Tom Johnson was a man with a mission, but that mission had nothing to do with telescopes, at least not at first. Johnson had formed a small company in the early 1960s in southern California, Valor Electronics, to produce power supplies and other items for the burgeoning hi-tech industries of the area, and he was focused on making his firm a success. There things would have stayed if he had not bought a small Newtonian reflector for himself and his children. That was the spark. Johnson became one of those rare people who discover their true calling, and he was soon a full-fledged amateur telescope maker. By 1963, he had finished building an innovative 18.75-inch Cassegrain telescope, a huge amateur instrument for the time, and was showing it off at area clubs and star parties. Tom Johnson and his absurdly big, portable, and advanced telescope were featured in a Sky & Telescope magazine cover story that year.
Johnson soon set his sights higher than just occasionally contributing to astronomy magazines, however. Seeing the tremendous response his big Cassegrain got at amateur gatherings, he began to wonder if he could sell telescopes like it to amateurs or maybe sell telescopes that were even more advanced and easier to use and transport. He was well aware of the Schmidt Cassegrain design and its potential advantages for the amateur astronomer. He was also aware of the problems involved in fabricating corrector lenses. While considering the problem, he ordered a 20-inch corrector plate from optics giant Perkin-Elmer, but by the time this massive, expensive piece of glass arrived he had already thought of an elegant and inexpensive way to make Schmidt Cassegrain telescope correctors.
It was obvious that hand grinding and figuring correctors would not be practical in a mass production setting. Schmidt's vacuum trick was the way to go, but something needed to be done to make the process easier. What he came up with was his "master block" process. In this method a master corrector "form" is first
R. Mollise, Choosing and Using a New CAT, »19
DOI: 10.1007/978-0-387-09772-5_3, © Springer Science + Business Media, LLC 2009
ground and polished into a Pyrex glass blank somewhat larger than the desired size of a finished corrector. The figure ground into this master is the exact inverse of the shape needed for the finished corrector, which is thick in the center, thinner between center and edge, and thick again at the edge (Figure 1). The master block's shape is the exact opposite.
When the painstaking process of making the master block is complete, a glass blank that is to become a corrector is placed on the master and pulled against it with a vacuum. The exposed surface of the blank is then ground and polished flat. When the vacuum is released, the blank springs back and assumes the opposite curve of the master, just as in Schmidt's original process.
The master block system is actually considerably more difficult and complex than described. In addition to the basic challenge of making a master block (luckily, a single master can produce many corrector plates), it is hard, for example, to pull the blank evenly against the master so the two pieces of glass are in perfect contact. The interface between the blank and the block also must be nearly clean-room clean as any dust between the master block and the corrector blank will show up as light-scattering depressions on the finished corrector plate. These problems are manageable, however, and the master block technique allowed Johnson to start cranking out SCTs.
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