Newtonian telescope

This article is about the Newtonian type of reflecting telescope. For the optical telescope at Roque de los Muchachos Observatory on La Palma, see Isaac Newton Telescope.
Newtonian Telescope

The Newtonian telescope is a type of reflecting telescope invented by the British 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 makes it very popular with amateur telescope makers.[2]

History

Newton's idea for a reflecting telescope was not a new one. 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]

A replica of Newton's second reflecting telescope, which he presented to the Royal Society in 1672.[7]

Newton built his reflecting telescope because he suspected it could prove his theory that white light is composed of a spectrum of colours.[8] 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.[9][10] 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[11] 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.[12] 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.[13] 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.[14] The size of reflecting telescopes subsequently grew rapidly, with designs doubling in primary mirror diameter about every 50 years.[15]

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).

Disadvantages of the Newtonian design

A large Newtonian reflector from 1873 with structure to access the eyepiece.

Variation

Jones-Bird

A Jones-Bird Newtonian telescope (sometimes called a Bird-Jones) is a mirror-lens (catadioptric) variation on the traditional 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[20] 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.[21]

Gallery

See also

Wikimedia Commons has media related to Newtonian telescopes.

Notes

  1. ↑ Isaac Newton: adventurer in thought, by Alfred Rupert Hall, page 67
  2. ↑ Telescope Basics - Mark T. VandeWettering, 2001
  3. ↑ Stargazer - By Fred Watson, Inc NetLibrary, Page 108
  4. ↑ The Galileo Project > Science > Zucchi, Niccolo
  5. ↑ Derek Gjertsen, The Newton handbook, page 562
  6. ↑ Isaac Newton By Michael White Page 169
  7. ↑ The History of the Telescope By Henry C. King, Page 74
  8. ↑ Isaac Newton By Michael White Page 170
  9. ↑ 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”
  10. ↑ Treatise on Optics, p. 112
  11. ↑ Reflecting Telescope Optics: Basic Design Theory and Its Historical Development By Ray N. Wilson Published by Springer, 2004 ISBN 3-540-40106-7, ISBN 978-3-540-40106-3.
  12. ↑ telescope-optics.net Reflecting Telescopes: Newtonian, two- and three-mirror systems
  13. ↑ amazing-space.stsci.edu - Hadley’s Reflector
  14. ↑ The complete Amateur Astronomer - John Hadley's Reflector
  15. ↑ Racine, René (2004). "The Historical Growth of Telescope Aperture". The Publications of the Astronomical Society of the Pacific 116 (815): 77. Bibcode:2004PASP..116...77R. doi:10.1086/380955.
  16. ↑ 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
  17. ↑ Knisely, David (2004). "Tele Vue Paracor Coma Corrector for Newtonians" (pdf). Cloudy Nights Telescope Review. Retrieved 29 November 2010.
  18. ↑ Alexius J. Hebra, The Physics of Metrology: All about Instruments: From Trundle Wheels to Atomic Clocks, page 258-259
  19. ↑ Antony Cooke, "Make Time for the Stars: Fitting Astronomy Into Your Busy Life", page 14
  20. ↑ 10.1.2. Sub-aperture corrector examples: Single-mirror systems - Jones-Bird
  21. ↑ TELESCOPES - OVERVIEW AND TELESCOPE TYPES, CATADIOPTRIC NEWTONIAN

References

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