How To Use A Reflector Telescope

Telescopes which utilize curved mirrors to course an image

24-inch convertible Newtonian/Cassegrain reflecting telescope on display at the Franklin Institute

A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. 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. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very big diameter objectives. Near all of the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may utilize extra optical elements to better image quality or place the image in a mechanically advantageous position. Since reflecting telescopes employ mirrors, the design is sometimes referred to as a catoptric telescope.

From the time of Newton to the 1800s, the mirror itself was fabricated of metal – usually speculum metallic. This type included Newton's first designs and even the largest telescopes of the 19th century, the Leviathan of Parsonstown with a one.8 meter wide metallic mirror. In the 19th century a new method using a block of drinking glass coated with very sparse layer of silver began to become more pop by the plough of the century. Mutual telescopes which led to the Crossley and Harvard reflecting telescopes, which helped establish a better reputation for reflecting telescopes as the metal mirror designs were noted for their drawbacks. Chiefly the metal mirrors only reflected about ii⁄3 of the low-cal and the metal would tarnish. After multiple polishings and tarnishings, the mirror could lose its precise figuring needed.

Reflecting telescopes became extraordinarily popular for astronomy and many famous telescopes, such as the Hubble Space Telescope, and popular amateur models use this design. In addition, the reflection telescope principle was applied to other electromagnetic wavelengths, and for example, Ten-ray telescopes too apply the reflection principle to make image-forming eyes.

History [edit]

A replica of Newton's 2d reflecting telescope that he presented to the Royal Social club in 1672

The great telescope of Birr, the Leviathan of Parsonstown. Modern day remnants of the mirror and back up structure.

The idea that curved mirrors comport like lenses dates back at least to Alhazen's 11th century treatise on optics, works that had been widely disseminated in Latin translations in early modern Europe.^[ane] Shortly after the invention of the refracting telescope, Galileo, Giovanni Francesco Sagredo, and others, spurred on by their knowledge of the principles of curved mirrors, discussed the thought of building a telescope using a mirror as the image forming objective.^[2] At that place were reports that the Bolognese Cesare Caravaggi had constructed one around 1626 and the Italian professor Niccolò Zucchi, in a afterwards work, wrote that he had experimented with a concave bronze mirror in 1616, but said it did not produce a satisfactory image.^[2] The potential advantages of using parabolic mirrors, primarily reduction of spherical aberration with no chromatic aberration, led to many proposed designs for reflecting telescopes.^[3] The almost notable being James Gregory, who published an innovative design for a 'reflecting' telescope in 1663. It would be x years (1673), before the experimental scientist Robert Hooke was able to build this type of telescope, which became known equally the Gregorian telescope.^[4] ^[5] ^[6]

Isaac Newton has been mostly credited with building the offset reflecting telescope in 1668.^[seven] It used a spherically basis metal primary mirror and a small-scale diagonal mirror in an optical configuration that has come up to be known as the Newtonian telescope.

Despite the theoretical advantages of the reflector design, the difficulty of construction and the poor functioning of the speculum metal mirrors being used at the time meant it took over 100 years for them to become pop. Many of the advances in reflecting telescopes included the perfection of parabolic mirror fabrication in the 18th century,^[8] silver coated glass mirrors in the 19th century (built by Léon Foucault in 1858),^[9] long-lasting aluminum coatings in the 20th century,^[10] segmented mirrors to permit larger diameters, and active optics to recoup for gravitational deformation. A mid-20th century innovation was catadioptric telescopes such equally the Schmidt camera, which employ both a spherical mirror and a lens (called a corrector plate) every bit primary optical elements, mainly used for wide-field imaging without spherical abnormality.

The late 20th century has seen the development of adaptive optics and lucky imaging to overcome the problems of seeing, and reflecting telescopes are ubiquitous on space telescopes and many types of spacecraft imaging devices.

Technical considerations [edit]

A curved primary mirror is the reflector telescope's basic optical element that creates an image at the focal plane. The distance from the mirror to the focal aeroplane is called the focal length. Picture show or a digital sensor may exist located hither to record the image, or a secondary mirror may be added to modify the optical characteristics and/or redirect the low-cal to moving picture, digital sensors, or an eyepiece for visual observation.

The primary mirror in about modernistic telescopes is composed of a solid glass cylinder whose front surface has been basis to a spherical or parabolic shape. A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective first surface mirror.

