Probably the most popular telescope design is the Schmidt-Cassegrain telescope (SCT). The SCT is a versatile design, good for both imaging and visual observation. It is compact for portability and ease of use, and is relatively inexpensive compared to other systems. SCTs offer the most versatility. They are good for visual observing, deep-sky photography and planetary photography.
Above: The optical layout of a Schmidt-Cassegrain telescope
In a Schmidt-Cassegrain, the light passes through a corrector lens to the primary mirror. The corrector lens is required to minimize the spherical aberration which arises from using spheroidal mirrors. (A true Classical Cassegrain telescope uses a paraboloidal primary and hyperboloidal secondary and does not require a corrector plate. However, this is a much more expensive design than the SCT due to the difficulties in making the aspheric mirrors.) The light is reflected from the primary mirror to a convex secondary mirror. This mirror then sends the light to a focus position behind the primary mirror for easy access with an eyepiece or camera. The convex secondary mirror amplifies the focal ratio of the telescope, making the short-tubed SCT act like an f/10 system while being as short as an f/2 system!
While f/10 is advantageous for visual observations and large-image-scale CCD imaging, what about wide field imaging? Focal reducer lenses can be attached to the rear of an SCT to decrease the focal length of the telescope. This provides a wider field of view as well as shorter exposure times. The most common focal reducer changes an f/10 system to f/6.3, making exposures 2.5 times faster. Faster f/3.3 reducers have been employed for CCD imaging, decreasing exposure times by a factor of 9, but they are limited in quality and the size CCD they can cover.
Above: Image of NGC253 taken with a 14″ f/7 SCT. The telescope is inherently f/11 but was made f/7 with the use of a focal reducer.
The most extreme example of focal reduction is the Starizona HyperStar system, available on Celestron SCTs. The primary mirror of a typical SCT has a focal ratio of f/2. The secondary mirror has a 5x amplification factor. By removing the secondary mirror and placing the CCD camera at the front of the telescope (with an appropriate correcting lens), imaging can be done at f/2, making exposures 25 times faster than at f/10. This super-fast imaging system has made CCD imaging possible for the average amateur who does not want to spend all night taking pictures, but would rather see results right away! The optical quality is also superior to most focal reducers, even though HyperStar is so much faster.
Above: M17 with the same 14″ telescope, but using a HyperStar lens to operate at f/1.9, yielding a larger field of view and faster imaging system.
There are advantages and disadvantages to every telescope system, and every combination of scope and CCD camera. But the SCT provides the most flexible system, capable of both high-resolution imaging and wide-field, fast imaging. The combination of portability, versatility and price make the SCT by far the most popular imaging and observing system.
The standard SCT design, which has been popular for 50 years, does suffer from two aberrations that can be problematic for astroimaging. These are coma and field curvature. Coma causes stars near the edge of the field of view to stretch into little comet shapes, lowering the image quality and resolution. A curved focal plane causes stars to not be focused across the flat surface of a camera sensor.
Meade began making coma-free SCTs which replaced the spherical secondary mirror with a hyperbolic one that corrects for coma, allowing for round stars at the edge of the field. Celestron went one step further with their Edge HD telescopes. These scopes still use spherical mirrors, but they add a pair of small lenses located within the primary baffle tube before the focal plane. These lenses correct for both coma and field curvature.
Above: The optical layout of a Schmidt-Cassegrain telescope
A related design is the Maksutov-Cassegrain. The Maksutov-Cassegrain uses a different corrector lens in place of the Schmidt corrector. The Maksutov corrector lens is easier to manufacture than the Schmidt corrector, but is much thicker and therefore requires a longer cool-down time, especially in larger sizes (7″ and larger). Maksutov-Cassegrains often have slower focal ratios (f/12 to f/15), making them less well suited to deep-sky imaging. Smaller Mak-Cass telescopes often use an aluminized spot on the back surface of the corrector as the secondary mirror, although this is not ideal optically. Higher-end design usually have a separate secondary mirror as in an SCT.
Above: The optical layout of a typical Maksutov-Cassegrain telescope
Full-Aperture Corrector Lenses
Both Schmidt-Cassegrains and Maksutov-Cassegrains use corrector lenses. These are full-aperture corrector lenses because they cover the entire aperture of the telescope (as opposed to a sub-aperture corrector such as a coma corrector for a Newtonian which is inserted along the light path just ahead of the focal plane).
A true Cassegrain telescope uses a paraboloidal primary mirror and a hyperboloidal secondary mirror. These aspheric mirrors are difficult and expensive to manufacture, but they are necessary to eliminate spherical aberration from a two-mirror optical design. However, by employing a refractive element (lens) in the design, it is possible to remove spherical aberration while still using easy-to-manufacture spheroidal mirrors. Two independent solutions were derived for such a telescope in the early 20th Century.
Bernhard Schmidt in Germany solved the problem by using an aspheric corrector plate, now called a Schmidt corrector. The Schmidt corrector is a thin plate of glass which has a very slight aspheric curve ground into the surface. This curve is so minute that a Schmidt corrector looks flat. But the curve is sufficient to eliminate spherical aberration and allow a Cassegrain to be built with spheroidal mirrors for much less money than a classical Cassegrain would cost. However, Schmidt correctors are somewhat difficult to manufacture (although Schmidt not only invented the corrector but also an ingenious method for making it).
Dmitri Maksutov in Russia also devised a similar solution. The Maksutov corrector is easier to manufacture because it is an all-spherical design. It is a highly curved meniscus lens, much thicker than the Schmidt corrector. This has the advantage of easy manufacture, but the disadvantage of inducing chromatic aberration into the system. With the right design, this can be minimized. Schmidt correctors suffer from chromatic aberration as well, but since they have very little optical power it is much less than that of a similar Maksutov. Some Maksutovs with all spherical mirrors also suffer from more higher-order spherical aberration that can decrease the resolution.
Above: On the left is a Schmidt aspheric corrector lens with the curve exaggerated about 500 times for clarity. On the right is a Maksutov meniscus corrector lens.