A simple description of telescope is an instrument designed to magnify distant objects so that they can be viewed more easily. Telescopes historically have been constructed of lenses and mirrors which concentrate visible light into a smaller and more defined image.
The telescope may have been the first device to extend the human senses and in that way it was at the forefront of the scientific revolution. Its contributions have been profound. Prior to the invention of the telescope, astronomers had made many inferences about the nature of the universe, noting the position of the stars and how they changed, the changing aspects of the Moon, a host of ideas abounded dating back to the Greeks and possibly further back to the Babylonians and the Egyptians. The telescope vastly extended the work that had previously been done with the unaided eye.
It was evidently a bit of a novelty in its initial stages but became an instrument of serious research in the hands of Galileo. How or when it was first invented is a bit obscure and its lineage is the subject of debate.
History of the development of the telescope
Leonard Digges (1520-1559)
Leonard Digges was a writer of mathematics and science in English, one of the first people to popularise work in either field. He was also a surveyor who invented a theodolite used in surveying. A few scholars, notably Ronan in 1991 and Gribbins in 2002, have concluded Leonard designed a reflecting telescope and, possibly, a refractive telescope. However subsequent publications from Leonard's son, Thomas, evidently do not mention their use nor were there any physical apparatus left to study. There is clear evidence that he and his son Thomas speculated about the design and function of telescopes and according to Colin Ronan and John Gribbin, their work was not mere speculation but evidence drawn from actual application of their design. Leonard's son claimed his father used lenses to make a sort of telescope, but also makes claims about the ability of the apparatus to make out details seven miles away. Given the technical skill available at the time, some authorities speculate that it is unlikely that Thomas was speaking about a reflective telescope although he may have been discussing a refractive telescope. Colin Ronan and Gilbert Satterthwaite constructed a device reported sometime later in the 16th century by a Mr. William Bourne which draws on the description of the device made by the Digges and concluded that observations at that distance using magnification of 8-power would have been sufficient to substantiate this claim. 
Hans Lipperhey is commonly credited with having invented the telescope but it is clear that he was not the first to have understood the design and its capabilities. It is also apparent that the Diggeses' telescope design may not have gained widespread use and as such, was by passed.
John Dee was a mathematician of renown and while he holds no claim to having invented the telescope, Dee was the guardian of Leonard Digges' son, Thomas Digges, following the death of Leonard, and had a hand in his education and support in his efforts. Dee is also a source for the origins of the telescope. Dee noted in a preface to Billingsley's translation of Euclid (1570) what he refers to as Perspective glasses, a term used by Thomas Digges in Pantometria (the word telescope was actually coined in the 17th century). In this preface Dee offers advice to the military commanders on how to obtain information about enemy forces:
- He may wonderfully helpe him selfe, by Perspective glasses. In which (I trust) our posterity will prove more skillfull and expert, and to greater purposes, than in these days, can (almost) be credited to be possible.
Thomas Digges (1543-1595)
In 1571, Thomas Digges published Leonard Digges's book on the telescope, Pantometria, twelve years after his father's death. Pantometria was the first publication to discuss the invention of the telescope in English. Thomas had extended, revised and enhanced the book and he wrote the preface. J J O'Connor and E F Robertson note that while the description of how lenses could be combined to construct a telescope, there is no known evidence that the Diggeses did actually make a telescope and use one. Some authorities take exception to this view and regard the issue to be resolved. How is it that the Diggeses knew how to construct a telescope and yet did not? Having constructed a telescope how is it they would not have used it? In his preface to the Pantometria, published the year after John Dee's Preface to Euclid, Thomas noted specifically how his father had observed things with "Perspective glasses' on numerous occasions from a considerable distance and with witnesses present. Thomas wrote:
- ..... my father by his continual pain-full practices [practical experiments], assisted with Demonstrations Mathematicall, was able and sundrie Times hath by proportionall Glasses duly situate in convenient angles, not onely discovered things farre off, read letters, numbered peeces of money with the very coyne and superscription thereof, cast by some of his freends of purpose uppon Downes in open fields, but also at seven miles declared what had been doon at that instant in private places.....