Some telescopes use primary mirrors which are made differently. Molten drinking glass is rotated to make its surface paraboloidal, and is kept rotating while information technology cools and solidifies. (See Rotating furnace.) The resulting mirror shape approximates a desired paraboloid shape that requires minimal grinding and polishing to reach the verbal figure needed.^[eleven]

Optical errors [edit]

Reflecting telescopes, just like any other optical organisation, do non produce "perfect" images. The demand to epitome objects at distances up to infinity, view them at dissimilar wavelengths of light, along with the requirement to have some manner to view the image the primary mirror produces, means there is always some compromise in a reflecting telescope's optical design.

Because the main mirror focuses light to a common point in front of its own reflecting surface almost all reflecting telescope designs have a secondary mirror, film holder, or detector well-nigh that focal point partially obstructing the light from reaching the primary mirror. Not just does this cause some reduction in the amount of calorie-free the arrangement collects, it also causes a loss in contrast in the epitome due to diffraction effects of the obstruction also as diffraction spikes caused by most secondary back up structures.^[12] ^[thirteen]

The use of mirrors avoids chromatic aberration only they produce other types of aberrations. A simple spherical mirror cannot bring light from a distant object to a common focus since the reflection of light rays striking the mirror almost its edge do not converge with those that reflect from nearer the center of the mirror, a defect called spherical aberration. To avert this problem most reflecting telescopes utilise parabolic shaped mirrors, a shape that can focus all the light to a common focus. Parabolic mirrors work well with objects most the center of the image they produce, (low-cal traveling parallel to the mirror's optical axis), but towards the border of that aforementioned field of view they suffer from off axis aberrations:^[14] ^[15]

Coma – an aberration where point sources (stars) at the center of the image are focused to a betoken but typically appears as "comet-like" radial smudges that get worse towards the edges of the image.
Field curvature – The all-time prototype aeroplane is in full general curved, which may non represent to the detector's shape and leads to a focus error across the field. It is sometimes corrected by a field flattening lens.
Astigmatism – an azimuthal variation of focus around the aperture causing signal source images off-axis to announced elliptical. Astigmatism is not usually a problem in a narrow field of view, but in a broad field image it gets apace worse and varies quadratically with field angle.
Baloney – Distortion does not affect epitome quality (sharpness) but does impact object shapes. It is sometimes corrected by image processing.

At that place are reflecting telescope designs that use modified mirror surfaces (such as the Ritchey–Chrétien telescope) or some class of correcting lens (such every bit catadioptric telescopes) that correct some of these aberrations.

Use in astronomical enquiry [edit]

Nearly all large inquiry-grade astronomical telescopes are reflectors. There are several reasons for this:

Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or in a catadioptric telescope.
In a lens the unabridged book of material has to be free of imperfection and inhomogeneities, whereas in a mirror, simply ane surface has to be perfectly polished.
Light of different wavelengths travels through a medium other than vacuum at different speeds. This causes chromatic aberration. Reducing this to acceptable levels usually involves a combination of 2 or three aperture sized lenses (meet achromat and apochromat for more than details). The toll of such systems therefore scales significantly with aperture size. An epitome obtained from a mirror does not suffer from chromatic aberration to begin with, and the cost of the mirror scales much more modestly with its size.
There are structural problems involved in manufacturing and manipulating large-discontinuity lenses. Since a lens tin can just be held in place by its edge, the eye of a large lens will sag due to gravity, distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter.^[16] In contrast, a mirror tin can be supported by the whole side reverse its reflecting face, assuasive for reflecting telescope designs that tin can overcome gravitational sag. The largest reflector designs currently exceed ten meters in diameter.

Reflecting telescope designs [edit]

Gregorian [edit]

Calorie-free path in a Gregorian telescope.

The Gregorian telescope, described by Scottish astronomer and mathematician James Gregory in his 1663 book Optica Promota, employs a concave secondary mirror that reflects the image back through a hole in the main mirror. This produces an upright image, useful for terrestrial observations. Some minor spotting scopes are still built this way. There are several big modern telescopes that use a Gregorian configuration such as the Vatican Advanced Engineering Telescope, the Magellan telescopes, the Large Binocular Telescope, and the Behemothic Magellan Telescope.

Newtonian [edit]

Light path in a Newtonian telescope.

The Newtonian telescope was the first successful reflecting telescope, completed past Isaac Newton in 1668. It usually has a paraboloid main mirror simply at focal ratios of virtually f/x or longer a spherical primary mirror can exist sufficient for high visual resolution. A flat secondary mirror reflects the light to a focal plane at the side of the acme of the telescope tube. It is one of the simplest and least expensive designs for a given size of primary, and is popular with amateur telescope makers as a home-build project.

