The market for commercially produced Schmidt-Cassegrains has been dominated by Celestron and Meade Instruments Corporation since the 1980s. Both companies now offer more costly derivatives of the Schmidt-Cassegrain design that reduce an optical aberration known as coma. The latter are primarily designed for for wide-field imaging with larger format CCD cameras; we believe that most visual-only users will be satisfied with the original Schmidt-Cassegrain designs.
More About Schmidt-Cassegrain Telescopes
Schmidt-Cassegrain telescopes are some of the most popular pieces of equipment on the market, favored for their large aperture in a relatively small, lightweight package. Perhaps the best way to describe a Schmidt-Cassegrain would be to label it as "the jack of all trades" among telescope designs. Simply stated, Schmidt-Cassegrains do everything at least moderately well or better. This includes lunar and planetary observation, astrophotography, and deep space observing, provided they are of large enough aperture. We recommend a minimum of six inches in aperture for deep space observing, but more is always better. Schmidt-Cassegrains equipped with a 90 degree star diagonal produce an image that is upright but right-left reversed, making them suitable for terrestrial observing as well.
Schmidt-Cassegrains are a type of catadioptric telescope with a folded light path using both mirrors and a lens to concentrate light at the focal plane. The result is a relatively compact telescope relative to the aperture, or the short and stubby appearance of these telescopes. Schmidt-Cassegrain telescopes, also called SCTs, use the following components to concentrate light at the focal plane:
- Schmidt Corrector Lens: Also known as a corrector plate, this lens is at the front end of an SCT and is where light passes through to enter the telescope. This Schmidt corrector lens is flat with a slight aspheric curve to correct the spherical aberration caused using spherical mirrors in the telescope.
- Concave Spherical Primary Mirror: This is the light that passes through the corrector lens and is reflected by a concave primary mirror at the back of the scope, close to the eyepiece. This concave mirror focuses the light toward the front of the telescope. Usually, these primary mirrors have a focal length around twice the diameter of the mirror. For example, a Celestron 8-inch CPC Schmidt-Cassegrain telescope would have an 8-inch primary mirror with a focal length of 16 inches.
- Convex Secondary Mirror: The primary mirror reflects light onto a convex secondary mirror close to the front of the telescope. This not only reflects the light back toward the primary mirror but the convex curvature of this mirror also effectively magnifies the focal length of the telescope, allowing the telescope to act much longer than it is. Typically, this magnification factor is around 5x. Following the previous example, this means that an 8-inch telescope with a primary mirror that produces a focal length of 16 inches would have an effective focal length of 80 inches, ten times the diameter of the primary mirror.
What is Coma and Should You Worry About it?
One minor drawback of the classic Schmidt-Cassegrain design is an optical aberration known as coma. Stars at the edge of the field will not be as round as those at the center of the field. The two primary manufactures of Schmidt-Cassegrain telescopes, Meade and Celestron, have more advanced offerings designed to minimize coma. Meade introduced their solution in 2005, using an aspherical secondary mirror to correct coma. Meade refers to these telescopes as Advanced Coma Free, or ACF. Celestron answered the call several years later with an entirely different approach. The Celestron design, known as EdgeHD, incorporates a coma correcting lens group into a standard Schmidt-Cassegrain. Both designs do produce superior star images towards the edge of field, with the affect being most noticeable in photographs. It takes a trained eye to notice the difference visually, and that difference may not be worth the price premium for a large number of visual observers. Our standard advice to beginners is that you are unlikely to notice the difference visually, if for no other reason than the fact that your eye is going to concentrate on the center of the field where coma is not present.