Refractor, Reflector or Catadioptric?

The refractor is the classic telescope design. It features a long, thin tube with a lens at one end and a focuser at the other. The front lens is called the primary objective. This lens collects and focuses light by refraction. Hence the name, refractor. Let's consider the refractor's relative strengths and weaknesses.

First, the strengths. The refractor is the only design without a central obstruction. This gives it an advantage in the contrast department. This is important because, in the dark, the human eye is primarily a contrast detector. A high contrast image makes faint objects and subtle details easier to see. Also, the refractor is a relatively simple, rugged design requiring very little maintenance. Refractors tend to be small and lightweight, making them very portable. Extremely portable "short tube" refractors produce wonderful low power, wide field views of the night sky. Finally, a refractor can be easily adapted for daytime nature observing.

Now, the weaknesses. The biggest is that refractors are expensive. You can easily pay several thousand dollars for a quality 6-inch refractor. A Newtonian with the same aperture (think: light-gathering power) would cost one-tenth that price. Refractors also suffer from chromatic aberration. One lens will not focus all light wavelengths to the same point. That's why you see colorful halos around bright stars and planets when using inexpensive refractors. Chromatic aberration doesn't just produce colorful halos, it also reduces image contrast. Chromatic aberration can be corrected through the use of multiple lens elements made of special (read: very expensive) glass. This makes top quality refractors the most expensive telescopes per inch of aperture.

English astronomer Sir Isaac Newton was brilliant man. He not only invented calculus and worked out the problem of gravity, Isaac Newton invented a new kind of telescope. The Newtonian reflector uses two mirrors to collect and focus light. The primary mirror is at the bottom of the optical tube. Its diameter determines the light-gathering power of the telescope. This primary mirror has a curved surface that reflects the light back toward the tube opening. A small, flat secondary mirror is positioned centrally within the tube not far from the opening. This secondary mirror redirects light out the side of the tube where it comes to focus.

The Newtonian's biggest advantage is that it costs less per inch of aperture than the other two designs. This gives the Newt an edge in the "bang for the buck" department. Your dollar buys more light-gathering power with a Newtonian than with any other telescope. This is particularly true of a specific kind of Newtonian reflector called a Dobsonian. Also, Newtonians do not suffer from chromatic aberration. You won't see blue halos around bright stars in a Newtonian reflector.

The biggest weakness of the Newtonian design is that the mirrors tend to lose alignment. The proper alignment of a telescope's optics is called collimation. Good collimation is essential to getting the best views from your telescope. Newts require regular checks of and subtle adjustments to collimation. This makes a reflector a high-maintenance telescope to own. Finally, Newtonians suffer from an optical aberration called coma. This is where stars outside the center of the image appear elongated and out-of-focus. Coma can be corrected with optical devices called field flatteners.

That's "Dob-," as in Bob, "-sonian," as in Smithsonian. The name refers to a mount design popularized by California amateur astronomer, John Dobson. The Dobsonian mount is an altazimuth design. You move the telescope up-and-down through altitude and around the horizon in azimuth. A Dob mount is best described as a rocker box on a Lazy Susan. It's simple, inexpensive and very effective. A 6- or 8-inch aperture Dobsonian offers moderate light-gathering power for under $500! That's a great bargain. Dobsonians are very popular with deep-sky observers who need as much aperture as they can get to see distant, faint galaxies. Also, Dobs as large as 8-inches in aperture are still very portable.

Catadioptric telescopes use a combination of lenses and mirrors to collect and focus light. The most common example is the Schmidt-Cassegrain telescope, also known as the SCT. The 8-inch SCT is the most popular amateur telescope in the world, according to one author. If not the most popular, the SCT is arguably the most versatile of the three designs. Schmidt-Cassegrains have short, broad optical tubes with a front corrector lens. A spherical primary mirror collects light and reflects it toward the corrector plate. A secondary mirror mounted within the front corrector redirects light back toward the primary. The light passes through a hole at the center of the primary and comes to focus behind the telescope. It's a complex design that offers several advantages.

The compact design of the SCT makes it the most portable of the three telescopes in apertures up to 8-inches. Although somewhat more expensive than a Newtonian, the SCT costs far less than moderate aperture refractors. SCTs also have a wide range of accessories available for astrophotography and CCD imaging. And like refractors, the Schmidt-Cassegrain can be easily adapted for daytime use as a spotting scope.

Another catadioptric design, the Maksutov-Cassegrain, is also fairly popular. These tend to come in smaller apertures, 90-mm to 175-mm being the typical range. Because the MCT design uses all spherical optical surfaces, these telescopes provide sharp, contrasty views. Also, they don't suffer from chromatic aberration or coma. As a result, the Maksutov-Cassegrain design is favored by many lunar/planetary observers.

The biggest weakness of the catadioptric design is found in its large central obstruction. The secondary mirror typically blocks 33% or more of the telescope's aperture. This produces significant light scatter which can severely reduce image contrast and quality. Also, catadioptrics become quite heavy in apertures greater than 8-inches. Finally, Maksutov-Cassegrain optical systems are expensive to produce. They can even rival refractors in cost.