....Optics Highlights

III. The Telescope

A Dutch eyeglass maker, Hans Lippershey, has been given credit for the invention of the telescope in 1608; when he offered it to the government for military use, they required that it be converted to binocular form. By 1610, Galileo announced the telescopic observations of the moon and planets which signaled the end of the Ptolemaic theory of the heliocentric solar system. Galileo’s telescope was a simple refractor, employing two lenses in a tube. Keppler invented the form of the refracting telescope which is the basis for modern refractors; it has a convex lens placed in back of the focus. The reflecting telescope invented by Isaac Newton used an on-axis planar mirror to move the focus of the parabolic reflector to a point outside the light collecting cylinder; reflector telescopes avoid the problem of lens chromatic aberration which affect refractors. Variations of the Newtonian reflector in which the light was reflected back through a hole in the primary mirror were invented by James Gregory in 1663 and by Cassegrain in 1672. A lens design which resolved the problem of chromatic aberration for refractor telescope was discovered in 1733 by Hall but kept secret until it was uncovered and used commercially in 1759 by John Dolland and his son. A third type of telescope, which uses a spherical mirror and a correcting lens, was invented in 1930 by Bernhard Schmidt. The Schmidt telescope serves astronomy as a wide angle camera.
Hans Lippershey (1570-1619), a native of Wesel, Germany who had settled in the Netherlands, applied for a patent for the telescope in 1608. At least two other Dutch spectacle makers made telescopes about the same time, and there were rumors of some such sort of magical device during the 16th century. Nevertheless, Lippershey was the first to describe the telescope in writing. He was paid handsomely by the government to construct several telescopes, but the patent was denied because it was felt that it could be not kept secret. Certainly, within a short time, three-power telescopes were being made and sold by Parisian eyeglass makers.
Galileo Galilei (1564 - 1643) was the oldest of seven children born to a gifted Pisan composer and mathematician. As an impoverished student, he turned from medicine to mathematics and science, and was known for both brilliance and trouble-making. He began his career as a professor in the University of Pisa, but was soon in conflict with the authorities and was forced to transfer to Padua. He never married but his mistress provided him with three children. He was always in financial difficulties of one sort or other and because he could not afford dowries for his illegitimate daughters he had them placed in convents. In 1609 Galileo learned of the invention of the telescope, built a three power instrument, which he quickly improved to eight, twenty and then thirty power. These were the most powerful instruments of his time and with them he made the discoveries that established Copernican system.
(Sir) Isaac Newton (1642 - 1727) occupies such a large niche in the history of science and mathematics that he now appears as an Olympian, god-like, figure. In fact, however, he was a man subject to acute anxieties that led him to defend his work in such an irrational and furious manner that his friends questioned his sanity (he did suffer a series of nervous breakdowns) and put him in a series of vicious conflicts with such important scientists as Robert Hooke, John Flamsteed (the royal astronomer) and Gottfried Leibniz. Newton’s paranoia is often ascribed to the fact that his father died before his birth and that at the age of two his mother remarried and left him with his grandmother. His work on color phenomena led him to conclude that light consists of distinct particles (of different sizes) with immutable refractive properties and that an achromatic lens was impossible; thus he was led to the invention and construction of the reflecting telescope in 1670. (Shown above). However, for a century, problems in mirror grinding and in maintaining an untarnished surface prevented the use of reflector telescopes.
James Gregory (1638 - 1675), born in Aberdeen, Scotland was a professor of mathematics at the University of St. Andrews (1669 - 1674) and later at the University of Edinburgh (1674 - 77). He left St. Andrews, writing that his salary was not paid and that students "were violently kept from me, contrary to their own and their parents wills, the masters persuading them that their brains were not able to endure it." Gregory was the first to publish a proof of the fundamental theorem of the calculus and letters show that he was the first to discover Taylor's theorem. Gregory also discovered the diffraction grating; he used a bird’s feather! In order to build an observatory at St. Andrews he stood outside the church in his hometown, Aberdeen, and took up a collection.
N. Cassegrain, a French scientist, proposed the telescope design which bears his name in 1672. Nothing else is known about him. One hundred years later, Jesse Ramsden, an English instrument maker who rose from poverty to membership in the Royal Society, discovered that the Cassegrain design may be used to reduce spherical aberration using a paraboidal reflector and a hyperboloid for the secondary reflector. Microwave reflector antennas often employ the Cassegrain design, with the feed horn between the primary and secondary reflectors.
Chester More Hall (1703 - 1771) was an English barrister and amateur mathematician who, like Euler, became convinced from the fact that the eye is achromatic that an achromatic lens could be built. He experimented with different types of glass until he found the correct combination of flint and crown lenses in 1730. Although he later appeared unconcerned by priority claims, he did attempt to keep the discovery secret by contracting the construction separately to two different firms; however, they subcontracted to the same lens maker, George Bass, who realized what was being built.
John Dolland (1706 - 1761) was the son of Huguenot silk weavers who set up an optical business in London with his son. By 1759, Dolland learned the secret of the achromatic lens (from George Bass, it is believed), patented the design, received a medal from the Royal Society and began commercial production which made him wealthy. When it was learned that Hall had invented the achromatic lens, Dolland's patent was contested, but the British court upheld his rights
Bernhard Schmidt (1879 - 1935) was born in Estonia, studied in Germany, and worked as an optical technician at the Bergedorf Observatory, Hamburg. In 1930 he invented the telescope which is named for him. It uses a spherical mirror, not a paraboidal reflector, and employs a correcting plate at the telescope aperture to compensate for spherical aberration, thus it is a combination reflector-refractor system. The Schmidt-Cassegrain telescope is the most popular among amateur astronomers because of its compact design and large aperture and because the optics are completely enclosed .
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