The Early Reflecting Telescope:Cassegrain and Mersenne

Over the last 120 years, almost all of the largest optical telescopes were reflecting telescopes with a "Cassegrain" configuration. The earliest champions of this design published their designs 30 years before the debut of Newton's telescope. Histories of the reflecting telescope typically skip over their contributions to start with Isaac Newton (1668) or James Gregory (1663). The early proponents of reflecting designs (Marin Mersenne, Bonaventura Cavalieri,Laurent Cassegrain) deserve a closer look. By coincidence, all three were Roman Catholic priests.

Marin Mersenne presented workable designs for reflecting telescopes that are still used today. But he was not the first to toy with the idea of using mirrors in place of the lenses of the telescope. Understanding the contributions of Mersenne, or for that matter Newton, requires some attention to the work that preceded them and that was being done by contemporaries.

Interest in the idea of building telescopes using mirrors instead lenses began shortly after the invention of refracting telescopes. The Jesuit priest, Niccolo Zucchi, even built a crude reflecting telescope as early as 1616. The motivation for this interest were some very serious problems with early refracting telescopes. With refracting telescopes, glass lenses are used to refract (redirect) light rays at the objective to a single focus point. The problem is that glass refracts the different wavelengths of light differently, resulting in different focus points for different colors. This is known as chromatic aberration. This could result in fuzzy edges to objects. Another problem was the actual quality of the glass. Bubbles in the glass were common.

The reflecting telescope did not have these problems. Several configurations of convex mirrors ,concave mirrors and lenses were proposed, some of which are still in use today. Different shapes were suggested for the mirrors (e.g. spherical, paraboloid). Early reflecting telescopes were made from these designs using the speculum metal then used for regular mirrors. This is an alloy of copper (2 parts) and tin (1 part). Silvered mirrors good enough to be used in telescopes would not be available for another 200 years (1856-1857).

Whatever problems refracting telescopes had at the time were small by comparison to those of reflectors. The tolerances for mirrors is four times more critical than for lenses [_1_] . A reflecting telescope that could compete with refracting telescopes would require a parabolic or hyperbolic shape. This was beyond the capability of the craftsmen of the day. While precision grinding and polishing of glass to create lenses had been practiced for centuries in Europe (i.e. circa 1300 ), shaping metal to this precision was something new. As a result, Isaac Newton's compromise of using a spherical shape had little hope of matching the sharpness of existing refracting telescopes.

Another problem with early reflecting telescopes was the speculum metal itself. It reflected only about two thirds of the light that hit it. Being metal, it expanded and contracted much more than glass with temperature changes. Worst of all, speculum metal tarnished. That meant it would need to be removed and repolished (potentially changing its shape). Spare mirrors would be needed if the telescope was to be used while a mirror was being repolished.

Interest in the use of mirrors to concentrate light dates back to at least 212 BCE. That is the date that Archimedes is thought to have assembled 'burning mirrors' in Syracusa to set a Roman fleet afire from several hundred metres. Burning mirrors, like reflecting telescopes, would be required to redirect incoming rays to a focus point. Today 'burning mirrors' are called solar concentrators and are typically used to generate electricity. Interestingly, one design for solar concentrators is the Cassegrain design so popular in astronomy.

The magnifying effect of concave mirrors was well known to both the Muslim and medieval European natural philosophers. Concave mirrors (known as reading mirrors) had been used as reading aids in Europe right up to sixteenth century (see University of Arizona). The image above is from a fresco of St. Isnardo clearly showing a reading mirror on the shelf. The fresco by Tomasso da Modena was completed in 1352 (see here). In the early 1500's, Leonardo da Vinci had used concave mirrors to study the planets(see Timeline of the Telescope: Year 1513). This means that that by the time the refracting telescope was 'invented' in 1609, it was widely known that concave mirrors behave much like convex or plano-convex lenses.

The earliest clearly documented attempt to build a reflecting telescope was by Father Niccolo Zucchi, an Italian Jesuit priest, around 1616. This was only a few years after the debut of refracting telescopes. 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. Recently, there have been claims that Leonard Digges may have built a reflecting telescope as early as 1556. The language he used to describe his device was vague so many doubt it was a reflecting device.

The fascination with burning mirrors lasted right up to Galileo's time. In 1632, Bonaventura Cavalier, an important mathematician and Catholic priest, wrote a book on burning mirrors titled Lo Specchio Ustorio. In it, Bonaventura Cavalieri 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 [_2_] . 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 Lo Specchio Ustorio in his private library [_3_] as well as a copy of Zucchi's Optica Philosophica.

In Lo Specchio, Cavalieri also proposed a device to amplify sound that employed a Cassegrain configuration (see image below).

The diagrams below show the Gregorian and Cassegrain configuration of mirrors used in modern reflecting telescopes. The diagrams are from Harmonie Universelle, published in 1636 by Father Marin Mersenne [_4_] . Mersenne's work on reflecting telescopes was very advanced. Today it is believed that neither he nor 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 [_5_] .

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. Mersenne intended to build telescopes to his designs but was dissuaded by Rene Descartes. Descartes, being the originator of analytical geometry, understood the challenges in shaping a mirror to the correct parabolic curve. Descartes was right; James Gregory could not find anyone competent to build a parabolic mirror for his telescope, and Newton gave up on parabolic mirrors and settled for a spherical mirror [_6_] .

Several nineteenth century encyclopedias, including the Encyclopedia Americana, credited Mersenne as the inventor of the reflecting telescope. Other sources, if they did not credit him with the invention of the reflecting telescope, did mention his contribution to its early development (see Google Books). By the middle of the twentieth century, Mersenne had largely disappeared from both academic and popular discussion of early telescopes [_7_] . Mersenne's fall from grace may be because he was a Catholic priest. Published work on the history of science of the early 20th century was biased heavily toward the theory that church and science conflict (see Sarton-A Case for Bias).

M. Cassegrain - Man without a Name

Answering this question is more difficult than it seems. If inventing means being the first to use a combination of mirrors and lenses to make distant objects appear magnified then the inventor is probably Leonard Digges(1556) or Nicolo Zucchi (1616). If inventing means being first to publish an effective design for a reflecting telescope then the inventor is Marin Mersenne (1636) or Bonaventura Cavalieri (1632). If inventing means the first to design and build a minimally functional reflecting telescope then the inventor is Isaac Newton (1668).

History is not always cut and dried. Leonard Digges descriptions of his devices were very vague so they may not have been reflecting telescopes. Bonaventura Cavalieri's reflecting design was designed to concentrate (magnify) sound, not light. Isaac Newton is considered the inventor of the reflecting telescope because his design was supposedly practical, but it wasn't practical when compared with refracting telescopes of the day. Newton discontinued his foray into telescopes after his second prototype. The technology and materials to build reflecting telescopes that could be competitive with refracting telescopes would not be available until many decades after Newton's death.

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).

Another interesting lesson from the early history of reflecting telescopes is the importance of "technology gaps". On paper, the reflecting telescope had many advantages over refracting telescopes. The promise of reflecting telescopes wouldn't fully be realized for centuries because the technology to build superior reflecting telescopes wasn't yet there.

Looking for the older page, it's here.