The Cassegrain design and its variations [edit]

Light path in a Cassegrain telescope.

The cassegrain telescope (sometimes chosen the "Classic Cassegrain") was first published in a 1672 pattern attributed to Laurent Cassegrain. Information technology has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the calorie-free back downwardly through a hole in the main. The folding and diverging upshot of the secondary mirror creates a telescope with a long focal length while having a brusque tube length.

Ritchey–Chrétien [edit]

The Ritchey–Chrétien telescope, invented by George Willis Ritchey and Henri Chrétien in the early 1910s, is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of coma and spherical aberration at a nearly flat focal plane if the primary and secondary curvature are properly figured, making it well suited for wide field and photographic observations.^[17] Almost every professional reflector telescope in the world is of the Ritchey–Chrétien design.

Three-mirror anastigmat [edit]

Including a third curved mirror allows correction of the remaining distortion, astigmatism, from the Ritchey–Chrétien pattern. This allows much larger fields of view.

Dall–Kirkham [edit]

The Dall–Kirkham Cassegrain telescope'due south pattern was created past Horace Dall in 1928 and took on the proper noun in an article published in Scientific American in 1930 following discussion betwixt apprentice astronomer Allan Kirkham and Albert One thousand. Ingalls, the mag editor at the time. Information technology uses a concave elliptical chief mirror and a convex spherical secondary. While this system is easier to grind than a classic Cassegrain or Ritchey–Chrétien system, it does not correct for off-centrality coma. Field curvature is actually less than a classical Cassegrain. Because this is less noticeable at longer focal ratios, Dall–Kirkhams are seldom faster than f/15.

Off-centrality designs [edit]

In that location are several designs that try to avert obstructing the incoming light by eliminating the secondary or moving any secondary element off the primary mirror'southward optical axis, commonly called off-axis optical systems.

Herschelian [edit]

Herschelian telescope

Schiefspiegler telescope

The Herschelian reflector is named after William Herschel, who used this design to build very large telescopes including the 40-foot telescope in 1789. In the Herschelian reflector the master mirror is tilted so the observer'southward head does not block the incoming light. Although this introduces geometrical aberrations, Herschel employed this design to avoid the apply of a Newtonian secondary mirror since the speculum metal mirrors of that time tarnished quickly and could but achieve 60% reflectivity.^[eighteen]

Schiefspiegler [edit]

A variant of the Cassegrain, the Schiefspiegler telescope ("skewed" or "oblique reflector") uses tilted mirrors to avoid the secondary mirror casting a shadow on the main. However, while eliminating diffraction patterns this leads to an increase in coma and astigmatism. These defects become manageable at large focal ratios — most Schiefspieglers use f/15 or longer, which tends to restrict useful observation to the Moon and planets. A number of variations are common, with varying numbers of mirrors of different types. The Kutter (named afterwards its inventor Anton Kutter) style uses a single concave primary, a convex secondary and a plano-convex lens between the secondary mirror and the focal plane, when needed (this is the instance of the catadioptric Schiefspiegler). One variation of a multi-schiefspiegler uses a concave master, convex secondary and a parabolic tertiary. One of the interesting aspects of some Schiefspieglers is that ane of the mirrors can be involved in the low-cal path twice — each light path reflects along a dissimilar meridional path.

Stevick-Paul [edit]

Stevick-Paul telescopes^[nineteen] are off-centrality versions of Paul 3-mirror systems^[xx] with an added apartment diagonal mirror. A convex secondary mirror is placed just to the side of the light entering the telescope, and positioned afocally so as to transport parallel light on to the tertiary. The concave 3rd mirror is positioned exactly twice as far to the side of the entering beam as was the convex secondary, and its ain radius of curvature distant from the secondary. Because the tertiary mirror receives parallel light from the secondary, it forms an epitome at its focus. The focal airplane lies inside the arrangement of mirrors, just is accessible to the centre with the inclusion of a flat diagonal. The Stevick-Paul configuration results in all optical aberrations totaling zero to the third-order, except for the Petzval surface which is gently curved.

