Laurent Cassegrain

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Light path in a Cassegrain reflector

Laurent Cassegrain was a Catholic priest born in the region of Chartres around 1629 and died at Chaudon (Eure-et-Loir) on August 31, 1693. At the time of his death he was working as a teacher giving science classes at the Collège de Chartres, a French lycée (that is, a high-school like institution). There is evidence that Laurent Cassegrain was the obscure namesake of the Cassegrain telescope.

In February of 1672, Isaac Newton reported his first invention, the Newtonian telescope to Christiaan Huygens, who promptly published it. Huygens also wrote to Jean Gallois to report the invention, and that letter was published in the February 29, 1672 issue of the French Journal des Sçavans. In England, Newton's invention appeared a month later, in the Philosophical Transactions of March 25, 1672 (number 81). The Journal des Sçavans was completed by the Recueil des mémoires et conférences concernant les arts et les sciences, published by Jean-Baptiste Denys. In the eighth such Mémoire, published on April 25, 1672, is found an extract from a letter written by M. de Bercé, writing from Chartres, where he acted as a representative for the Académie des sciences —scholars of Chartres submitted manuscripts for publication through him.

M. de Bercé reported on a man named Cassegrain who had described, in a manuscript submitted to him, a type of reflecting telescope where a secondary convex mirror is suspended above the centre of a primary concave mirror, the light going through a hole in the centre of the latter to reach the eyepiece. A century later, Jesse Ramsden would praise this design for its cancellation of spherical aberration. Most modern telescopes are Cassegrain designs or variations thereof. A similar design had been published nine years earlier by James Gregory in his Optica Promota (Oxford, 1663), but Gregory's had a concave secondary mirror, and he did not attempt the construction of a prototype. On June 13, 1672, Huygens wrote about the Cassegrain design and critiqued it harshly. The reaction seems extreme to us moderns —maybe Huygens felt Newton's design was being "imperiled" by this alternative. Whatever the motives, the storm of controversy that followed had one lasting effect: Cassegrain's name was forgotten.

Who was Cassegrain? His only publication is a memoir on the megaphone. For a long time, reference works were forced to report his first name as "not conclusively known". The Encyclopaedia Britannica (15th edition, 1974), for example, only goes as far as listing "Cassegrain, N." (this, in turn, seems to come from Hoefer's Nouvelle biographie générale, Paris, 1855). Other sources have suggested the "N." stood for Nicolas.

Some sources (such as La grande encyclopédie, 9, 696) claim his name to be Guillaume, a metal-caster and sculptor who is mentioned in the accounts of king Louis XIV's buildings between 1684 and 1686, and also in a Paris notarized act from 1693. Another name put forward is Jacques, a chirurgeon or doctor, who is mentioned in the Mémoires de l’Académie des sciences as having found, in 1691, a piece of magnet in the steeple of Chartres Cathedral, then being repaired after being damaged by unclement weather. One dictionary mentions a third name, Jean, and gives his birth and death dates as 1625-1712; these dates (and the surname) are highly suspicious as they match those of the astronomer Jean-Dominique Cassini, whose entry immediately follows.

After a long and meticulous investigation, including a search for unpublished manuscripts and the analysis of parish registers in the places where Cassegrain lived (Chartres first and then Chaudon, near Nogent-le-Roi), the man was finally identified as the catholic priest Laurent Cassegrain, born in the region of Chartres around 1629, died at Chaudon (Eure-et-Loir) on August 31, 1693. He was working as a teacher giving science classes at the Collège de Chartres, a French lycée, i.e. a high-school like institution.

Cassegrain Telescope: First developed in 1672, this type of reflector is a combination of a prime concave and a secondary convex mirror, both aligned axially. The prime mirror is usually featuring a hole in the centre thus permitting the light to reach an eyepiece, a camera, or a light detector. Concave Mirror: Paraboloid type Convex Mirror: Hyperboloid type

A Cassegrain reflecting telescope consists of primary and secondary reflecting mirrors. In a traditional Newtonian reflector, light from the primary mirror is deflected by a flat mirror to the eye-piece, placed on the side of the telescope body. In a Cassegrain telescope, there is a hole in the primary mirror; light from the primary mirror is reflected by the secondary mirror through the hole in the primary to reach the eyepiece, placed behind the telescope.

There are three basic types of telescopes: refractors, reflectors and combined lens-mirror systems or catadioptric sensors. Catadioptrics are a combination of a refractor and reflector telescope, using both mirrors and lens to focus the incoming light. There are two popular catadioptrics designs: the Schmidt-Cassegrain and the Maksutov-Cassegrain; both are similarly designed.

See also

External link


sl:Laurent Cassegrain


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