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Schmidt-Cassegrain Telescopes

Several variations of this type of telescope exist. The Schmidt-Cassegrain is the most popular, if for no other reason than it is mass-produced. For this reason they are readily available and reasonably priced. Several factors conspire to make these instruments considered by many to be the best all-around telescope for your money. Because of folded optics a long focal length can be fit into a compact space. Large apertures, up to 14 and even 16 inches become manageable. The 8 inch version is most popular because of cost and easy transportability.

The primary mirror is spherical, with a lens "corrector plate" over the front that holds the secondary mirror. The light is sent back through a hole in the primary to a diagonal mirror, allowing for easy behind the telescope viewing. The system is sealed so the internal optics stay clean and the system stays free of image disabling tube currents. SCT's seldom need collimation.

Two concerns keep it from being the "Perfect" telescope. First, the secondary is overly large to accommodate the short tube. This results in scattered light that softens the images and degrades detail. The images are not as sharp as in a long-focus reflector or refractor. Second, the large glass corrector plate is a dust magnet, and a dew collector. This requires constant attention, and the use of a battery operated heat tape wrapped around the front to warm the lens.

Nevertheless, SCT's make excellent telescopes for both visual and photographic astronomy. Two companies manufacture most of these telescopes. Celestron and Meade instruments are available through dealers in many camera stores and telescope shops.