Laurent Cassegrain, a French instrument maker, died Aug. 31, 1693, at the age of about 64. Cassegrain, whose name is well known to amateur telescope makers, is the most shadowy of historical figures; we didn’t even learn his first name until 1997. In 1672, Cassegrain invented a new kind of reflecting telescope. Isaac Newton and James Gregory had preceded him in offering designs for a reflecting telescope.
The reflecting telescope has certain advantages over the refracting telescope that Galileo used. A reflecting telescope, or reflector, does not exhibit chromatic aberration, which means the colors do not get split apart as they do when passing through a lens. A reflector can also be quite a bit shorter, and therefore easier to use, than the long unwieldy refractors that were common by 1660. The main disadvantage of the reflector is that it was hard to properly shape a metal mirror, and the metal tarnished rapidly, reducing the amount of light gathered.
The reflecting telescope was invented in either 1663, when James Gregory had the idea and published it, or 1668, when Newton was the first to build an actual instrument. All three telescopes – Newtonian, Gregorian, and Cassegrain – use the same basic principle: light enters the telescope tube, is reflected off a curved mirror at the back, and the image is magnified and viewed with an eyepiece. The difference lies in how the image from the main mirror is transmitted to the eyepiece. In the Newtonian telescope (the link is to a diagram of how light rays pass through a Newtonian telescope), the image is reflected out of the tube near the entrance by a flat secondary mirror, so the observer has to stand near the top of the tube and looks sideways through the eyepiece.
In the Gregorian telescope, there is a small concave mirror mounted within the tube, which focuses light back though a central hole in the main mirror, to the eyepiece. The image in a Gregorian telescope is erect, meaning if you are looking at a ship, it will appear right-side up. Here is a diagram of the Gregorian light path.
The Cassegrain works on the same general principle as the Gregorian, except that the small mirror is not concave, but convex, which produces a superior optical path. To use the technical term, the Cassegrain secondary mirror is hyperboloid. Our second image, just below, shows the light path through a Cassegrain reflector.
One advantage of the Cassegrain design versus the Newtonian is that folding the light back on itself makes for a very compact design that is easily portable, even for 5- or 6-inch mirrors. Another advantage is psychological; with the Cassegrain design, it seems as if you are looking “through” the telescope directly at the object, as with a refractor, even though you really are not.
The Cassegrain telescope was first described (and diagramed) in a short paper in the Journal des Scavans in 1672 (first image), based on a letter sent by Cassegrain, shortly after Newton’s design was published in the Philosophical Transactions (1672). We have both Cassegrain’s and Newton’s papers in our History of Science Collection, as well as James Gregory’s Optica promota (1663), in which the Gregorian reflector was first described. Nearly all compact reflectors these days use some variation of Cassegrain’s invention.
The Rolls-Royce of small personal telescopes, the Questar 3.5, uses a modified Cassegrain design. If you are a movie buff, you have seen this telescope at least twice – it was a major appliance in Brian de Palma’s gory-noir film, Body Double (1984), and it also made an extended appearance in the lengthy (and dialogue-less) opening scene of the Charles Bronson thriller, The Mechanic (1972). The Questar built in 1972 is not very different from the ones being sold today.
Dr. William B. Ashworth, Jr., Consultant for the History of Science, Linda Hall Library and Associate Professor emeritus, Department of History, University of Missouri-Kansas City. Comments or corrections are welcome; please direct to firstname.lastname@example.org.