The Early Reflecting Telescope:Cassegrain, Mersenne, and Cavalieri

Isaac Newton is often credited as the inventor of the reflecting telescope. In fact, a crude reflecting telescope had been built and used more than half a century before Newton's invention. The design for the Mersenne telescope, a reflecting telescope in common use today, was published before Newton was born. The early history of the reflecting telescope has its surprises. The builder of that first crude telescope, Nicolo Zucchi, and the scientist who published the Mersenne telescope design, Marin Mersenne, were both Catholic priests. The involvement of church scientists in the early development of both the reflecting and refracting telescopes is often missed (see Fathers of the Telescope).

Isaac Newton developed the design for his reflecting telescope in 1668 (see Timeline of the Telescope). Newton's idea of building telescopes using mirrors instead of lenses was not new. 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). Leonardo da Vinci had used concave mirrors to study the planets more than a century earlier (see Timeline of the Telescope: Year 1513). The earliest known attempt to build a reflecting telescope was by Father Niccolo Zucchi, an Italian Jesuit, around 1616. This was only a few years after refracting telescopes started being used for astronomy. Zucchi stated that he used it to view objects both "celestial and terrestrial". Zucchi was not happy with the result so he went back to building and using refracting telescopes.

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 of Galileo's contemporaries 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 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 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. Marin Mersenne proposed a similar design in Harmonie Universelle (1636).

Specchio Ustorio Plate	Cassegrain Telescope Design

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 both Cavalieri's and Nicolo Zucchi's work on telescopes early in his career. His friend and mentor, Isaac Barrow, had a copy of the Cavalieri's Specchio Ustorio in his private library [_2_] as well as a copy of Zucchi's Optica Philosophica.

You don'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 prototypical forms of the Gregorian and Cassegrain telescopes. The image below shows Mersenne's diagrams of his telescopes (image was taken from [_3_] ).

There is a side story to Mersenne's and Cavalieri's works. 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. Today it is doubted 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 correct. It would be more than a century 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_] .

Why are Mersenne's contributions ignored? It is difficult to find his name in popular histories of the telescope. 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 ( see Google Books). Mersenne fell from grace in the twentieth century. Mersenne was not even mentioned in a 1943 survey of the early development of the telescope by the historical journal, Isis [_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).

On April 15, 1672 , the Journal de Scavans 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.

This author 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 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_] .

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).