Newtonian telescope

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Newton telescope
A replica of Newton's second reflecting telescope, which he presented to the Royal Society in 1672.
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Newtonian telescope design

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. [1] The Newtonian telescope's simple design has made it very popular with amateur telescope makers. [2]



Newton's idea for a reflecting telescope was not new. Galileo Galilei and Giovanni Francesco Sagredo had discussed using a mirror as the image forming objective soon after the invention of the refracting telescope, [3] and others, such as Niccolò Zucchi, claimed to have experimented with the idea as far back as 1616. [4] Newton may even have read James Gregory's 1663 book Optica Promota which described reflecting telescope designs using parabolic mirrors [5] (a telescope Gregory had been trying unsuccessfully to build). [6]

Newton built his reflecting telescope because he suspected it could prove his theory that white light is composed of a spectrum of colours. [7] Colour distortion (chromatic aberration) was the primary fault of refracting telescopes of Newton's day, and there were many theories as to what caused it. During the mid-1660s with his work on the theory of colour, Newton concluded this defect was caused by the lens of the refracting telescope behaving the same as prisms he was experimenting with, breaking white light into a rainbow of colours around bright astronomical objects. [8] [9] If this were true, then chromatic aberration could be eliminated by building a telescope which did not use a lens – a reflecting telescope.

In late 1668 Isaac Newton built his first reflecting telescope. He chose an alloy (speculum metal) of tin and copper as the most suitable material for his objective mirror. He later devised means for shaping and grinding the mirror and may have been the first to use a pitch lap [10] to polish the optical surface. He chose a spherical shape for his mirror instead of a parabola to simplify construction; even though it would introduce spherical aberration, it would still correct chromatic aberration. He added to his reflector what is the hallmark of the design of a Newtonian telescope, a secondary diagonally mounted mirror near the primary mirror's focus to reflect the image at a 90° angle to an eyepiece mounted on the side of the telescope. This unique addition allowed the image to be viewed with minimal obstruction of the objective mirror. He also made the tube, mount, and fittings. Newton's first version had a primary mirror diameter of 1.3 inches (33 mm) and a focal ratio of f/5. [11] He found that the telescope worked without colour distortion and that he could see the four Galilean moons of Jupiter and the crescent phase of the planet Venus with it. Newton's friend Isaac Barrow showed a second telescope to a small group from the Royal Society of London at the end of 1671. They were so impressed with it that they demonstrated it to Charles II in January 1672. Newton was admitted as a fellow of the society in the same year.

Like Gregory before him, Newton found it hard to construct an effective reflector. It was difficult to grind the speculum metal to a regular curvature. The surface also tarnished rapidly; the consequent low reflectivity of the mirror and also its small size meant that the view through the telescope was very dim compared to contemporary refractors. Because of these difficulties in construction, the Newtonian reflecting telescope was initially not widely adopted. In 1721 John Hadley showed a much-improved model to the Royal Society. [12] Hadley had solved many of the problems of making a parabolic mirror. His Newtonian with a mirror diameter of 6 inches (150 mm) compared favourably with the large aerial refracting telescopes of the day. [13] The size of reflecting telescopes subsequently grew rapidly, with designs doubling in primary mirror diameter about every 50 years. [14]

Advantages of the Newtonian design

Newtonian optical assembly showing the tube (1), the primary mirror (2), and the secondary diagonal mirror support (also called a "spider support") (3). Newtonianscope-inside.JPG
Newtonian optical assembly showing the tube (1), the primary mirror (2), and the secondary diagonal mirror support (also called a "spider support") (3).

Disadvantages of the Newtonian design

A large Newtonian reflector from 1873 with structure to access the eyepiece. Lanature1873 telescope lassel.png
A large Newtonian reflector from 1873 with structure to access the eyepiece.



