Early Telescope Designers: Laurent Cassegrain

 

Compound Telescope Beginnings

So little is known is known about Laurent Cassegrain that even his name has been disputed. It is known that Cassegrain was born in 1629. He was the son of a surgeon and grew up to become a Catholic priest who taught science classes at the College de Chartes. It has been suggested that Cassegrain purposely kept a low profile to avoid the dire consequences that could easily befall a man of both science and the church.

The idea was to to use a parabolic primary mirror along with a hyperbolic secondary, allowing a clear view without the primary mirror obstruction inherent to standard reflector telescopes. Another advantageous effect of the "folded" light path would be a telescope with a long focal length housed in a short tube. In 1672 Cassegrain's idea was published in the French science journal Recueil des mémoires et conférences concernant les arts et les sciences. It was his only publication and he never developed a working model.

Although Cassegrain's design was at first ridiculed by no less than Isaac Newton and Christian Huygens, the ideas proposed by he and James Gregory eventually became the basis for nearly every catadioptic telescope built in the 20th and 21st centuries. Both Schmidt-Cassegrain and Maksutov-Cassegrain telescopes are the descendants of Laurent Cassegrain's optical configuration. Though Cassegrain's name is now associated with so many modern telescopes, his idea was neither unique nor nor the first such proposal. Roughly ten years before, James Gregory had written of a very similar design.