design, though the image remains inverted. The design gained practical adoption in the 1630s through astronomers like Christoph Scheiner. Early constructions of these telescopes featured simple wooden or leather-covered tubes. Galilean models, such as Galileo's own instruments from 1609–1610, had tube lengths of approximately 3 feet (e.g., 927 mm for a 21× example with a 37 mm objective) and were compact enough for handheld use, including as glasses achieving up to 20× . Keplerian telescopes required longer tubes, often 15–20 feet by the mid-17th century, to accommodate the separation of the two positive focal lengths, as seen in ' 23-foot, 100× instrument from 1656. The design offers simplicity and lower cost, making it suitable for terrestrial viewing with its , but it suffers from distortion, narrow field, and inability to support accessories like crosshairs. The Keplerian variant excels in astronomical applications due to its brighter, wider-field views and compatibility with reticules, despite the inverted orientation, which poses minimal issue for celestial targets.

Achromatic and apochromatic refractors

The achromatic refractor employs a two-element objective lens, typically comprising a convex crown glass element paired with a concave element, a design patented by John Dollond in 1758. This configuration corrects for two wavelengths—usually red and blue—by balancing the dispersion properties of the glasses, thereby minimizing the violet-blue fringing that plagued earlier single-lens refractors and enabling sharper, color-corrected images. Dollond's innovation, which built on earlier concepts but was the first commercially viable implementation, earned him the from the Royal Society and revolutionized telescope optics. These instruments dominated 18th- and 19th-century astronomy, with apertures commonly ranging from 2 to 6 inches in early examples and extending up to 12 inches in later professional models, such as those produced by the Dollond firm and successors like Alvan Clark. Air-spaced doublets were the standard to optimize monochromatic aberrations alongside chromatic correction, allowing for practical use in both visual and early photographic applications despite residual secondary spectrum effects. Achromats proved suitable for visual astronomy up to approximately 150× , where chromatic fringing becomes noticeable on bright objects like the or under average seeing conditions. Apochromatic refractors advance this further with three-element objectives that correct for three wavelengths—typically red, green, and blue—substantially reducing the secondary spectrum and higher-order aberrations for even crisper images. Peter Dollond, building on his father's legacy, further refined apochromatic principles in the late with three-lens configurations that minimized residual color errors. Design evolution progressed to oil-spaced doublets and air-spaced triplets in the , enhancing correction while managing and mechanical stability; modern apochromats often feature or extra-low dispersion (ED) elements in 4- to 6-inch apertures, making them ideal for amateur visual observing and . These scopes excel in high-end applications, delivering sub-1 arcsecond resolution for planetary and imaging, limited primarily by atmospheric seeing rather than optical flaws.

Specialized variants

Terrestrial refractors are adapted versions of standard refractor designs that incorporate erecting prisms or additional lenses to produce upright, laterally correct images, addressing the inverted view inherent in basic Keplerian configurations. These modifications make them suitable for daytime observation of landscapes, wildlife, and distant objects, where an is essential for natural orientation. Erecting prisms, often in the form of Amici roof prisms or Porro prisms, are inserted between the objective and to flip and revert the image without significant loss of light transmission. Such designs are commonly employed in spotting scopes, which are compact refractors typically ranging from 50mm to 100mm in , offering magnifications of 20x to 60x for applications like or surveillance. also frequently utilize similar erecting prism systems within a refractor framework, providing stereoscopic upright views in portable formats. Monocentric designs represent an early specialized refractor variant focused on achieving wide-field views with minimal distortion, featuring s composed of concentric spherical lenses that form a curved focal surface. Invented by Hugo Adolf Steinheil in 1883, the monocentric consists of three solid glass elements sharing a common , which inherently corrects for and provides a sharp field of about 32 degrees while maintaining achromatic and orthoscopic performance. This configuration excels in low-distortion panoramic observation, making it particularly valuable in 19th-century periscopes, where it enabled wide-angle with reduced edge blurring compared to conventional flat-field s. Although now largely obsolete for general use due to advancements in multi-element optics, monocentric principles influenced early wide-field adaptations for tactical viewing in confined spaces. Petzval lenses, originally developed in 1840 by Joseph Petzval for , have been adapted into specialized refractor telescopes featuring a four-element configuration—typically a doublet objective followed by a cemented doublet field flattener—that achieves fast focal ratios around f/3 to f/5 with a flat focal plane. This design minimizes field curvature and , allowing for sharp, high-contrast images across a wide field without requiring additional flatteners, which is advantageous for imaging applications. In astronomical contexts, Petzval refractors have been employed for lunar and planetary observation, where their rapid focal ratios enable shorter exposure times and detailed capture of surface features like craters and atmospheric bands on . Modern examples, such as the William Optics RedCat series, leverage this layout in compact apochromatic refractors for portable planetary imaging, delivering well-corrected views rivaling slower traditional designs. Boundary cases like catadioptric hybrids blur the line between pure refractors and reflector systems, but pure refractor variants emphasize all-lens ; for instance, the Maksutov design integrates a meniscus lens corrector with mirrors. Oil-immersion techniques, common in for enhancing resolution via high-refractive-index fluids between lens and specimen, have limited application in refractors. Modern portable refractors often incorporate extra-low dispersion (ED) or singlets, such as in Takahashi's FS-series, where a single element provides superior over standard crown , enabling lightweight scopes under 5 pounds for travel astronomy with minimal chromatic fringes on bright objects. Solar refractors represent a niche variant equipped with narrowband filters, particularly hydrogen-alpha (H-alpha) etalons, to safely observe chromospheric features like prominences without full-disk white-light projection. These telescopes use objective lenses of 50mm to 150mm combined with bandpass filters tuned to 0.5–1 Å in the 656.3 nm line, allowing transmission of only solar emission from ionized to reveal dynamic plasma loops and filaments at the solar limb. Devices like the DayStar eyepiece filters attach to standard refractors, converting them into prominence viewers with energy rejection front filters to block harmful and UV , achieving safe magnifications up to 100x for detailed prominence structure. Such systems prioritize limb viewing over central disk details, providing astronomers with insights into solar activity cycles.