A Jones-Bird reflector telescope (sometimes called a Bird-Jones) is a mirror-lens (catadioptric) variation on the traditional Newtonian design sold in the amateur telescope market. The design uses a spherical primary mirror in place of a parabolic one, with spherical aberrations corrected by sub-aperture corrector lens [19] usually mounted inside the focusser tube or in front of the secondary mirror. This design reduces the size and cost of the telescope with a shorter overall telescope tube length (with the corrector extending the focal length in a "telephoto" type layout) combined with a less costly spherical mirror. Commercially produced versions of this design have been noted to be optically compromised due to the difficulty of producing a correctly shaped sub-aperture corrector in a telescope targeted at the inexpensive end of the telescope market. [20]

See also


  1. Hall, A. Rupert (1992). Isaac Newton: Adventurer in Thought. Cambridge University Press. p. 67. ISBN   9780521566698.
  2. Ingalls, Albert G., ed. (1935). Amateur Telescope Making (4th ed.). Munn and Co., Inc.
  3. Fred Watson (2007). Stargazer: The Life and Times of the Telescope. Allen & Unwin. p. 108. ISBN   978-1-74176-392-8.
  4. The Galileo Project > Science > Zucchi, Niccolo
  5. Derek Gjertsen (1986). The Newton Handbook. Routledge & Kegan Paul. p. 562. ISBN   978-0-7102-0279-6.
  6. Michael White (1999). Isaac Newton: The Last Sorcerer. Basic Books. p. 169. ISBN   978-0-7382-0143-6.
  7. Michael White (1999). Isaac Newton: The Last Sorcerer. Basic Books. p. 170. ISBN   978-0-7382-0143-6.
  8. Newton thought little could be done to correct aberration short of making lenses that were f/50 or more."the object-glass of any telescope cannot collect all the rays which come from one point of an object, so as to make them convene at its focus in less room than in a circular space, whose diameter is the 50th part of the diameter of its aperture
  9. Stephen Parkinson (1870). A Treatise on Optics. Macmillan. p.  112.
  10. Raymond N. Wilson (2007). Reflecting Telescope Optics I: Basic Design Theory and its Historical Development. Springer Science & Business Media. p. 9. ISBN   978-3-540-40106-3.
  11. Reflecting Telescopes: Newtonian, two- and three-mirror systems
  12. – Hadley’s Reflector
  13. The complete Amateur Astronomer – John Hadley's Reflector
  14. Racine, René (2004). "The Historical Growth of Telescope Aperture". The Publications of the Astronomical Society of the Pacific. 116 (815): 77–83. Bibcode:2004PASP..116...77R. doi: 10.1086/380955 .
  15. Sacek, Vladimir (2006-07-14). "8.1.1. Newtonian off-axis aberrations" . Retrieved 2009-09-29. off-axis performance of the paraboloidal mirror drops so quickly with the increase in relative aperture beyond ~ƒ/6
  16. Knisely, David (2004). "Tele Vue Paracor Coma Corrector for Newtonians" (PDF). Cloudy Nights Telescope Review. Retrieved 29 November 2010.
  17. Alex Hebra (2010). The Physics of Metrology: All about Instruments: From Trundle Wheels to Atomic Clocks. Springer Science & Business Media. pp. 258–259. ISBN   978-3-211-78381-8.
  18. Antony Cooke (2009). Make Time for the Stars: Fitting Astronomy into Your Busy Life. Springer Science & Business Media. p. 14. ISBN   978-0-387-89341-9.
  19. 10.1.2. Sub-aperture corrector examples: Single-mirror systems – Jones-Bird

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Ritchey–Chrétien telescope Specialized Cassegrain telescope

A Ritchey–Chrétien telescope is a specialized variant of the Cassegrain telescope that has a hyperbolic primary mirror and a hyperbolic secondary mirror designed to eliminate off-axis optical errors (coma). The RCT has a wider field of view free of optical errors compared to a more traditional reflecting telescope configuration. Since the mid 20th century, a majority of large professional research telescopes have been Ritchey–Chrétien configurations; some well-known examples are the Hubble Space Telescope, the Keck telescopes and the ESO Very Large Telescope.

History of the telescope Aspect of history

The history of the telescope can be traced to before the invention of the earliest known telescope, which appeared in 1608 in the Netherlands, when a patent was submitted by Hans Lippershey, an eyeglass maker. Although Lippershey did not receive his patent, news of the 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. By 1655, astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces.