Yolo [edit]

The Yolo was adult by Arthur Southward. Leonard in the mid-1960s.^[21] Like the Schiefspiegler, information technology is an unobstructed, tilted reflector telescope. The original Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the master axis. Most Yolos use toroidal reflectors. The Yolo design eliminates blackout, merely leaves significant astigmatism, which is reduced by deformation of the secondary mirror by some grade of warping harness, or alternatively, polishing a toroidal figure into the secondary. Similar Schiefspieglers, many Yolo variations have been pursued. The needed amount of toroidal shape can be transferred entirely or partially to the primary mirror. In big focal ratios optical assemblies, both master and secondary mirror can exist left spherical and a spectacle correcting lens is added between the secondary mirror and the focal plane (catadioptric Yolo). The improver of a convex, long focus third mirror leads to Leonard'due south Solano configuration. The Solano telescope doesn't comprise whatever toric surfaces.

Liquid-mirror telescopes [edit]

One blueprint of telescope uses a rotating mirror consisting of a liquid metal in a tray that is spun at constant speed. Every bit the tray spins, the liquid forms a paraboloidal surface of substantially unlimited size. This allows making very big telescope mirrors (over 6 metres), but unfortunately they cannot be steered, as they always signal vertically.

Focal planes [edit]

Prime number focus [edit]

A prime focus telescope design. The observer/camera is at the focal indicate (shown as a red X).

In a prime focus design no secondary eyes are used, the prototype is accessed at the focal indicate of the main mirror. At the focal betoken is some type of structure for property a film plate or electronic detector. In the past, in very large telescopes, an observer would sit inside the telescope in an "observing cage" to directly view the image or operate a camera.^[22] Present CCD cameras allow for remote functioning of the telescope from almost anywhere in the globe. The space available at prime focus is severely limited by the demand to avoid obstructing the incoming light.^[23]

Radio telescopes often have a prime focus design. The mirror is replaced by a metal surface for reflecting radio waves, and the observer is an antenna.

Cassegrain focus [edit]

For telescopes built to the Cassegrain pattern or other related designs, the image is formed behind the chief mirror, at the focal point of the secondary mirror. An observer views through the rear of the telescope, or a camera or other instrument is mounted on the rear. Cassegrain focus is normally used for amateur telescopes or smaller research telescopes. Even so, for big telescopes with correspondingly large instruments, an instrument at Cassegrain focus must motion with the telescope every bit it slews; this places additional requirements on the strength of the musical instrument support structure, and potentially limits the movement of the telescope in order to avoid collision with obstacles such as walls or equipment inside the observatory.

Nasmyth and coudé focus [edit]

Nasmyth/coudé light path.

Nasmyth [edit]

The Nasmyth design is similar to the Cassegrain except the lite is not directed through a pigsty in the primary mirror; instead, a 3rd mirror reflects the calorie-free to the side of the telescope to allow for the mounting of heavy instruments. This is a very common design in large research telescopes.^[24]

Coudé [edit]

Adding farther optics to a Nasmyth-way telescope to deliver the light (unremarkably through the declination axis) to a stock-still focus point that does not move as the telescope is reoriented gives a coudé focus (from the French word for elbow).^[25] The coudé focus gives a narrower field of view than a Nasmyth focus^[25] and is used with very heavy instruments that do not need a wide field of view. One such application is loftier-resolution spectrographs that have big collimating mirrors (ideally with the same diameter equally the telescope's primary mirror) and very long focal lengths. Such instruments could not withstand being moved, and adding mirrors to the light path to class a coudé railroad train, diverting the light to a fixed position to such an musical instrument housed on or beneath the observing floor (and usually built as an unmoving integral part of the observatory building) was the only option. The 60-inch Hale telescope (1.five m), Hooker Telescope, 200-inch Hale Telescope, Shane Telescope, and Harlan J. Smith Telescope all were built with coudé foci instrumentation. The development of echelle spectrometers allowed high-resolution spectroscopy with a much more compact musical instrument, one which tin can sometimes be successfully mounted on the Cassegrain focus. Since inexpensive and adequately stable reckoner-controlled alt-az telescope mounts were developed in the 1980s, the Nasmyth pattern has by and large supplanted the coudé focus for large telescopes.

