Last updated on
4/11/
2006

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Cassegrain Optics (OTAs)

As noted on the Cassegrain Telescope Basics page, cassegrain scopes combine a spherical primary light-gathering mirror at the rear of the scope with a glass correcting plate at the front.  On the back of the correcting plate is a secondary (hyperbolic-shaped) mirror that reflects light from the primary back through a hole in the center of the primary, through which you observe the image.  Therefore this optical design “folds” the light path back on itself and that dramatically shortens the length of the scope and reduces its weight (and thus the weight of the mount required to support it), making it a lot more portable. 

The front correcting plate is needed because a spherical mirror does not focus light exactly correctly - only a parabolic mirror does - but a high-quality spherical mirror is less expensive to manufacture than a parabolic especially in larger sizes.  The cost savings of the spherical primary is offset by the need to manufacture the glass correcting plate so cassegrain scopes aren’t necessarily less expensive than simpler newtonian designs. Their advantage is really in their reduced length and weight. 

There are three main cassegrain designs: Schmidt-Cassegrains, Maksutof-Cassegrains, or Schmidt Cameras. (The optical design differences among these are largely related to the shape of the correcting lens.) A Schmidt-Cassegrain is somewhat less expensive than the other two and is optically faster (typically an f/10), but has lower contrast due to the somewhat larger size of the secondary mirror. A Maksutof has a smaller secondary mirror and thus is regarded as providing higher-contrast, crisper images but is optically much slower (typically f/15).  Schmidt-Cassegrains are therefore more popular than Maksutofs for astrophotography, while the Maksutofs are popular for visual use or planetary observing. (The exquisite, expensive Questar scope you may have admired in the ads that have run for decades in National Geographic, is a Maksutof.)  For both Schmidt-Cassegrain and Maksutof designs the secondary mirror tends to be a larger obstruction than in a newtonian design.  Thus serious planetary viewing or planetary astrophotography may be better done with a newtonian or refractor design.  (Although Maksutofs have less of a problem with reduced contrast because of their smaller secondary mirror, their optical speed is so low that they still present a bit of a problem for planetary photography with film, although not so much with CCDs.)

A Schmidt Camera is a specialized instrument.  (The design, invented by Bernard Schmidt, is actually the precursor to the modern Schmidt-Cassegrain design.)  It also has a primary mirror at the rear and a glass correcting plate at the front. However, it cannot be used for visual observation. The mirror focuses an image on a curved plane in front of the correcting plate. You mount a sheet of film in a curved film-holder at the focal plane, and aim and guide the scope with another auxiliary finder scope.
 

Schmidt Cameras have traditionally been used for some of the most serious professional astrophotography and research. You can purchase one from Meade and it’s a wonderful 12” f/2.2 instrument - click here to see it.

Ooops, I forgot to mention that it sells for about $26,000 plus tax and shipping, and it is heavy enough that it must be mounted on a permanent pier in an observatory.  You say you didn’t quite have that much in your budget? Well, for the price of a Celestron NexStar GPS or CGE scope plus the Fastar (or Hyperstar) lens system, you can have a functional equivalent of a Schmidt camera. In all honesty there are important differences: the NexStar 8 GPS has an 8” primary mirror rather than a 12” and you must use a CCD camera instead of 120-format film so you won’t get the same detail in your photographs. But you pay for what you get - the NexStar 8” does cost roughly $22,000 less (even with the CCD camera included) and you won’t need to guide the scope for 30-45 minutes with the CCD camera like you would need to do with film.  Plus, do you have a darkroom in which to develop 120-format film?  (Note that the larger Celestron 11” and 14” scopes that also use the Hyperstar system, provide more light-gathering power than an 8” and still are substantially less expensive than the Meade Schmidt camera.)
 

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