Spherical aberration Optical aberration

In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strike a spherical surface off-centre are refracted or reflected more or less than those that strike close to the centre. This deviation reduces the quality of images produced by optical systems.

Refracting telescope

A refracting telescope is a type of optical telescope that uses a lens as its objective to form an image. 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.

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 visual inspection, to make a photograph, or to collect data through electronic image sensors.

Reflecting telescope Telescopes which utilize curved mirrors to form an image

A reflecting telescope 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 large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. 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 use mirrors, the design is sometimes referred to as a catoptric telescope.

Coma (optics) Aberration inherent to certain optical designs or due to imperfection in the lens

In optics, the coma, or comatic aberration, in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted, appearing to have a tail (coma) like a comet. Specifically, coma is defined as a variation in magnification over the entrance pupil. In refractive or diffractive optical systems, especially those imaging a wide spectral range, coma can be a function of wavelength, in which case it is a form of chromatic aberration.

Catadioptric system Optical system where refraction and reflection are combined

A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors (catoptrics). Catadioptric combinations are used in focusing systems such as searchlights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses. Other optical systems that use lenses and mirrors are also referred to as "catadioptric", such as surveillance catadioptric sensors.

Maksutov telescope

The Maksutov is a catadioptric telescope design that combines a spherical mirror with a weakly negative meniscus lens in a design that takes advantage of all the surfaces being nearly "spherically symmetrical". The negative lens is usually full diameter and placed at the entrance pupil of the telescope. The design corrects the problems of off-axis aberrations such as coma found in reflecting telescopes while also correcting chromatic aberration. It was patented in 1941 by Russian optician Dmitri Dmitrievich Maksutov. Maksutov based his design on the idea behind the Schmidt camera of using the spherical errors of a negative lens to correct the opposite errors in a spherical primary mirror. The design is most commonly seen in a Cassegrain variation, with an integrated secondary, that can use all-spherical elements, thereby simplifying fabrication. Maksutov telescopes have been sold on the amateur market since the 1950s.

Schmidt–Cassegrain telescope

The Schmidt–Cassegrain is a catadioptric telescope that combines a Cassegrain reflector's optical path with a Schmidt corrector plate to make a compact astronomical instrument that uses simple spherical surfaces.

Vixen is a Japanese company that makes telescopes, binoculars, spotting scopes and accessories for their products.

Cassegrain 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.

Schmidt–Newtonian telescope

A Schmidt–Newtonian telescope or Schmidt–Newton telescope is a catadioptric telescope that combines elements from both the Schmidt camera and the Newtonian telescope. In this telescope design, a spherical primary mirror is combined with a Schmidt corrector plate, which corrects the spherical aberration and holds the secondary mirror. The resulting system has less coma and diffraction effects than a Newtonian telescope with a parabolic mirror and a "spider" secondary mirror support. The design uses a 45° flat secondary mirror to view the image, as in a standard Newtonian telescope.

Lurie–Houghton telescope

The Houghton telescope or Lurie–Houghton telescope is a catadioptric telescope. Houghton's original design was patented in 1944. Instead of the fairly hard to make Schmidt and heavy meniscus (Maksutov) corrector lenses, the corrector for the Houghton is relatively easy to make. It consists of two lenses: a positive and a negative, set at the front of the telescope which fixes the telescope's aperture. All lens and mirror surfaces are spheroidal, which eases construction. These lenses are relatively thin, though not as thin as the Schmidt corrector. With a good anti-reflective coating, light loss and "ghost" reflections are minimal.

Mangin mirror Catadioptric reflector for search lights

In optics, a Mangin mirror is a negative meniscus lens with the reflective surface on the rear side of the glass forming a curved mirror that reflects light without spherical aberration. This reflector was invented in 1876 by a French officer Alphonse Mangin as an improved catadioptric reflector for search lights and is also used in other optical devices.

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 the Newtonian telescope. There were some early prototypes and also modern replicas of this design.

Aerial telescope

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.

Meniscus corrector

A meniscus corrector is a negative meniscus lens that is used to correct spherical aberration in image-forming optical systems such as catadioptric telescopes. It works by having the equal but opposite spherical aberration of the objective it is designed to correct.