Fibre-fed spectrographs [edit]

For instruments requiring very high stability, or that are very large and cumbersome, it is desirable to mount the musical instrument on a rigid structure, rather than moving it with the telescope. Whilst transmission of the full field of view would crave a standard coudé focus, spectroscopy typically involves the measurement of only a few detached objects, such equally stars or galaxies. Information technology is therefore feasible to collect light from these objects with optical fibers at the telescope, placing the instrument at an capricious distance from the telescope. Examples of fiber-fed spectrographs include the planet-hunting spectrographs HARPS^[26] or ESPRESSO.^[27]

Additionally, the flexibility of optical fibers allow calorie-free to be collected from any focal airplane; for case, the HARPS spectrograph utilises the Cassegrain focus of the ESO three.vi m Telescope,^[26] whilst the Prime Focus Spectrograph is connected to the prime number focus of the Subaru telescope.^[28]

References [edit]

^ Fred Watson (2007). Stargazer: The Life and Times of the Telescope. Allen & Unwin. p. 108. ISBN978-1-74176-392-viii.
^ ^a ^b Fred Watson (2007). Stargazer: The Life and Times of the Telescope. Allen & Unwin. p. 109. ISBN978-1-74176-392-8.
^ theoretical designs past Bonaventura Cavalieri, Marin Mersenne, and Gregory amongst others
^ Fred Watson (2007). Stargazer: The Life and Times of the Telescope. Allen & Unwin. p. 117. ISBN978-1-74176-392-eight.
^ Henry C. King (2003). The History of the Telescope. Courier Corporation. p. 71. ISBN978-0-486-43265-vi.
^ "Explore, National Museums Scotland". Archived from the original on 2017-01-17. Retrieved 2016-11-15 .
^ A. Rupert Hall (1996). Isaac Newton: Charlatan in Thought . Cambridge University Press. p. 67. ISBN978-0-521-56669-8.
^ Parabolic mirrors were used much earlier, but James Short perfected their construction. See "Reflecting Telescopes (Newtonian Type)". Astronomy Department, University of Michigan. Archived from the original on 2009-01-31.
^ Lequeux, James (2017-01-01). "The Paris Observatory has 350 years". L'Astronomie. 131: 28–37. Bibcode:2017LAstr.131a..28L. ISSN 0004-6302.
^ Silvering on a reflecting telescope was introduced by Léon Foucault in 1857, come across madehow.com - Inventor Biographies - Jean-Bernard-Léon Foucault Biography (1819–1868), and the adoption of long lasting aluminized coatings on reflector mirrors in 1932. Bakich sample pages Affiliate 2, Page 3 "John Donavan Strong, a young physicist at the California Plant of Technology, was one of the first to coat a mirror with aluminum. He did information technology by thermal vacuum evaporation. The first mirror he aluminized, in 1932, is the earliest known example of a telescope mirror coated by this technique."
^ Ray Villard; Leonello Calvetti; Lorenzo Cecchi (2001). Large Telescopes: Inside and Out. The Rosen Publishing Group, Inc. p. 21. ISBN978-0-8239-6110-8.
^ Rodger West. Gordon, "Central Obstructions and their effect on prototype contrast" brayebrookobservatory.org
^ "Obstacle" in optical instruments
^ Richard Fitzpatrick, Spherical Mirrors, farside.ph.utexas.edu
^ "Vik Dhillon, reflectors, vikdhillon.staff.shef.ac.united kingdom of great britain and northern ireland". Archived from the original on 2010-05-05. Retrieved 2010-04-06 .
^ Stan Gibilisco (2002). Physics Demystified . Mcgraw-hill. p. 515. ISBN978-0-07-138201-4.
^ Sacek, Vladimir (July 14, 2006). "8.2.2 Classical and aplanatic 2-mirror systems". Notes on AMATEUR TELESCOPE OPTICS . Retrieved 2009-06-22 .
^ catalogue.museogalileo.it - Found and Museum of the History of Science - Florence, Italia, Telescope, glossary
^ Stevick-Paul Telescopes by Dave Stevick
^ Paul, Thou. (1935). "Systèmes correcteurs pour réflecteurs astronomiques". Revue d'Optique Théorique et Instrumentale. 14 (5): 169–202.
^ Arthur S. Leonard THE YOLO REFLECTOR
^ W. Patrick McCray (2004). Giant Telescopes: Astronomical Appetite and the Hope of Engineering. Harvard University Press. p. 27. ISBN978-0-674-01147-2.
^ "Prime Focus".
^ Geoff Andersen (2007). The Telescope: Its History, Technology, and Future . Princeton Academy Press. p. 103. ISBN978-0-691-12979-2.
^ ^a ^b "The Coude Focus".
^ ^a ^b "HARPS Instrument Description".
^ "ESPRESSO Instrument Description".
^ "Subaru PFS Instrumentation".

External links [edit]

Who was James Gregory? Reflecting Telescopes, Explore, National Museums Scotland Archived 2017-01-17 at the Wayback Machine