This is clear evidence to some authorities that Thomas was describing a telescope that had indeed been constructed and used. The absence of specific notes on the use of a telescope are therefore not to be regarded as proof that the Diggeses, father and son, never made nor used a telescope.
Colin Ronan (1920-1995) and Gilbert Satterthwaite (Department of Physics, Imperial College) built a working telescope in the late 1980s from a description provided in a report on military and naval inventions, written in 1578 William Bourne, for Lord Burghley, Elizabeth I's Secretary of State. Essentially, this shows that it was an invention that predates the claims for the invention by the Hans Lipperhey by at least 30 years. Considering that the Pantometria was published 12 years after Leonard's death it is not unreasonable to assume that the telescope may have been invented more than 40 years before Hans Lipperhey.
Colin Ronan's perspective on this matter has been refuted by Fred Watson. Essentially, Watson is not saying that mainstream scholarship does not credit the Diggeses with inventing a telescope at all. He is saying that the technology to make a more complicated telescope was beyond the Diggeses in the 16th century.
What actually constitutes the invention of the telescope? Isaac Newton is credited with building the first functioning reflective telescope and Galileo was using a refractive telescope of his own construction in 1609 that was powerful enough and with sufficient resolution to provide him a useful perspective of the lunar surface. Reflective telescopes are a more economical design and can produce much higher resolutions in a smaller space but their precision is more exacting. It is important to note however, that the means existed and the diagrammes published in several sources in the 16th century show that whether or not the result was more than novelty, it is clear that the invention of the telescope predates the 17th century.
Whether Thomas used telescopes further is unclear. One would suspect that if his father's work had been truly useful, his later publications would have born evidence of their employment in his work. Thomas’s publication, Alae seu scalae mathematicae, in 1573, was a Latin text prompted by the new star of 1572, a supernova. Thomas's observations were of such quality that his research was employed by Tycho Brahe in his work. The supernova created quite a stir worldwide and certainly in Europe. There was a tremendous increase in astronomical and astrological work and publications. Tycho Brahe's supernova was significant because it encouraged astronomers in the 16th-century to question their perception that the heavans were immutable, that is, unchanging. Thomas's contribution was to determined the nova's position and his conclusion that its appearance was a challenge to traditional cosmology of the day. 
Hans Lipperhey (d. 1619)
Lipperhey (or Lippershey) was a German born spectacle maker who, while working in the Dutch province of Zealand, filed what is thought to be the first patent for the refractive telescope in 1608. At that time there were several other patents pending. Lipperhey was apparently employed to make two lenses, one convex and the other concave. When the client appeared to take possession of the lens he positioned them to show how they magnified distant objects when used in tandem. Another claimant to the patent was the son of Sacharias Janssen. Janssen later noted that his father already had a telescope of Italian manufacture, dated 1590. These events predate Lipperhey’s claims.
Galileo first heard of the telescope in 1609 and was able construct his own model from the description given him about the device patented by a spectacle maker in the Netherlands, Hans Lipperhey. He made several models including an 8-power telescope and a 20-power. Galileo went on to make observations of the Moon, the Sun, the planets and stars.
Zik (2001) notes that before the telescope scientific observation relied on instruments such as Heron's diopter, Levi Ben Gershom's cross-staff, Egnatio Danti's torqvetto astronomico, Tycho's quadrant, Galileo's geometric military compass, and Kepler's ecliptic instrument. Galileo not only had to adapt the telescope to astronomy, he also had to create a system by which it could be integrated into scientific knowledge. To do so Galileo showed its images were real and not caused by defects in lenses nor illusions in the eye of the observer. More importantly, he had to adapt or invent measurement techniques and provide a way to process data while recognizing the issue of measurement error. He realized that the accurate measurement of natural phenomena is a challenge and that suitable protocols had to be established and agreed to by the community of astronomers. Historians of science explore the linkage established by Galileo among theory, method, and instrument, in his case the telescope. Although the telescope was invented independently of astronomical science, Galileo's innovative optics married the machinery and the theory to close the gap between image in the eyepiece and scientific language--that is, between drawing what was seen and reporting physical facts. He thus bridged the gap between merely sketching the sky and actually describing it and created a scientific methodology using the new instrument that all astronomers came to follow.
Types of telescopes
Optical telescopes were the first type used. Used to view only visible light, they incorporate lenses made of glass and reflective mirrors. They are of varied design.
