User:LouScheffer/Sandbox

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Telescopes (and the optically similar[1] microscopes) are based on the science of optics and the technology of mirrors and lenses, and the history of the telescope is interwined with advances in these fields. Crude optics and crude lenses had been known since antiquity, but the scientific study of optics started in the 10th and 11th centuries by scholars such as Alhazen[2][3] and Ibn Sahl[4], and lenses good enough for telescopes were developed by the spectacle makers of 13th century Italy and Germany.[2][5]. The combination of two lenses in a single device (compound optics) was first used by Giambattista della Porta in 1589[6] and the telescope and microscope soon followed. Although there is fragmentary evidence for earlier designs[7] the first historically accepted telescopes appear in the Netherlands in 1608. The invention of the telescope is generally credited to three individuals: Hans Lippershey and Zacharias Janssen of Middelburg (who also invented the microscope), and Jacob Metius of Alkmaar.[8]

Galileo greatly improved upon these designs the following year and is generally credited with being the first to use a telescope for astronomical purposes. Galileo's telescope used Hans Lippershey's design of a convex objective lens and a concave eye lens and this design has come to be called a Galilean telescope. Johannes Kepler proposed an improvement on the design[9] that used a convex eyepiece, often called the Keplerian Telescope. Theory was of little help in the development of refractors.[10] Isaac Newton, in a famous mistake, thought achromatic lenses impossible, delaying for decades[11] the development of the achromatic lens.[12] Aspheric lenses, though understood early on, could not be constructed until the 19th century. The 20th century addition of anti-reflective coating leads to the refractive telescopes of today.

For reflecting telescopes, which use a curved mirror in place of the objective lens, theory proceeded practice. The potential advantages (primarily lack of chromatic aberration) and optical theory were known by 1632, including the Cassegrain and Gregorian configurations.[13][14], but the technology of fabricating and testing the required mirrors was the limiting factor. Isaac Newton developed pitch lap[15] for grinding spherical mirrors and has been generally credited with constructing the first practical reflecting telescopes, the Newtonian telescope, in 1669. Other important developments in reflecting telescopes were paraboloidal metal mirrors over 100 years later[16], silver coated glass mirrors in the 19th century, long-lasting aluminum coatings in the 20th century[17], segmented mirrors to allow larger diameters, and active optics to compensate for gravitational deformation. Finally, recent reflecting telescopes use sophisticated optical designs to simultaneously optimize both light gathering power and field of view, while minimizing off-axis optical aberrations.

A mid-20th century innovation was catadioptric telescopes such as the Schmidt Cassegrain, which uses both a lens and mirror as primary optical elements. The lens, though aspheric, is very weak and serves mainly to correct the spherical aberration of the mirror. This allows very compact optical configurations, provides support for the secondary, and keeps dust out of the optical path.

The resolution of Earth-bound telescopes is determined by seeing and not the diffraction limit. One way around this problem is a space based telescope such as the Hubble space telescope. However, space telescope are extremely expensive and of limited size, so adaptive optics for ground-based telescopes has been developed as well.

Until the mid 20th century, all telescopes were optical, and were called simply 'telescopes'. However, starting with the radio telescope, new devices to record emanations of different forms from the heavens were called telescopes as well. Now the phrase 'optical telescope' (a retronym) is used for a telescope that gathers visible light, to distinguish it from telescopes of other types, ranging from other electromagnetic bands (radio through gamma ray) as well as other forms such as neutrinos, cosmic rays, and gravitational waves. The ancient Greeks knew of the focusing properties of parabolic mirrors [18]

As of 2008, telescopes of less than about 10cm in diameter are typically refractors. Larger telescopes are typically reflectors.

