Cassegrain (c. 1650–1700)

French inventor of the system of mirrors used within many modern reflecting telescopes and sometimes used in large refraction telescopes.

Nothing is known for certain about Cassegrain's life – not even his first name. Believed to have been a professor at the College of Chartres, he is variously credited with having been an astronomer, a physician, and a sculptor at the court of Louis XIV.

Cassegrain's telescope, an improvement on a design by English scientist Isaac Newton, used an auxiliary convex mirror to reflect the image through a hole in the objective – that is, through the end of the telescope itself. One intention behind this innovation was further to increase the angular magnification. A century later it was noted that this also partly cancelled out the spherical aberration (see aberration, optical).

Cassegrain also submitted a scientific paper concerning the megaphone to the Academy of Sciences in Paris.