A reflecting telescope (also called a reflector) is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. An optical telescope is a Telescope which is used to gather and focus light mainly from the visible part of the Electromagnetic spectrum A curved mirror is a Mirror with a curved reflective surface which may be either convex (bulging outward or concave (bulging inward Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 The Reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. A refracting or refractor telescope is a dioptric Telescope that uses a lens as its objective to form an image In Optics, chromatic aberration is caused by a lens having a different Refractive index for different Wavelengths of Light Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives and is used almost exclusively in major research telescopes. An objective in Optics is the lens or Mirror in a Microscope, Telescope, camera or other optical instrument Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes uses mirrors, the design is sometimes referred to as a "catoptric" telescope. A mirror is an object with a surface that has good Specular reflection; that is it is smooth enough to form an Image. Catoptrics deals with the phenomena of reflected light and image-forming optical systems using Mirrors From the Greek κατοπτρικός (specular
Contents |
History
The Italian professor Niccolò Zucchi is credited with making the first reflector in 1616. Niccolò Zucchi ( December 6 1586 - May 21 1670) was an Italian Jesuit astronomer and physicist. With it, in 1630 he observed two bands on Jupiter, and in 1640 observed spots on Mars. But his inability to shape the concave mirror accurately, and the lack of means of viewing the image without blocking the mirror, meant others did not adopt Zucchi's design. In 1663 James Gregory published Optica Promota which described the first practical design of a reflector using two concave mirrors. James Gregory (November 1638 &ndash October 1675 was a Scottish Mathematician and Astronomer. A working example was not built until 10 years later by Robert Hooke. Robert Hooke, FRS (18 July 1635 – 3 March 1703 was an English Natural philosopher and Polymath who played an important role in the Sir Isaac Newton is credited with constructing the first "practical" reflecting telescope after his own design in 1668. Sir Isaac Newton, FRS (ˈnjuːtən 4 January 1643 31 March 1727) Biography Early years See also Isaac Newton's early life and achievements [1] Newton's added a smaller "diagonal" mirror near the primary mirror's focus to reflect the image at 90° angle. This allowed the user to view the image without obstructing the incoming light. Newton invented his reflector to solve the problem of chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses. In Optics, chromatic aberration is caused by a lens having a different Refractive index for different Wavelengths of Light A refracting or refractor telescope is a dioptric Telescope that uses a lens as its objective to form an image An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and Spherical aberration.
Technical considerations
A curved primary mirror is the reflector telescope's basic optical element and creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. The focal length of an optical system is a measure of how strongly it converges (focuses or diverges (diffuses Light. Film or a digital sensor may be located here to record the image, or an eyepiece for visual observation or a mirror that reflects the image to an eyepiece. For the device for looking through a camera see Viewfinder. An eyepiece, or ocular lens, is a type of lens that is attached
The primary mirror in most modern telescopes is composed of a solid glass cylinder whose front surface has been ground to a spherical or parabolic shape. A curved mirror is a Mirror with a curved reflective surface which may be either convex (bulging outward or concave (bulging inward A parabolic reflector (or dish or mirror) is a Parabola -shaped reflective device used to collect or distribute Energy such as A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective front surface. WikipediaNaming Vacuum deposition or vacuum coating is a family of processes used to deposit layers atom-by-atom or molecule-by-molecule at sub- Atmospheric pressure ( Vacuum Early reflecting telescopes used a metal objective called a speculum. Speculum metal is a very hard white Alloy of four parts Copper to one part Tin, or according to other sources three parts copper to one part tin some
Mirrors eliminate the risk of chromatic aberration but may still produce other types of aberrations: In general, on axis they may produce spherical aberration, in which case the outer and inner zones of the telescope do not share a common focus. In Optics, chromatic aberration is caused by a lens having a different Refractive index for different Wavelengths of Light Aberrations are departures of the performance of an optical system from the predictions of Paraxial optics. This was the construction flaw in the Hubble Space Telescope mirrors. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into Spherical aberration can be eliminated with aspheric (non-spherical) mirrors. Off axis, additional aberrations may become apparent:
- Coma - a variation of telescope magnification with radial zone on the mirror typically appears as a radial smudging of the images which gets worse at the edges of the field. In Optics (especially Telescopes, the coma (aka comatic aberration) in an optical system refers to aberration inherent to certain optical Spherical aberration and coma are eliminated in two mirror Ritchey Chretien designs.
- The best image plane is in general curved, which may not correspond to the detector's shape and leads to a focus error across the field.
- Astigmatism, an azimuthal variation of focus around the aperture. See also Aberration in optical systems, Astigmatism (eye An Optical system with astigmatism is one where rays that propagate Azimuth ( is a mathematical concept defined as the angle usually measured in degrees (° between a reference plane and a point. Near the center of the field astigmatism is not usually a problem, but it gets rapidly worse once it becomes apparent - it varies quadratically with field angle.