Technical Aspects

Lens fabrication and materials

The fabrication of lenses for refracting telescopes has evolved significantly since the early 17th century, when pioneers like relied on manual hand-grinding and polishing techniques using rudimentary tools such as copper or bronze laps and abrasives like emery or sand. These methods involved shaping glass blanks by hand to approximate spherical surfaces, a labor-intensive process that limited lens quality and size due to inconsistencies in curvature and surface smoothness. By the , advancements in machine tools, including lathes and precision grinders, enabled more uniform grinding and polishing, allowing for larger and more accurate lenses, such as those in the 1-meter Yerkes refractor. In modern production, computer numerical control (CNC) machines facilitate the fabrication of aspheric surfaces through and computer-controlled grinding, which generate complex profiles with sub-micrometer precision, essential for correcting aberrations in high-performance refractors. Material selection for refractor lenses prioritizes like (n) and dispersion to minimize aberrations while ensuring durability. Crown glass, typically with a low around 1.52 and high (indicating low dispersion), forms the basis for objective lenses in simple refractors, providing good transmission across visible wavelengths. Flint glass, with a higher of about 1.62 and greater dispersion (lower ), is paired with crown glass in achromatic doublets to counteract by balancing the dispersion of different wavelengths. For apochromatic designs, materials like (, CaF₂, n ≈ 1.43) offer exceptionally low dispersion, enabling sharper focus across a broader without fringing. Extra-low dispersion (ED) glasses, such as those with anomalous partial dispersion (e.g., Ohara FPL series, s >90), further enhance correction in multi-element objectives by reducing residual chromatic errors beyond what alone achieves. Fabricating high-quality telescope lenses presents several challenges to achieve optical performance. Surface figure errors must be controlled to less than λ/4 (where λ is the of , typically 550 nm for green ) to avoid wavefront distortion and maintain diffraction-limited imaging, requiring iterative polishing with finer abrasives and tools like interferometers. Anti-reflection (AR) coatings, often multi-layer stacks, are applied to reduce surface reflectivity from about 4% per air-glass interface to under 1%, boosting transmission to over 98% across the visible band and minimizing images. In compound lenses, matching the coefficients of between elements (e.g., ~8-9 × 10⁻⁶ K⁻¹ for borosilicate crown and flint) prevents stress-induced or during temperature fluctuations in environments. The practical size limit for refractor objective lenses is around 1 meter in diameter, constrained by the immense weight of (density ~2.5 g/cm³) and gravitational sagging, which deforms the lens figure and introduces aberrations. For a , the central deflection δ due to self-weight approximates δ ∝ D^4 / t^3, where D is the diameter and t is the thickness; increasing D beyond 1 m requires thicker lenses to limit deflection, exacerbating weight (scaling approximately as D^{10/3} if thickness scales to maintain constant deflection) without proportional gains in resolution due to atmospheric seeing limits. This is exemplified by the Yerkes Observatory's 40-inch (1.02 m) refractor, the largest ever built, beyond which reflectors became preferable.

