Refracting telescope

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Image of a refracting telescope from the Cincinnati Observatory in 1848 Refractor Cincinnati observatory.jpg
Image of a refracting telescope from the Cincinnati Observatory in 1848

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 spy glasses 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were very popular in the second half of the 19th century, for most research purposes the refracting telescope has been superseded by the reflecting telescope which allows larger apertures.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]

Optical telescope Telescope for observations with visible light

An optical telescope is a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct view, or to make a photograph, or to collect data through electronic image sensors.

Lens (optics) optical device which transmits and refracts light

A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis. Lenses are made from materials such as glass or plastic, and are ground and polished or molded to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly focus or disperse waves and radiation other than visible light are also called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses.

Objective (optics) optical element that gathers light from the object being observed and focuses the light rays to produce a real image

In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be a single lens or mirror, or combinations of several optical elements. They are used in microscopes, telescopes, cameras, slide projectors, CD players and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.

Contents

Invention

Refractors were the earliest type of optical telescope. The first practical refracting telescopes appeared in the Netherlands about 1608, and were credited to three individuals, Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar. Galileo Galilei, happening to be in Venice in about the month of May 1609, heard of the invention and constructed a version of his own. Galileo then communicated the details of his invention to the public, and presented the instrument itself to the Doge Leonardo Donato, sitting in full council. [2] [3]

Netherlands Constituent country of the Kingdom of the Netherlands in Europe

The Netherlands is a country located mainly in Northwestern Europe. The European portion of the Netherlands consists of twelve separate provinces that border Germany to the east, Belgium to the south, and the North Sea to the northwest, with maritime borders in the North Sea with Belgium, Germany and the United Kingdom. Together with three island territories in the Caribbean Sea—Bonaire, Sint Eustatius and Saba— it forms a constituent country of the Kingdom of the Netherlands. The official language is Dutch, but a secondary official language in the province of Friesland is West Frisian.

Hans Lippershey German-Dutch spectacle-maker

Hans Lippershey, also known as Johann Lippershey or Lipperhey, was a German-Dutch spectacle-maker. He is commonly associated with the invention of the telescope, because he was the first one who tried to obtain a patent for it. It is, however, unclear if he was the first one to build a telescope.

Zacharias Janssen Dutch scientist

Zacharias Janssen was a Dutch spectacle-maker from Middelburg associated with the invention of the first optical telescope. Janssen is sometimes also credited for inventing the first truly compound microscope. However, the origin of the microscope, just like the origin of the telescope, is a matter of debate.

Refracting telescope designs

Kepschem.png

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.

Eyepiece type of lens attached to a variety of optical devices such as telescopes and microscopes; usually the lens that is closest to the eye when someone looks through the device, placed near the focal point of the objective to magnify the image

An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.

Brightness perception of light level

Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. It is not necessarily proportional to luminance. This is a subjective attribute/property of an object being observed and one of the color appearance parameters of color appearance models. Brightness refers to an absolute term and should not be confused with Lightness.

Magnification process of enlarging something only in appearance, not in physical size

Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to a reduction in size, sometimes called minification or de-magnification.

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]

Refraction refraction of light

In physics refraction is the change in direction of a wave passing from one medium to another or from a gradual change in the medium. Refraction of light is the most commonly observed phenomenon, but other waves such as sound waves and water waves also experience refraction. How much a wave is refracted is determined by the change in wave speed and the initial direction of wave propagation relative to the direction of change in speed.

Light electromagnetic radiation in or near visible spectrum

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is the visible spectrum that is visible to the human eye and is responsible for the sense of sight. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared and the ultraviolet. This wavelength means a frequency range of roughly 430–750 terahertz (THz).

In geometry, parallel lines are lines in a plane which do not meet; that is, two lines in a plane that do not intersect or touch each other at any point are said to be parallel. By extension, a line and a plane, or two planes, in three-dimensional Euclidean space that do not share a point are said to be parallel. However, two lines in three-dimensional space which do not meet must be in a common plane to be considered parallel; otherwise they are called skew lines. Parallel planes are planes in the same three-dimensional space that never meet.

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.

Galileo's telescope

Optical diagram of Galilean telescopey - 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 Galileantelescope.png
Optical diagram of Galilean telescopey – 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 in 1609 is commonly called a Galilean telescope. 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 and upright image.

Galileo Galilei Italian polymath (1564-1642)

Galileo Galilei was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Galileo has been called the "father of observational astronomy", the "father of modern physics", the "father of the scientific method", and the "father of modern science".

Galileo's best telescope magnified objects about 30 times. Because of flaws in its design, such as the shape of the lens and the narrow field of view, the images were blurry and distorted. Despite these flaws, the telescope was still good enough for Galileo to explore the sky. The Galilean telescope could view the phases of Venus, and was able to see craters on the Moon and four moons orbiting Jupiter.

Phases of Venus

The phases of Venus are the variations of lighting seen on the planet's surface, similar to lunar phases. The first recorded observations of them were telescopic observations by Galileo Galilei in 1610. Although the extreme crescent phase of Venus has been observed with the naked eye, there are no indisputable historical pre-telescopic records of it being observed.

Impact crater Circular depression on a solid astronomical body formed by a hypervelocity impact of a smaller object

An impact crater is an approximately circular depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth.

Moon Earths natural satellite

The Moon, also known as Luna, is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, and the largest among planetary satellites relative to the size of the planet that it orbits. The Moon is after Jupiter's satellite Io the second-densest satellite in the Solar System among those whose densities are known.

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.

The final image (y″) is a virtual image, located at infinity and is the same way up as the object.

