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.


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. . 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 .

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