Mounting systems and mechanics

Mounting systems for refracting telescopes provide the structural support necessary to point the instrument accurately and track celestial objects while minimizing vibrations and flexure. The two primary types are alt-azimuth and equatorial mounts. Alt-azimuth mounts allow movement in altitude (up-down) and (left-right) directions, offering simplicity and compactness suitable for smaller refractors used in casual observing. In contrast, equatorial mounts align one axis parallel to Earth's rotational axis (the polar axis), enabling sidereal tracking by rotating only around the (RA) axis to compensate for the apparent motion of stars due to . This design became essential for precise astronomical observations, particularly with the introduction of clock drives in the , which automated tracking via gear mechanisms powered by weights or motors, as pioneered in Joseph von Fraunhofer's Great Dorpat Refractor in 1824. Among equatorial mounts, the German equatorial design—featuring a polar axis supported at one end and a axis perpendicular to it—emerged as the standard for large refractors due to its accessibility for attaching instruments and counterweights. mounts, which use a U-shaped to support the tube and create a virtual axis, are preferred for smaller refractors for their balanced stability and reduced . For refractors with apertures of 30 inches or larger, such as the Yerkes Observatory's 40-inch instrument, massive piers and foundations are required to dampen vibrations and support immense weights; the Yerkes pier, for instance, rises 65 feet tall with a foundation extending 40 feet into the ground, constructed from , , and to isolate the mount from seismic and environmental disturbances. Weight distribution is critical in these systems, with counterweights and balanced tube designs preventing that could misalign optics during tracking. Key mechanical components enhance operational reliability. Focusers, which adjust the eyepiece or detector position for sharp imaging, commonly employ rack-and-pinion mechanisms for precise geared movement in amateur and mid-sized refractors, while Crayford focusers use a friction-driven sliding carriage for smoother, backlash-free operation in higher-end models. shields, tubular extensions fitted over lens, reduce dew formation by limiting radiant cooling and blocking , thereby extending observing sessions in humid conditions. Finderscopes, small auxiliary refractors mounted parallel to the main tube, provide a wide-field view with crosshairs to locate targets before fine-pointing the primary optic. Operationally, lens collimation ensures the objective elements remain aligned within their cell, achieved by adjusting tilt screws to center defocused star images, preventing aberrations from misalignment. Large refractor objectives, often weighing hundreds of pounds due to their glass composition, require several hours to reach with ambient air to avoid turbulence-induced image degradation from internal temperature gradients.

Applications and Limitations

Astronomical and scientific uses

Refracting telescopes excel in visual astronomy, particularly for high-contrast imaging of solar system objects such as and the , where their unobstructed apertures deliver sharp, detailed views without the effects introduced by secondary mirrors in reflectors. For example, a quality 4-inch apochromatic refractor can resolve Jupiter's equatorial cloud bands and at magnifications around 200x, revealing fine atmospheric structures that benefit from the instrument's inherent contrast. Similarly, lunar observations with refractors highlight crater rims and maria with exceptional clarity, as the sealed optical tube minimizes internal reflections and formation. In double-star observing, refractors provide superior performance through high Strehl ratios—often exceeding 0.9 in modern apochromats—which concentrate light efficiently into the , enabling the resolution of close visual binaries with angular separations as small as 0.5 arcseconds under good seeing conditions. This advantage stems from the full aperture utilization and minimal wavefront errors in well-figured lenses, making refractors a preferred choice for splitting colorful pairs like or . Astrometry has long benefited from refractors' stable optics and precise focusing, facilitating accurate position measurements essential for determining stellar es and proper motions. The 40-inch refractor, for instance, was instrumental in early 20th-century parallax programs, yielding distances for hundreds of stars through photographic plates exposed over multiple years. Additionally, refractors support when fitted with slit attachments at the focal plane, allowing the isolation of stellar or planetary light for and composition analysis, as demonstrated in historical setups at observatories like . Key historical achievements underscore refractors' role in scientific discovery; in 1846, Johann Galle used the 9-inch Fraunhofer refractor at Berlin Observatory to confirm Neptune's position based on Urbain Le Verrier's predictions, marking a triumph of predictive celestial mechanics. Prior to CCD detectors, refractors enabled extensive visual and photographic asteroid surveys. In contemporary astronomy, refractors continue as versatile tools, often serving as off-axis guide scopes for large reflector or segmented-mirror telescopes to maintain precise tracking during long exposures. Among amateurs, they remain popular for visual work due to their portability, low maintenance, and contrast superiority for planetary and double-star viewing, though limited by smaller apertures compared to reflectors for faint deep-sky objects.

