Schmidt-Cassegrains
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 are technically known as aplanatic Schmidt-Cassegrains. These premium offerings reduce an optical aberration known as coma. They are primarily designed for for wide-field imaging with larger format CCD cameras. Coma is visibly noticeable on the outer edge of the field at very low magnification. 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, lending to 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.
Learn More - Telescope Optical Designs
Newtonian Reflector Telescopes
Newtonian reflector telescopes are an optical design that use a mirror to focus light into a smaller area. Most of the larger telescopes in use today utilize mirrors for economic reasons. It is less expensive to produce a mirror than a lens, and that advantage grows with the size of the telescope. The main advantage of a Newtonian Reflector is cost per unit aperture. The disadvantage is the images they produce are upside down, meaning that they are not suitable for terrestrial observing.
Dobsonian Telescopes
A Dobsonian telescope is a Newtonian reflector mounted on a simple yet stable alt-azimuth (lazy Susan style) mount. These telescopes are named for their designer, John Dobson, who sought to build a low cost large aperture telescope. No other design has had a greater impact on amateur astronomy in the past half century. Dobsonian telescopes are popular because they offer the most available aperture for the dollar. Dobsonian telescopes may appear unconventional to the first time buyer, but the fact is that they dominate the market for amateur telescopes over eleven inches in aperture. Wikipedia has a fine article on the history and origins of the Dobsonian Telescope.
Refractors
Refractors or refracting telescopes are a design that use only lenses to focus the light; every other type of telescope relies on one or more mirrors to do that job. The long, slender optical tube of a refractor is the image that most people associate with a telescope. Refractor telescopes have the advantage of having an unobstructed light path, which produces higher visual contrast than other optical designs. Stars appear as pinpoints against an inky black background when viewed in a refractor. Refractors have the disadvantage of being the most costly per unit of aperture, which is why refractors tend to be smaller than all other designs.
Schmidt-Cassegrain Telescopes
Schmidt-Cassegrain telescopes use both mirrors and a lens (corrector plate) to focus light into a smaller area. Schmidt-Cassegrains are popular due to their compact size relative to their aperture. These are the Jack of all trades in the telescope business. They are moderately well adapted to a wide variety of applications, including lunar, planetary, deep space, and even terrestrial observing. Variations on the Schmidt-Cassegrain design include Celestron's EdgeHD and Meade's Advanced Coma Free (ACF) optical systems. The primary advantage of a Schmidt-Cassegrain telescope is that it tends to offer the greatest degree of portability per unit of aperture. The disadvantage is they are more costly than Newtonian Reflectors.
Maksutov Cassegrain Telescopes
Maksutov-Cassegrain telescopes are similar to Schmidt-Cassegrains, using both mirrors and a lens to focus light into a smaller area. Maksutov-Cassegrains replace the flat corrector plate of the Schmidt-Cassegrain with a convex meniscus lens. The secondary mirror is actually a silvered spot on the inside of the meniscus lens. This in itself leads to one major advantage: Maksutovs do not require periodic collimation of the optics as do Schmidt-Cassegrains and Newtonians. Maksutov-Cassegrains offer superior contrast to a Schmidt-Cassegrain, but they are also more expensive per unit of aperture.
Solar Telescopes
Solar telescopes offer a safe way to observe and study our nearest star: the sun. This is a highly specialized field and the number of suppliers is considerably smaller than that for general purpose telescopes. We offer solar telescopes from Lunt Solar Systems, Coronado, and DayStar Filters, that are filtered for the Hydrogen-Alpha and Calcium-K spectral lines. Often it is more cost effective to purchase a dedicated solar telescope than it is to add a solar filter to an existing one.
Feeling Overwhelmed?
Don't worry, this happens to a lot of people. If this is your first time shopping for a telescope, please take a few minutes to read our article Advice for First Time Telescope Buyers before proceeding any further. We think that you will be glad that you did.