Telescopes incorporating only glass lens are refractive in that they magnify light by bending it through two or more successive lenses.
Early descriptions of telescopes employed mirrors but the earliest known practically operable telescopes for which we have resultant views of the heavens are those of Galileo from the year 1609. Galileo employed a convex and a concave lens placed in tandem in a tube of opaque material. Galileo initially constructed two refractive telescopes, a three power and a nine power. The images this combination produces are upright.
In 1611, in his book Dioptrice,Kepler suggested using two convex mirrors. The resultant image is upside down but it produced a larger field of view and higher magnification than the Galilean type.
"Achromatic" simply means "without colour." The simple convex lenses used in early telescopes produced coloured fringes (chromatic aberrations) around the object being viewed which tended to distort the image. A simple convex lens will focus different wavelengths at different points, literally separating the wavelengths and distorting the image. Red and blue fringes on photographed objects are still apparent today. A common example of colour separation are 3-D images which are deliberately separated to be viewed with special glasses.
The achromatic lenses combined two lenses made from different types of glass which eliminated the coloured fringes by focusing different wavelengths, notably red and blue, at the same point, decreasing distortions. Achromatic lenses were evidently first perfected by Chester Moore Hall nearly forty years after Gregory proposed the idea. In 1758 however, John Dolland, an optical instrument maker in London, was the first to take out a patent for producing achromatic lenses commercially. The improvement on image was enhanced by the fact that achromatic lenses allowed telescopes to have shorter focal lengths.
Apochromatic means "free of chromatic and spherical aberration" (e.g. an apochromatic lens).
Reflective telescopes employ mirrors to magnify and reflect the images through an ocular lens.
Catadioptric telescopes are a combination of lenses and mirrors
for a more detailed guide see the Bibliography subpage
- Brunier, Serge, and Anne-Marie Lagrange. Great Observatories of the World (2005) 240pp; covers 56 observatories excerpt and text search
- King, Henry C. The History of the Telescope (2003) 480pp
- McCray, W. Patrick Giant Telescopes: Astronomical Ambition and the Promise of Technology (2nd ed. 2006) excerpt and text search
- Van Helden, A. "Telescope and Authority From Galileo to Cassini." Osiris 1994. Vol. 9:9-29 in JSTOR
- Zik, Yaakov. "Science and Instruments: the Telescope as a Scientific Instrument at the Beginning of the Seventeenth Century." Perspectives on Science 2001 9(3): 259-284. Issn: 1063-6145 Fulltext: Project Muse
- Zimmerman, Robert. "More Light!" American Heritage of Invention & Technology 2002 18(2): 14-23. Issn: 8756-7296, on 1970s Fulltext: online
- Zirker, J. B. An Acre of Glass: A History and Forecast of the Telescope Johns Hopkins U. Press, 2005. 343 pp. excerpt and text search
-  Ask Oxford; Shorter Oxford English Dictionary on Historical Principles (2007) Sixth Edition, Vol. 2, Oxford University Press, New York; Also telo (τηλó), for example, télothen (τηλóθν), “from a distance” télourós (τηλουρóς), “far off.”
- ,  &  S.C. Woodhouse (1910) Woodhouse's English-Greek Dictionary, The University of Chicago Library
-  from World Book at NASA for students adapted from "Telescope." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.
- Telescope Richard S. Westfall (1995) Department of History and Philosophy of Science, Indiana University for the Galileo Project, Rice University
- Gribbin, J. (2002) Science: A history. London: Penguin
- Thomas Digges: Gentleman and mathematician Stephen Johnston (1994) chapter 2 (pp. 50-106) of, ‘Making mathematical practice: gentlemen, practitioners and artisans in Elizabethan England’ Ph.D. thesis, Cambridge. Available through University of Oxford, Museum of History of Science; It is interesting to note that Johnson does not mention the word "telescope" in his article.
- Thomas Digges O'Connor, J. J. and Robertson, E. F. (2002) MacTutor History of Mathematics Archive, School of Math and Statistics, University of St. Andrews. The authors note, "The completed work [Pantometria] contains Digges' description of how lenses could be combined to make a telescope."