Terrestrial applications and constraints

Refracting telescopes find practical use in various terrestrial applications where high and clear imaging of earthly objects are required. In , theodolites incorporate refracting telescopes with erecting lenses to provide upright images for precise angle measurements in land mapping and . These instruments, often featuring plungeable telescopes for direct and reverse observations, achieve resolutions down to 0.1 arcseconds over distances up to 2 km, correcting for and curvature effects. For birding and , spotting scopes based on refractor designs offer portable, high-contrast views with typical zoom ranges of 20-60x, using objective lenses of 50-100 mm to capture details of distant animals without disturbing them. Examples include models with extra-low dispersion (ED) glass for reduced aberrations, such as the ATX/STX 85 mm system, which provides sharp, color-accurate images in straight or angled configurations. In military reconnaissance, refractors have historically enabled aerial ; during , they were mounted on for detecting enemy movements from afar, while versions integrated powerful lenses with cameras for high-resolution imaging of bases and territories. Navigation relies on compact refracting s in tools like sextants, which measure angular distances between the horizon and celestial bodies for determining position at sea. The sextant's aligns the reflected image of a or the Sun with the horizon, allowing mariners to read altitudes up to 130° from a graduated arc for calculations. Marine s, evolved from early 17th-century refractors, further aid in identifying ships and landmarks, with historical designs featuring convex objectives and concave eyepieces for extended-range viewing. These applications often employ configurations to produce erect images, essential for orienting terrestrial scenes. Despite these uses, refracting telescopes face significant constraints in terrestrial settings. , where different wavelengths focus at varying points, intensifies at high magnifications, causing colored fringes that blur fine details unless mitigated by achromatic or apochromatic designs. Atmospheric seeing, due to , limits resolution to approximately 1 arcsecond in typical conditions, preventing larger apertures from achieving their diffraction-limited potential. Cost and scaling issues make large refractors uneconomical compared to reflectors, as fabricating defect-free lenses becomes prohibitively expensive beyond modest sizes, while reflectors allow cheaper, larger mirrors for equivalent performance. Modern limitations include portability trade-offs, where apochromatic refractors under 100 mm balance compactness and performance for field use, as larger models grow cumbersome. in urban areas reduces contrast and visibility, diminishing utility for observing faint terrestrial features like distant or survey markers. Additionally, correcting aberrations requires higher f-ratios in refractors, resulting in longer tube lengths that hinder mobility relative to compact reflectors.