Here are some potentially missing points, in no particular order:

  • Reflecting telescopes were guided by theory, but the mirror technology is hard. Note that the optics used in the Hubble were understood in 1632, and the mirror mis-manufactured in the 1980s. The Hale (Palomar) 5 meter was the last big parabolic mirror, since Hale thought the project risky enough already. All big telescopes since then are Ritchey-Chrétien telescope or more sophisticated yet.
  • In comparison, theory did not help much with refractors, and in fact set them back. In particular, Newton thought an achromatic lens was impossible, and this discouraged research for many years.
  • On the other hand, Newton invented the pitch lap, the key to polishing reflectors for the next few hundred years.
  • In many cases there are three contributors, the person who first discovered it, the person who popularized it, and the person who made it work. Cavalieri (1632) and Mersenne (1636) invented the Cassegrain and Gregorian reflectors, Gregory (1663) and Cassegrain (1672) made them popular (all before Newton's reflector) but could not make them work, but later Hooke (1674, Gregorian) and (???, Cassegrain) figured out how to make the mirrors and make them practical.
  • Many modern telescopes are optimized for etendue as well as diameter. For many research programs, field of view is as important as mirror size (since in a large angle investigation or survey, one can be traded for the other.)
  • Modern telescopes have no need to accomodate a human observer at the focus, though this was a requirement for many years.


  1. ^ Plant Microtechnique and Microscopy, Steven E. Ruzin, Published by Oxford University Press, 1999 ISBN 0195089561, 9780195089561, p. 10
  2. ^ a b The History of the Telescope By Henry C. King, Page 27
  3. ^ BEST IDEA; EYES WIDE OPEN 23rd paragraph
  4. ^ Designing the perfect lens
  5. ^ galileo.rice.edu The Galileo Project > Science > The Telescope by Al Van Helden
  6. ^ Inside the Camera Obscura – Optics and Art under the Spell of the Projected Image Max Planck Institute for the History of Science, 2007, PREPRINT 333, Wolfgang Lefèvre (ed.), p. 42
  7. ^ There is some documentary evidence that the principles of telescopes were known to Roger Bacon, Leonard Digges, Taqi al-Din, and Giambattista della Porta
  8. ^ galileo.rice.edu The Galileo Project > Science > The Telescope by Al Van Helden "The Hague discussed the patent applications first of Hans Lipperhey of Middelburg, and then of Jacob Metius of Alkmaar... another citizen of Middelburg, Sacharias Janssen had a telescope at about the same time but was at the Frankfurt Fair where he tried to sell it"
  9. ^ See his books Astronomiae Pars Optica and Dioptrice
  10. ^ Descartes and the Hyperbolic Quest: Lens Making Machines and Their Significance in the Seventeenth Century By D. Graham Burnett Published by DIANE Publishing, 2005 ISBN 0871699532, 9780871699534
  11. ^ Wrong for the Right Reasons By Jed Z. Buchwald, Allan Franklin Published by Springer, 2005 ISBN 1402030479, 9781402030475
  12. ^ Sphaera - Peter Dollond answers Jesse Ramsden - A review of the events of the invention of the achromatic doublet with emphasis on the roles of Hall, Bass, John Dollond and others.
  13. ^ Stargazer, the Life and Times of the Telescope
  14. ^ Reflecting telescopes were proposed several times by Bonaventura Cavalieri, Marin Mersenne, Niccolò Zucchi, and James Gregory
  15. ^ "pitch+lap"+newton+invented Reflecting Telescope Optics: Basic Design Theory and Its Historical Development By Ray N. Wilson Published by Springer, 2004 ISBN 3540401067, 9783540401063.
  16. ^ John Hadley
  17. ^ Silvering was introduced by by Léon Foucault in 1857, see madehow.com - Inventor Biographies - Jean-Bernard-Léon Foucault Biography (1819-1868). The adoption of long lasting aluminized coatings on reflector mirrors in 1932. Bakich sample pages Chapter 2, Page 3 "John Donavan Strong, a young physicist at the California Institute of Technology, was one of the first to coat a mirror with aluminum. He did it by thermal vacuum evaporation. The first mirror he aluminized, in 1932, is the earliest known example of a telescope mirror coated by this technique."
  18. ^ Thomas Heath. A history of Greek Mathematics. Page 201
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