- Distortion over the field of view. The field of view (also field of vision) is the angular extent of the observable world that is seen at any given moment Distortion does not affect image quality (sharpness) but does affect object shapes. It can be corrected by image processing.
There are reflector designs and modifications such as catadioptrics that correct some of these aberrations. A catadioptric optical system is one which contains both lenses and Mirrors Catadioptric systems are commonly used in Telescopes and in lightweight
Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this:
- In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished. A lens is an optical device with perfect or approximate Axial symmetry which transmits and refracts Light, converging or diverging
- Light of different wavelengths travels through a medium other than vacuum at different speeds. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. This causes chromatic aberration in uncorrected lenses and creating an aberration-free large lens is a costly process. In Optics, chromatic aberration is caused by a lens having a different Refractive index for different Wavelengths of Light A mirror can eliminate this problem entirely.
- 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 catadioptric.
- There are structural problems involved in manufacturing and manipulating large-aperture lenses. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces. Gravitation is a natural Phenomenon by which objects with Mass attract one another The largest practical lens size in a refracting telescope is around 1 meter[2]. In contrast, a mirror can be supported by the whole side opposite its reflecting face, allowing for reflecting telescope designs that can overcome gravitational sag. The largest reflector designs currently exceed 10 meters in diameter.
While the Newtonian focus design is still used in amateur astronomy, professionals now tend to use prime focus, Cassegrain focus, and coudé focus designs. Amateur astronomy, a subset of Astronomy, is a Hobby whose participants enjoy studying and observing celestial objects
Reflecting telescope designs
Newtonian
The Newtonian usually has a paraboloid primary mirror but for small apertures, say 12 cm or less, if the focal ratio is f/8 or longer a spherical primary mirror is sufficient for high visual resolution. The Newtonian telescope is a type of Reflecting telescope invented by the British scientist Sir Isaac Newton (1643-1727 using a parabolic primary mirror The Newtonian telescope is a type of Reflecting telescope invented by the British scientist Sir Isaac Newton (1643-1727 using a parabolic primary mirror A flat secondary mirror reflects the light to a focal plane at the side of the top 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 field of amateur telescope making is considered an offshoot of the Amateur astronomy community
See also
The Cassegrain design and its variations
The Cassegrain (sometimes called the "Classic Cassegrain") has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. The optical design of the Schmidt-Newton telescope combines elements from both the Schmidt camera and the Newtonian telescope. The Cassegrain reflector is a combination of two coaxial reflectors used in Cassegrain telescopes and radio antennas First developed in 1672 by Laurent Cassegrain The Cassegrain reflector is a combination of two coaxial reflectors used in Cassegrain telescopes and radio antennas First developed in 1672 by Laurent Cassegrain Folding the optics makes this a compact design. On smaller telescopes, and camera lenses, the secondary is often mounted on an optically flat, optically clear glass plate that closes the telescope tube. This support eliminates the "star-shaped" diffraction effects caused by a straight-vaned support spider. Diffraction spikes are lines radiating from bright light sources in Reflecting telescope images The closed tube stays clean, and the primary is protected, at the cost of some loss of light-gathering power.
Ritchey-Chrétien
The Ritchey-Chrétien is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). The Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope designed to eliminate coma, thus providing a relatively large field of view The Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope designed to eliminate coma, thus providing a relatively large field of view It is free of coma and spherical aberration at a flat focal plane if the primary and secondary curvature are equal, making it well suited for wide field and photographic observations. In Optics (especially Telescopes, the coma (aka comatic aberration) in an optical system refers to aberration inherent to certain optical Almost every professional reflector telescope in the world is of the Ritchey-Chrétien design. It was invented by George Willis Ritchey and Henri Chrétien in the early 1910s. George Willis Ritchey ( December 31 1864 &ndash November 4 1945) was an American Optician and telescope maker and Astronomer Henri Jacques Chrétien ( February 1, 1879, Paris – February 6, 1956, Washington) was a French astronomer
Dall-Kirkham
The Dall-Kirkham cassegrain telescope's design was created by Horace Dall in 1928 and took on the name in an article published in Scientific American in 1930 following discussion between amateur astronomer Allan Kirkham and Albert G. Scientific American is a Popular science magazine, published (first weekly and later monthly since August 28, 1845, making it Ingalls, the magazine editor at the time. It uses a concave elliptical primary mirror and a convex spherical secondary. In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe While this system is easier to grind than a classic Cassegrain or Ritchey-Chretien system, it does not correct for off-axis coma and field curvature so the image degrades quickly off-axis. Because this is less noticeable at longer focal ratios, Dall-Kirkhams are seldom faster than f/15.