Notable Examples

Largest refracting telescopes

The largest refracting telescope ever constructed for astronomical research is the 40-inch (1.02 m) instrument at in Williams Bay, Wisconsin, USA, completed in 1897 by Alvan G. Clark & Sons. This achromatic refractor, with a of 19.3 m and a tube length of 19 m, remains operational and was primarily used for high-resolution of stars and planets, contributing to early 20th-century studies of stellar atmospheres (as of 2025, used for public outreach). The second-largest is the 36-inch (0.91 m) Great Lick Refractor at Lick Observatory on Mount Hamilton, California, also built by Alvan G. Clark & Sons and dedicated in 1888. As the first major refractor mounted on a mountain site, it facilitated detailed planetary observations, including measurements of Jupiter's satellites and Saturn's rings, with a focal length of 17.37 m (as of 2025, operational for research and public viewing). In , the Meudon Great Refractor (Grande ), featuring an 83 cm (32.7-inch) visual lens and a companion 62 cm photographic lens on the same mounting, was completed in 1891 at Observatory near , , with lenses by the Henry Brothers and mounting by Gautier. This twin instrument specialized in solar research, enabling detailed spectroheliography of the Sun's (as of 2025, operational following 2023 restoration). Other notable large refractors include the 76 cm (30-inch) telescope at Pulkovo Observatory in St. Petersburg, Russia, built in 1885 by Alvan G. Clark & Sons, which advanced double-star and planetary photography until its destruction in World War II. The 60 cm (24-inch) double refractor at Bosscha Observatory in Lembang, Indonesia, completed in 1928 by Carl Zeiss, was the last major large refractor built before World War II and supported variable star and double-star observations in the southern hemisphere (as of 2025, operational).
RankTelescopeApertureYearBuilderLocationStatus (as of 2025)Primary Use
1Yerkes Great Refractor102 cm (40 in)1897Alvan G. Clark & SonsYerkes Observatory, USAOperationalSpectroscopy
2Lick Great Refractor91 cm (36 in)1888Alvan G. Clark & SonsLick Observatory, USAOperationalPlanetary imaging
3Meudon Grande Lunette83 cm (33 in) visual + 62 cm photographic1891Henry Brothers (lenses) & Gautier (mounting)Meudon Observatory, FranceOperational (restored 2023)Solar spectroscopy
4Potsdam Great Refractor80 cm (31.5 in) photographic + 50 cm visual1899Repsold & SonsPotsdam Observatory, GermanyOperational (public outreach)Meridian observations
5Nice Observatory Refractor77 cm (30 in)1886Henry & GautierNice Observatory, FranceDecommissionedAstrometry
6Pulkovo Refractor76 cm (30 in)1885Alvan G. Clark & SonsPulkovo Observatory, RussiaDestroyed (1941)Double stars
7Greenwich Great Refractor71 cm (28 in)1893Chance BrothersRoyal Greenwich Observatory, UKDecommissionedGeneral astronomy
8Vienna University Observatory Refractor69 cm (27 in)1880GrubbUniversity Observatory Vienna, AustriaDecommissionedPlanetary work
9Newall Telescope64 cm (25 in)1871Chance BrothersNational Observatory of Athens, Greece (relocated)OperationalGeneral astronomy
10US Naval Observatory Refractor66 cm (26 in)1873Alvan Clark & SonsUS Naval Observatory, USADecommissionedAstrometry
Note: The table focuses on historical general-purpose refractors larger than 60 cm; many were decommissioned after the as reflecting telescopes became dominant. Rankings by largest ; double instruments specified. The era of large refractors peaked around , after which construction declined sharply due to the prohibitive cost of fabricating and supporting massive, high-quality lenses, as well as inherent limitations like chromatic aberrations that worsened with scale and lens sagging under gravity. Reflecting telescopes, which avoided these issues by using mirrors, offered greater s at lower cost, rendering large refractors obsolete for most research by the mid-20th century.

Iconic historical instruments

One of the earliest and most influential refracting telescopes was constructed by in 1609, shortly after the device's invention in the . Galileo's instrument featured a convex objective lens and a concave eyepiece, achieving magnifications of about 20 to 30 times with apertures of roughly 1 to 2 inches. These simple refractors enabled groundbreaking observations, including the four largest , the , and the detailed topography of the Moon's surface, as documented in his 1610 publication . In the mid-17th century, Dutch scientist advanced refractor design through his personally crafted telescopes, which had apertures exceeding 2 inches and focal lengths up to 23 feet, providing magnifications around 100 times. Using these instruments starting in 1655, Huygens discovered Titan, Saturn's largest moon, and provided the first accurate description of Saturn's in his 1659 work Systema Saturnium. To address optical limitations, Huygens invented the two-lens in 1662, which compensated for and became a standard component in subsequent refractors. Johannes Hevelius, a Polish astronomer, pushed the limits of early refractor construction in the 1670s by building exceptionally long aerial s without tubes to reduce weight and flexure. His most notable instrument was a 150-foot refractor with an 8-inch , equivalent to an f/225 , designed to minimize chromatic blur through extreme length rather than advanced . This contributed to Hevelius's precise lunar mappings and stellar observations, detailed in works like Selenographia (updated editions) and Machina Coelestis (1673), though its impracticality for precise tracking highlighted the challenges of pre-achromatic designs. A pivotal advancement occurred in 1758 when English optician John Dollond patented the achromatic refractor, combining and lenses to correct chromatic and spherical aberrations. Dollond's early models, such as 4-inch instruments with focal lengths around 63 inches, offered sharper images and higher magnifications without the color fringing of single-lens designs, revolutionizing practical astronomy. These telescopes were commercially produced and used extensively, including by royal observatories, establishing Dollond's firm as a leader in optical instrumentation. In the , the 36-inch Great Lick Refractor, completed in 1888 at in , represented the zenith of large-scale refracting technology. Built by Alvan Clark & Sons with a 36-inch and 57-foot , it was the world's largest operational refractor upon commissioning and facilitated key discoveries in stellar and double-star measurements (as of 2025, still operational). Its equatorial mounting and advanced clock drive allowed for stable, long-duration observations, underscoring the refractor's role in late historical astronomy before reflectors dominated.

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