Keplerian telescope

Woodcut illustration of a 46 m (150 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius. Houghton Typ 620.73.451 - Johannes Hevelius, Machinae coelestis, 1673.jpg
Woodcut illustration of a 46 m (150 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius.

The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design. [8] 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 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 to overcome aberrations the simple objective lens needs to have a very high f-ratio (Johannes Hevelius built one with a 46-metre (150 ft) focal length, and even longer tubeless "aerial telescopes" were constructed). The design also allows for use of a micrometer at the focal plane (used to determine the angular size and/or distance between objects observed).

Achromatic refractors

The achromatic refracting lens 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 limit the effects of 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. 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.

Apochromatic refractors

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 up to 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. [9] Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture.

Technical considerations

The 1.0-meter (40 in) refractor, at Yerkes Observatory, the largest achromatic refractor ever put into astronomical use Yerkes Observatory Astro4p7.jpg
The 1.0-meter (40 in) refractor, at Yerkes Observatory, the largest achromatic refractor ever put into astronomical use

Refractors suffer from residual chromatic and spherical aberration. This affects shorter focal ratios more than longer ones. A 100 mm (4 in) f/6 achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 100 mm (4 in)f/16 has little 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). [10]

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.

List of the largest refracting telescopes

A list of the largest refracting telescopes over 60 cm (24 in) diameter.

See also

Related Research Articles

In optics, aberration is a property of optical systems such as lenses that causes light to be spread out over some region of space rather than focused to a point. Aberrations cause the image formed by a lens to be blurred or distorted, with the nature of the distortion depending on the type of aberration. Aberration can be defined as a departure of the performance of an optical system from the predictions of paraxial optics. In an imaging system, it occurs when light from one point of an object does not converge into a single point after transmission through the system. Aberrations occur because the simple paraxial theory is not a completely accurate model of the effect of an optical system on light, rather than due to flaws in the optical elements.

Chromatic aberration Failure of a lens to focus all colors on the same point

In optics, chromatic aberration is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wavelength of light. The refractive index of most transparent materials decreases with increasing wavelength. Since the focal length of a lens depends on the refractive index, this variation in refractive index affects focusing. Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image.

History of the telescope aspect of history

The earliest known telescope appeared in 1608 in the Netherlands when an eyeglass maker named Hans Lippershey tried to obtain a patent on one. Although Lippershey did not receive his patent, news of the new invention soon spread across Europe. The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. Galileo improved on this design the following year and applied it to astronomy. In 1611, Johannes Kepler described how a far more useful telescope could be made with a convex objective lens and a convex eyepiece lens and by 1655 astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces.

Newtonian telescope

The Newtonian telescope, also called the Newtonian reflector or just the Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton (1642–1727), using a concave primary mirror and a flat diagonal secondary mirror. Newton's first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope. The Newtonian telescope's simple design makes it very popular with amateur telescope makers.

Apochromat

An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses.

Barlow lens

The Barlow lens, named after Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, increases the effective focal length of an optical system as perceived by all components that are after it in the system. The practical result is that inserting a Barlow lens magnifies the image. A real barlow lens is not a single glass element, because that would generate chromatic aberration, and spherical aberration if the lens is not aspheric. More common configurations use three or more elements for achromatic correction or apochromatic correction and higher image quality.

Cassegrain reflector type of telescope reflector

The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to the optical system's primary mirror entrance aperture. This design puts the focal point at a convenient location behind the primary mirror and the convex secondary adds a telephoto effect creating a much longer focal length in a mechanically short system.

Achromatic telescope type of refracting telescope

The achromatic telescope is a refracting telescope that uses an achromatic lens to correct for chromatic aberration.

Newtons reflector

The first reflecting telescope built by Sir Isaac Newton in 1668 is a landmark in the history of telescopes, being the first known successful reflecting telescope. It was the prototype for a design that later came to be called a newtonian telescope.

Aerial telescope very long focal length refracting telescope without a tube, built in the 17th century; the objective was mounted on a pole, tree, etc. on a swivel ball-joint; the observer stood on the ground and maneuvered the eyepiece to aim at celestial objects

An aerial telescope is a type of very long focal length refracting telescope, built in the second half of the 17th century, that did not use a tube. Instead, the objective was mounted on a pole, tree, tower, building or other structure on a swivel ball-joint. The observer stood on the ground and held the eyepiece, which was connected to the objective by a string or connecting rod. By holding the string tight and maneuvering the eyepiece, the observer could aim the telescope at objects in the sky. The idea for this type of telescope may have originated in the late 17th century with the Dutch mathematician, astronomer and physicist Christiaan Huygens and his brother Constantijn Huygens, Jr., though it is not clear if they actually invented it.

Galileoscope

The Galileoscope is a small mass-produced refractor telescope, designed with the intention of increasing public interest in astronomy and science. It was developed for the International Year of Astronomy 2009. The small telescope has an aperture of 50 mm and a focal length of 500 mm, yielding a relatively long focal length of f/10.

Great refractor Wikipedia disambiguation page

Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an equatorial mount. The preeminence and success of this style in observational astronomy was an era in telescope use in the 19th and early 20th century. Great refractors were large refracting telescopes using achromatic lenses. They were often the largest in the world, or largest in a region. Despite typical designs having smaller apertures than reflectors, Great refractors offered a number of advantages and were favored for astronomy.

In optics, a relay lens is a lens or lens group that inverts an image and extends the optical tube. Relay lenses are found in refracting telescopes, endoscopes and periscopes for the purpose of extending the length of the system, and before eyepieces for the purpose of inverting an image. They may be made of one or more conventional lenses or achromatic doublets, or a long cylindrical gradient-index of refraction lens.

References

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