See also:
- Schmidt-Cassegrain telescope
- Maksutov telescope (Cassegrains)
Gregorian
The Gregorian telescope, invented by James Gregory, employs a concave, not convex, secondary mirror and in this way achieves an upright image, useful for terrestrial observations. The Schmidt-Cassegrain is a Catadioptric Telescope. It combines a folded optical path with a corrector plate to make a compact astronomical instrument The Maksutov is a Catadioptric Telescope design that employs a full diameter meniscus lens (commonly called a "corrector plate" to correct The Gregorian telescope is a type of Reflecting telescope designed by Scottish Mathematician and Astronomer, James Gregory in the The Gregorian telescope is a type of Reflecting telescope designed by Scottish Mathematician and Astronomer, James Gregory in the James Gregory (November 1638 &ndash October 1675 was a Scottish Mathematician and Astronomer. Whereas the design has largely fallen in disfavour, some small spotting scopes are still built this way. A spotting scope is a portable Telescope, optimized for the observation of terrestrial objects
Off-axis designs
There are several designs that try to avoid obstructing the incoming light by eliminating the secondary or moving any secondary element off the primary mirror's optical axis, commonly called off-axis optical systems. In Optics, the term optical axis is used to define a direction along which there is some degree of Rotational symmetry. An off-axis optical system is an optical system in which the Optical axis of the Aperture is not coincident with the mechanical center of the aperture
Herschelian
The Herschelian reflector is named after William Herschel, who used this design to build very large telescopes including a 49½ inche (126 cm) diameter telescope in 1789. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British The design is similar to Niccolò Zucchi's early attempts to build a reflector where the observer stands in front of the objective to view the image. In the Herschelian reflector the primary mirror is tilted so the observers head does not block the incoming light. Although this introduces geometrical aberrations, Herschel employed this design to avoid the use of a newtonian secondary mirror since the fast tarnishing speculum metal mirrors of that time could only achieve 60% reflectivity[3]. Tarnish is a layer of Corrosion that develops over Copper, Brass, Silver, Aluminum as well as a degree of semi-reactive metals as Speculum metal is a very hard white Alloy of four parts Copper to one part Tin, or according to other sources three parts copper to one part tin some
Schiefspiegler
A variant of the Cassegrain, the Schiefspiegler telescope ("skewed" or "oblique reflector"), which uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary. 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 style uses a single concave primary and a convex secondary. One variation of a multi-schiefspiegler uses a concave primary, convex secondary and a parabolic tertiary. One of the interesting aspects of some Schiefspieglers is that one of the mirrors can be involved in the light path twice - each light path reflects along a different meridional path.
Yolo
The Yolo was developed by Arthur S. Leonard in the mid 1960's [1]. Like the Schiefspiegler, it is an unobstructed, tilted reflector telescope. The Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the main axis. The Yolo design eliminates coma, but leaves significant astigmatism, which is reduced by deformation of the secondary mirror by some form of warping harness, or alternatively, polishing a toroidal figure into the secondary.
Focal planes
Prime focus
In a prime focus design in large observatory telescopes, the observer sits inside the telescope, at the focal point of the reflected light. Track listing Child – 516 All I Need – 355 In the past this would be the astronomer himself, but nowadays CCD cameras are used. A charge-coupled device ( CCD) is an analog Shift register, that enables the transportation of analog signals (electric charges through successive stages (capacitors The space available at prime focus is severely limited by the need to avoid obstructing the incoming light.
Radio telescopes often have a prime focus design. A radio telescope is a form of directional Radio antenna used in Radio astronomy and in tracking and collecting data from Satellites The mirror is replaced by a metal surface for reflecting radio waves, and the observer is an antenna. Radio waves are electromagnetic waves occurring on the Radio frequency portion of the Electromagnetic spectrum. An antenna is a Transducer designed to transmit or Receive electromagnetic waves In other words antennas convert electromagnetic waves into
See also:
Nasmyth and Coudé focus
Nasmyth
The Nasmyth design is similar to the Cassegrain except no hole is drilled in the primary mirror; instead, a third mirror reflects the light to the side. The Nasmyth telescope is a Reflecting telescope developed by James Nasmyth.
Coudé
Adding further optics to a Nasmyth style telescope that deliver the light (usually through the declination axis) to a fixed focus point that does not move as the telescope is reoriented gives you a Coudé focus. In Astronomy, declination (abbrev dec or δ) is one of the two coordinates of the Equatorial coordinate system, the other being either This design is often used on large observatory telescopes, as it allows heavy observation equipment, such as spectrographs, to be more easily used. A spectrometer is an Optical instrument used to measure properties of Light over a specific portion of the Electromagnetic spectrum, typically used
References
- ^ Reflecting Telescope Optics I, R. N. Wilson, A&A Library, Springer, 1996 (ISBN 0941-7834)
- ^ "Physics Demystified" By Stan Gibilisco, ISBN 0071382011, page 515
- ^ brunelleschi.imss.fi.it - Institute and Museum of the History of Science - Florence, Italy, Telescope, glossary
See also
- List of largest optical reflecting telescopes
- Large liquid mirror telescope
- Catadioptric
- Astrograph
- Refracting telescope
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic