Putting it All Together The Schmidt Cassegrain

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By the middle of the twentieth century, the two pieces of the Schmidt Cassegrain puzzle, the classical Cassegrain telescope and the Schmidt camera, were lying around waiting for someone to assemble. It was also at about this time that amateur astronomers began to be in need of a telescope of a new type.

Two things were changing the amateur's world as the 1960s arrived: light pollution and an interest in picture taking. The unchecked growth of the suburbs and the brightening of most astronomers' home skies meant more and more observers had to travel to get good views of the night sky. Also, quite a few of the more serious amateurs were trying to take pictures of what they saw. The average amateur's traditional instrument, the long-tubed Newtonian reflector, did not fit in well with either of these new realities. Long Newts were not easy to haul around to dark sites and often had to be rebuilt if not redesigned before they could be used for astrophotography.

It was becoming more and more obvious that something like a Cassegrain with its short tube and convenient eyepiece (or camera) position would be an ideal telescope for amateur astronomers. Many amateurs did build or buy classical Cassegrains at this time. Unfortunately, home-built Cassegrains were often a bust. The convex secondary mirror was considerably harder to make than it seemed. Store-bought

Casses? Even if made perfectly, the optical problems inherent in the design discouraged even the most forgiving amateurs.

A few brilliant amateur telescope makers thought they had a better idea. They had been experimenting with a design that combined the Schmidt camera and the classical Cassegrain. This "Schmidt Cassegrain" took the Schmidt camera's spherical primary mirror and corrector plate and added the Cassegrain's convex secondary and behind-the-primary eyepiece arrangement (Figure 1). In most designs, the secondary, like the primary, was spherical in shape. These mirrors' curves were figured so the secondary could be placed in a holder suspended near the corrector end of the tube or even attached to the corrector itself. This SCT design would be easy enough to make—two spherical mirrors are easy for even a novice "glass pusher"—if only a way could be found to produce that nasty corrector easily.

The stumbling block for amateur telescope makers was the same thing that gave Bernhard Schmidt fits (literally) 30 years before: the corrector lens. Some advanced and enterprising amateurs tried their hand at SCTs nevertheless. A few of the most talented workers were able to grind and polish correctors by hand. Most, however, tried Schmidt's vacuum trick. Some were successful, but most found the Schmidt trick hard to execute without a well-equipped optical shop at their disposal. There things remained for a while. An SCT would occasionally show up at Stellafane, the big U.S. amateur telescope-making yearly convention, but these CATs were curiosities. The SCT's impact on the average amateur was nil. Correctors would never be practical for most people to produce at home, and commercially made telescopes that required these labor-intensive lenses would, it seemed, be far too expensive for the average amateur to afford.

That's what everybody thought, anyway.


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