- Leonard Digges Richard S. Westfall, Department of History and Philosophy of Science, Indiana University for the Galileo Project, Rice University; Westfall says Digges invented a theodolite but mentions no telescope. He does however address this possibility in his briefs on "Telescope" and "Digges, Thomas" included here.
- Telescope Richard S. Westfall (1995) Department of History and Philosophy of Science, Indiana University for the Galileo Project, Rice University; In this article Westfall allows that " It is possible that in the 1570s Leonard and Thomas Digges in England actually made an instrument consisting of a convex lens and a mirror, but if this proves to be the case, it was an experimental setup that was never translated into a mass-produced device." and further notes that, "The claim for an "Elizabethan telescope" has recently been made by Colin Ronin, who has demonstrated an instrument based on the writings of Thomas Digges and William Bourne."
- Did the reflecting telescope have English origins? Colin A Ronan (1991). Leonard and Thomas Digges. Journal of the British Astronomical Association, 101, 6. Ronan did not accept the Digges invention in his 1969 history, but published an unrefereed paper late in life making the claim. The address contained herein, was published in Journal of the British Astronomical Association, 101, 6, 1991. which is a refereed Journal.
- Fred Watson (2006) Stargazer:The life and times of the telescope. "From Pantometria and from writings of another Elizabethan mathematicians, William Bourne (also a contemporary of Thomas Digges), it is clear that the instrument described by the two Diggeses incorporated a dished mirror though probably unlike the modern reflecting telescope. . . . " "But what might be called mainstream scholarship upholds and opposing view, namely that the first practical reflecting telescope was constructed by Issac Newton in 1668."
- sometimes referred to as Tycho's Supernova See reference to NASA/ESA Space Telescope cited below.
- Thomas Digges Richard S. Westfall, Department of History and Philosophy of Science, Indiana University for the Galileo Project, Rice University. Westfall states that: "Like his father Digges was skilled in so-called "perspective glasses."
- heic0415: Stellar survivor from 1572 A.D. NASA/ESA Space Telescope. On Nov. 11, 1572, Tycho Brahe observed a star in the constellation Cassiopeia as bright as Jupiter which eventually equaled Venus in brightness. It was visible during daylight for about two weeks and eventually faded from unaided view altogether after about 16 months.
- Hans Lipperhey Richard S. Westfall (1995) Department of History and Philosophy of Science, Indiana University for the Galileo Project, Rice University; The patent was denied on the grounds that the device could not be kept secret.
- Drake, Stillman (1973). "Galileo's Discovery of the Law of Free Fall". Scientific American v. 228, #5: 84-92
- Galilei, Galileo (1610). The Starry Messenger (Sidereus Nuncius). In Drake (1957):22-58
- The Dioptra was a surveying instrument similar to a theodolite. Heron was active in the first century A.D. Evangelos Papadopoulos, "Heron of Alexandria" online; also Ancient Greek Technology - Measuring Instruments (2004)
- David P. Stern "The Cross Staff" (2003) Gershom (France, early 14th century) is credited with a device that allowed navigators to estimate latitude while at sea.
- Tycho's 1576 Brass Azimuthal Quadrant High Altitude Observatory (HAO), Earth and Sun Systems Laboratory (ESSL), National Center for Atmospheric Research (NCAR)
- Yaakov Zik, "Science and Instruments: the Telescope as a Scientific Instrument at the Beginning of the Seventeenth Century." Perspectives on Science 2001 9(3): 259-284.
- Blackboard optics UCLA Physics department has an excellent interactive lens and mirror applet designed by Wolfgang Christian that can be used to explore various parameters such as lens, mirror, aperture, beam, objective, and source.
- The First Telescopes Center for History of Physics, American Institute of Physics
- Optical aberrations Mortimer Abramowitz et al (2003) Florida State University
- professor of mathematics University of Edinburgh, and Savilian Professor of Astronomy at University of Oxford
- in his Edinburgh lectures published in 1695 as an optics textbook, Catoptricae et dioptricae sphaericae elementa
- Papers of David Gregory Navigational Aids for the History of Science, Technology & the Environment, University of Edinburgh
- Achromatic Lenses National Maritime Museum, Greenwich, London
- Apochromatic Merriam-Webster Medical Dictionary