The Early Reflecting Telescope:Cassegrain, Mersenne, and Cavalieri

It is striking how popular histories of the reflecting telescope mirror those of the 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 important work 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 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, Niccolo 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 Niccolo 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 that put mirrors in place of lenses. Even Galileo recognized this 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 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 is shown below on the right . The Hubble Space Telescope site has another diagram of the Cassegrain in the Hubble Space Telescope. Both Cavalieri and Mersenne proposed designs that combined perforated primary reflectors and smaller secondary reflectors. The image on the left is a figure from Bonaventura Cavalieri's Specchio Ustorio 1632(modified from the Max Planck Institute Digital Library). Cavalieri's work may have been missed because he proposed the combination to amplify sound.

Cavalieri's Reflector Cassegrain Telescope
Specchio Ustorio Plate Cassegrain Telescope Design

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 proposed configurations of mirrors that could be used to produce either telescopic effects or burning mirrors. These configurations were clearly prototypical forms of the Gregorian and Cassegrain telescopes. The image below shows Mersenne's diagrams of his telescopes (image was taken from [_3_] ).

Mersenne Telescopes-Harmonie Universelle

There is an amusing irony relating to Harmonie Universelle and Lo Specchio. Harmonie Universelle was a book about music but it is important to the history of telescopes. Lo Specchio was a book about mirrors and telescopes but is probably more important to the history of classical mechanics. Lo Specchio contained the first published description of the parabolic nature of projectile motion. Cavalieri's publication of this important concept even caused a short-lived rift in his friendship with Galileo Galilei. Both Galileo Galilei and Thomas Harriot had described projectile motion in private notes but had never published.

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 [_4_] . 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 [_5_] .

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. Several nineteenth century encyclopedias, including the Encyclopedia Americana , 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. You can find many books from the nineteenth century that reference Mersenne's contribution to the telescope at Google Books. 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 [_6_] . 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).

M. Cassegrain - Man without a Name

On April 15, 1672 , the Journal de Scravans published an excerpt of a letter from a M. de Berce describing a telescope design proposed by a M. Cassegrain. It had a large (primary) concave mirror which reflected light onto a smaller (secondary) convex mirror which then reflected the light back through a hole in the primary mirror to an eyepiece. The diagram that was used to illustrate the device is shown below. Variations of this type of design would dominate the construction of research telescopes from the start of the twentieth century onward. In spite of the importance of the design, the true identity of this M. Cassegrain would not be known for more than three centuries, when it was discovered in 2000. There was a good reason for this obscurity. By the end of 1672, there would be no-one who would want to be this M. Cassegrain or take ownership of the design.

Cassegrain's Original Design

The originator of this design has the dubious honour of receiving a very public humiliation for his design from two of the greatest scientists of all time, Isaac Newton and Christian Huygens. Christian Huygens was especially caustic. Isaac Newton confidently predicted that the design would never be of any use.

You see therefore, that the advantages of this design are none, but the disadvantages so great and unavoidable, that I fear it will never be put in practise with good effect [_7_] .

Both the great scientists were wrong (although some of their criticisms were well-founded). The Cassegrain design is the basis for many of the most famous twentieth century telescopes including the Hubble Space Telescope and the 200 inch Hale Telescope on Mt. Palomar.

The identity of M. Cassegrain was finally discovered in 2000. He was Laurent Cassegrain, a French Catholic priest from the region around Chartres, France. He was a teacher at a lycee (high school). There is an interesting irony here. A common theme in popular literature is the clash between religion and science. The history of the telescope does not do much to support this. Most of the history of astronomical telescopes has been dominated by either Keplerian refractors or Cassegrainian reflectors. The Cassegrain telescope was proposed by a priest. The Keplerian telescope was first proposed by Johannes Kepler, but first built by the Catholic priest, Christopher Scheiner. Many of the important early advances in refractors were also contributed by priests (see Timeline of the Telescope).

Copyright Joseph Sant (2014).
Cite this page (APA).

1. Arriotti, Piero E., , Bonaventura Cavalieri, Marin Mersenne, and the Reflecting Telescope, Vol. 66, No. 3 (Sep.,1975), , 302-321
This article focuses on Bonaventura Cavalieri's published work on the use of mirrors to produce burning mirrors and telescopes. His proposed telescope designs include concave mirrors and flat mirrors as featured in the design of Newtonian telescopes. Cavalieri even suggested a configuration similar to a modern Cassegrain telescope but for concentrating sound not light. He also discusses how two of Marin Mersenne's designs were precursors to the Gregorian and Cassegrain telescopes.

2. Harrison, John, Cambridge University Press, The Library of Isaac Newton, , 62
. Here Harrison discusses the fact that Isaac Newton's library seems to be missing books that would seem important to his work (e.g. very few works by either Galileo or Kepler). He introduces the possibility that this was because his library was a working library, and that many important works were available in Isaac Barrows library. These books include the Specchio and unidentified works by Mersenne. Harrison does mention that the exact date of the start of the friendship cannot be determined exactly but it was definitely before the end of 1669.

3. Watson,Fred, Allen & Unwin, 2007, Stargazer: The Life and Times of the Telescope, , 115
. Fred Watson discusses Mersenne's all mirror telescopes and includes the diagrams captured here.

4. Wilson, R.N, Springer, Reflecting Optics 1 - Second Edition, , 4-6
In this book on optics, Wilson discusses 6 major advances in the optics of reflecting telescopes that can be attributed to Mersenne. The technical nature of most of these points is generally beyond the scope of this article however. He states that with some properties of Mersenne telescopes, it would have been only possible to fully understand the properties with Schwarzschild's work from the early twentieth century.

5. Graham, James, U.C. Berkeley, Historical Introduction-Early Period, , 4-6
Good overview of early history of telescopes at University of California, Berkeley.

6. Allen, Phyllis, , Problems Connected with the Development of the Telescope(1609-1687) Vol. 34, No. 4 (Spring,1943), , 302-311
This is a survey of early telescopes, their inherent problems, and how these problems were solved. In it Mersenne, Cavalieri or Zucchi are never mentioned. The history of the reflecting telescope therefore starts in 1663, with James Gregory's proposal for a reflecting telescope.

7. Newton, Isaac, The Newton Project, The Newton Project, http://www.newtonprojec... ,