Reflecting on History

Early Reflecting Telescopes

It is striking how popular histories of the reflecting telescope mirror those of the astronomical (or refracting) telescope. With the refracting telescope, one scientist, Galileo Galilei, is given credit for its development even though at least a half dozen other scientists from his time had made equally important contributions (see Fathers of the Telescope). With the reflecting telescope, one scientist, Isaac Newton, is given credit for its development even though at least a half dozen other early scientists made equally important contributions. In both histories, the critical theoretical contributions of other great historical figures are ignored. With the refracting telescope it is Johannes Kepler who is ignored. With reflecting telescopes, it is Rene Descartes. Descartes work on the geometry of spherical mirrors would prove to be a critical resource in the design of reflecting telescopes. In both cases as well, the critical contributions of church scientists are also ignored.

It is widely taught that Isaac Newton invented the reflecting telescope around 1668. In fact, Isaac Newton was neither the first to propose or build a reflecting telescope. This had been done decades before. It was Father Bonaventura Cavalieri and Father Marin Mersenne who proposed the basic geometry for the construction of reflecting telescopes that is still used today. The magnifying effect of concave mirrors had been put to practical use as reading aids by medieval monks centuries before (see Timeline of the Telescope: Year 1300). The possibility of using the telescopic effect of parabolic mirrors for astronomy was discussed centuries before by Leonardo Da Vinci (see Timeline of the Telescope: Year 1513). This possibility was recognized in Galileo's time as well. It was a Jesuit, Nicolo Zucchi, who set out to prove that it was possible by actually building a crude reflecting telescope using a borrowed parabolic mirror and some lenses. This was more than a half-century before Newton's famous telescope (see Jesuits and the Telescope). What Newton did was develop one configuration of reflecting telescope that would eventually prove more practical than some of the other designs from the seventeenth century. This was not the only seventeenth-century design that would prove practical; the Cassegrain design used in the Hubble Space Telescope and most large research telescopes was originally proposed during Newton's time. Laurent Cassegrain, the originator of this design, was also a Roman Catholic cleric.

Newton:Inventor or Innovator

Isaac Newton developed the design for his reflecting telescope in 1668 (see Timeline of the Telescope). The idea of building telescopes using mirrors instead of lenses had been discussed almost as soon as the refracting telescope became popular in the early seventeenth century. The earliest known attempt to prove a telescopic effect could be achieved using parabolic mirrors and lenses was by Father Nicolo Zucchi, an Italian Jesuit, around 1616. This was only a few years after refracting telescopes started being used for astronomy.

There was a good reason for this early interest in building telescopes using mirrors; there were many serious problems with early refracting telescopes. The lenses of the early telescopes were not made in billion-dollar computer-controlled Japanese factories. They were hand-blown by artisans. By today's standards the glass used was very poor for optics, being plagued by both bubbles and various types of distortions (specifically chromatic and spherical aberration).

Several scientists contemporary with Galileo either built reflecting telescopes or proposed designs for reflecting telescopes. Even Galileo himself agreed that using mirrors to achieve the same effect as a lense was a possibility. Two of these contemporaries, Father Bonaventura Cavalieri and Father Marin Mersenne, anticipated the work on reflecting telescopes that would be done in the latter half of the century. Father Bonaventura Cavalieri and Father Marin Mersenne suggested designs for constructing telescopes using parabolic mirrors instead of lenses. It was well-known that a parabolic mirror could have the same effect as a convex lense...but it wouldn't have the problems associated with chromatic aberration.

Most modern research telescopes use a combination of lenses and mirrors where a large (primary) mirror reflects light onto a smaller (secondary) mirror which then reflects the light back through a hole (perforation) in the primary mirror to an eyepiece. This type of telescope is named after Laurent Cassegrain, the young Catholic priest who proposed the design shortly after Newton proposed his design. A diagram of this type of telescope modified from the Hubble Space Telescope site is shown on the left of the diagrams below. Both Cavalieri and Mersenne proposed designs that combined perforated primary reflectors and smaller secondary reflectors. The image on the right is a figure from Bonaventura Cavalieri's Specchio Ustorio 1632(from the Max Planck Institute Digital Library). Cavalieri's work may have been missed because he proposed the combination to amplify sound.


Cassegrain Telescope Design	Specchio Ustorio Plate

Marin Mersenne in Harmonie Universelle (1636) discussed a similar configuration for telescopic designs. In Specchio Ustorio, Bonaventura Cavalieri also proposed a telescope design using a flat secondary mirror angled at a diagonal, just as Newton had. Since Cavalieri did not provide a figure for this design , it is difficult to know for certain how close his design would have been to Newton's. [_1_] . Newton would have had easy access to Cavalieri's work on telescopes early in his career. His friend and mentor, Isaac Barrow, had a copy of the Specchio Ustorio in his private library [_2_] .

Marin Mersenne and the Telescope

Today, we wouldn't expect to find major advances in the design of telescopes in a book on music. In 1636, Father Marin Mersenne published Harmonie Universelle, a mathematical study of music. It also contained a discussion of configurations of mirrors that could be used to produce either telescopic effects or burning mirrors. These configurations, illustrated by diagrams, were clearly prototypical forms of the Gregorian and Cassegrain telescopes still used today. Marin Mersenne may have more clearly described and illustrated the construction of these forms than Cavalieri but he was careful to share credit with Cavalieri.

Mersenne's work on reflecting telescopes was very advanced. Modern specialists in optics doubt that either he or any of his contemporaries (including Descartes and Galileo) understood the full significance of his work. A full understanding of the advanced nature of Mersenne's work would have to wait until the twentieth century. An indication of this is that the Mersenne telescope, still being produced today, is largely a development of the twentieth century. Mersenne, went further than simply presenting configurations that are used in modern telescopes; his designs featured the strong telephoto effect critical to modern photographic lenses. This all happened 30 years before Newton's telescope [_3_] . Mersenne actually never did build telescopes to his designs. Oddly, he was dissuaded from building them by Rene Descartes. Descartes felt that reflecting telescopes were impractical. Given the technology of the day, he was probably correct. It would be centuries before reflecting telescopes were competitive with refractors. The mirrors of the time were made of polished metal which tended to tarnish. Also the tolerances for mirrors is four times more critical than for lenses [_4_] .

A question is raised by Mersenne's work; Why is his work so rarely mentioned in modern histories of the telescope. This includes educational websites and many books. This is not easy to answer. Mersenne was widely considered to be the inventor of the reflecting telescope in the nineteenth century. Nineteenth century encyclopedias such as the Encyclopedia Americana, the Popular Encyclopedia, and the Icenographic Encyclopedia of Science identified Mersenne as the inventor of the reflecting telescope. Others, if they did not credit him with the invention of the reflecting telescope, did mention his contribution to its early development. Yet by the middle of the twentieth century Mersenne had largely disappeared from the discussion of the early history of the telescope. In 1943, a survey of the early development of the telescope was published in Isis, a respected journal of the history of science. Mersenne and Cavalieri are not to be found amongst the more than two dozen seventeenth century scientists mentioned. The article begins its discussion of reflecting telescopes with James Gregory in 1663 [_5_] . The importance given to a historical figure sometimes depends as much on the dominant biases of the day as on their contributions (see Sarton-A Case for Bias).

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