The Cassegrain Telescope

A Frenchman named Cassegrain came up with a clever design for a reflecting telescope in 1672, only a few months after Sir Isaac wowed the members of London's Royal Society with his Newtonian. What is surprising about Cassegrain is that, considering the impact his idea has had on astronomy over the last four centuries, we know so little about him. Historians are not even sure of the man's first name. Maybe it was "Jacques," or, perhaps, "Guillaume" or "Giovanni." Some historians think his first name was "Laurent." All we know for sure is that his telescope design was so innovative that it, rather than the Newtonian, is the basis for almost all professional telescopes in use today, including the Hubble space telescope. Unlike Isaac Newton, though, it seems Cassegrain never actually built one of his scopes. The Cassegrain existed only on paper for many years, perhaps because it took optical skills a while to catch up with Cassegrain's brilliant conception.

Cassegrain's idea, like Newton's, is simple and seems intuitive once you have heard it. Make a concave mirror with a shape identical to that used in Newtonians. Cut a hole in the center of this mirror. As in the Newtonian, place a secondary mirror at the opposite end of the tube, which will direct light to an eyepiece. Unlike the Newt's secondary, which is flat, the Cassegrain's secondary is convex in shape and is parallel to the primary and positioned so it reflects light back down the tube and through the hole in the primary mirror, as shown in the CAT diagram (Figure 1).

Cassegrain's arrangement has a number of advantages over the Newtonian design. Since viewing is done at the rear of the telescope, as in a refractor, the eyepiece is almost always in a comfortable position. The Newtonian's ocular, in contrast, is fixed at the top of a long tube and may be placed in inconvenient positions as the telescope moves across the sky. The Cassegrain's secondary design offers another advantage: It can reduce the length of the telescope's tube. Since the mirror is a convex shape, it does not just redirect light down the tube of the Cassegrain; it magnifies the image. Because of that, a Cassegrain can pack a long focal length into a short tube. A 6-inch (150-mm) Newtonian with a focal length of 60-inches (1,500-mm) will be nearly 60-inches long. A 6-inch Cassegrain of the same focal length may have a tube half that long or even less, and the shorter the tube, the better. Short telescope tubes dramatically reduce problems involved in designing and building solid yet light mountings.

Is the Cassegrain the perfect telescope? Not exactly. The design is brilliant, but it has some serious failings. One is that, since it usually uses a relatively short focal length parabolic-shaped primary mirror, it suffers from severe coma. What that means to the observer is that objects in the center of a Cassegrain's eyepiece field are sharp, but those on the edge appear out of focus. Stars may look more like comets than pinpoints at the field periphery. Astigmatism, another optical fault common to Cassegrains, may reduce sharpness at both the center and the edges of the field of view. Because of these inherent problems, it is rare to see a pure "classical" Cassegrain telescope today.

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