Cassegrain telescope

noun

Cas·​se·​grain telescope ˈka-sə-ˌgrān- 

variants or less commonly Cassegrain or Cassegrainian telescope

ˌka-sə-ˈgrā-nē-ən- 

: a reflecting telescope that has a paraboloidal primary mirror and hyperboloidal secondary mirror, is equivalent in its optical effects to a telephoto lens, and usually has the light brought to a focus through a perforation in the center of the primary mirror

Word History

Etymology

after Laurent Cassegrain †1693 French priest and teacher credited with its invention

Note: The first description of a telescope of this type was published in a learned journal edited by the physician and man of letters Jean-Baptiste Denis, Recoeuil des memoires et conferences sur les arts & les scienses, presentées à Monseigneur le Dauphin, Huitieme memoire …Le 15. Avril, 1672, pp. 121-23, appended to the Journal des Sçavans for 1672. Denis reports on correspondence directed to the journal from a Monsieur de Bercé writing from Chartres; in an extract from the correspondence Bercé in his turn reports on a letter which he had received from a certain "M. Cassegrain," containing the description of the telescope—Denis, presumably at second hand based on Bercé's letter, proceeds to reproduce the description along with a sketch. Nothing further is said about Cassegrain, who has longed remained a figure of some mystery. The French astronomers André Baranne and Françoise Launay, investigating local records in Chartres and Chaudon, have determined that the most likely designer of the telescope was Laurent Cassegrain, a priest and teacher at a secondary school, the College de Chartres, who was born ca. 1629 and died in 1693 (see Baranne and Launay, "Cassegrain: un célèbre inconnu de l'astronomie instrumentale," Journal of Optics, vol. 28 [1997], pp. 158-72).

First Known Use

1806, in the meaning defined above