Above: Optical layout of a typical Schmidt-Cassegrain
Schmidt-Cassegrains telescopes (SCTs) have become one of the most popular types of telescopes. This is due to the versatility and compactness of the optical design. SCTs are only about twice as long as their aperture, making them highly portable. They are also well-suited to a number of applications including photography and terrestrial observing as well as stargazing. Most new SCTs are computerized, adding to their ease of use and portability.
How Schmidt-Cassegrains Work
Schmidt-Cassegrain telescopes are a catadioptric design, meaning they use both lenses and mirrors. SCTs are primarily reflecting telescopes, but they use a corrector lens to eliminate aberrations that would result from the mirror design alone. In an SCT the incoming light passes through the Schmidt corrector lens (also called a corrector plate) at the front of telescope. It is reflected from a concave primary mirror at the back of the scope which focuses the light to the front of the telescope where it is reflected again by a smaller, convex secondary mirror. Finally, the light travels back through a hole in the primary mirror to the rear of the scope where an eyepiece is located for visual observing (or a camera for photographing). By folding the light in this manner, an SCT can be made much smaller than an equivalent Newtonian or refractor.
At first it seems that an SCT should have a focal length twice as long as the length of the tube, since the light travels twice the length of the tube. However, SCTs actually have focal lengths about 5 times longer than the tube length. This is due to the convex curvature of the secondary mirror. This effectively magnifies the focal length, making the scope act much longer than it is. The diagram below shows how a Cassegrain type telescope (such as an SCT or similar design) works to increase the effective focal length. The primary mirror of an SCT usually has a focal length only about twice its diameter. For example, an 8" telescope would have a primary mirror focal length of about 16". This focuses the light to a point 16" in front of the mirror. By placing a convex secondary mirror between the primary mirror and this focal point, the light is reflected back toward the primary mirror but at a less steep angle. If you trace this light cone back to where it reaches the full width of the telescope's apertureyou can see how long the effective focal length is. Most SCTs have secondary mirrors that provide a 5x magnification factor. In this example that would make the effective focal length 80", or ten times the diameter of the primary mirror, in a telescope only about 16" long.
Above: How a Cassegrain telescope creates a long effective focal length in a short tube. The actual length of the telescope is barely longer than the separation between the two mirrors.
Varieties of Cassegrain Telescopes
So what makes a Schmidt-Cassegrain different from a Maksutov-Cassegrain, or just a plain old Cassegrain? The answer has to do with the corrector lens at the front of the scope. In an all-reflective Cassegrain telescope (usually called a Classical Cassegrain), there is no corrector lens. To eliminate aberrations, this type of telescope must have aspherical mirrors, which are difficult and expensive to manufacture. By placing a corrector lens in front of the telescope, these aberrations can be minimized or eliminated while using spherical mirrors which are cheaper and easier to make. As the name implies, Schmidt-Cassegrains use a Schmidt corrector lens, while a Maksutov-Cassegrain uses a Maksutov corrector lens.
Schmidt corrector lenses have an aspherical shape. The curve on the lens is very slight, and the glass looks perfectly flat to the eye. The curve is drawn greatly exaggerated in the diagram at the top of the page.
With both mirrors left spherical, an SCT suffers from coma, an aberration affecting sharpness of the stars at the edge of the field. By making the secondary mirror aspheric, coma can be eliminated. Meade's Advanced Ritchey-Chrétien telescopes are of this latter design, simulating the effect of true Ritchey-Chrétien optics in an SCT configuration.
The Popularity of Schmidt-Cassegrains
Why are SCTs so popular? One word: versatility. Most amateur astronomers enjoy many different aspects of our hobby, from observing planets to hunting for deep-sky objects to photographing the heavens. SCTs are good for all of these things. While certain telescopes might be better suited for one particular application, SCTs can do it all. For the money, a Dobsonian will provide more aperture and hence be better for deep-sky observing, but it cannot be used for photography. A high-quality refractor can give some of the best planetary views of any type of telescope, but a large-aperture refractor capable of competing with an 8" or 10" SCT in light-gathering ability is a huge, painfully expensive telescope. SCTs combine a compact telescope with good moderate-aperture optics which are well-suited to visual observing and imaging.
Almost all current Schmidt-Cassegrain models are computerized goto telescopes. SCTs are typically the flagship models of a manufacturer's product line and usually incorporate state-of-the-art technology. Goto mounts also lend themselves to compact design and add to the portability of SCTs.
As mentioned above, most SCTs are goto telescopes. They come in a variety of sizes from about 5" to 16". Within each size there are often several models with varying features, mostly in terms of mount features such as tracking capabilities. The mount will determine the telescope's photographic abilities. Some less-expensive designs eliminate the high-precision motors necessary for photography; they are still suitable for visual observing but will not be as good for imaging. Most SCTs are fork-mounted, making them easy to setup and use, but requiring an equatorial wedge for long-exposure imaging. Some high-end SCTs are sold instead with heavy-duty German equatorial mounts, which are typically more stable than fork-mounted telescopes, but are also more expensive and require more setup time. Some inexpensive SCTs are also sold on German equatorial mounts, but these smaller mounts are not particularly well-suited to long-exposure photography.
The most popular SCTs are 8" in aperture. An 8" SCT on a goto mount starts around $1400. The same telescope with a mount capable of tracking accurately for long-exposure photography will start around $2000. Smaller SCTs start around $1000. Larger SCTs can cost up to around $10,000.
SCTs are also sold as optical tube assemblies (OTAs) without any mount for those observers who already have a German equatorial mount. 8" OTAs cost about $1000, with 14" to 16" OTAs costing $4000-6000.
Is a Schmidt-Cassegrain Best for Me?
For observers who want to be able to do a little of everything, from observing to photographing, SCTs are a great all-around telescope. They are very portable (at least in 11" and smaller sizes), and easy to setup and use. They often have the latest and greatest features in terms of computer systems and other high-tech gadgetry. If your only interest is in visual observing, you can get a larger-aperture Dobsonian for the price. But most observers have varied interests and that makes the Schmidt-Cassegrain a very popular choice.