Common Optical Configurations

Although a large number of optical configurations is possible, and many can be found on telescopes here and there, most amateur scopes fall within the following categories. DISCLAIMER: I'm not an optical designer, optician nor optical expert. The following information is generic and should remain accurate, however please let me know if you come across any glaring errors.

Newtonian

Named after its inventor, Sir Isaac Newton, the Newtonian is probably (?) the most popular optical configuration among amateurs. Technically, it's a reflector, but its proper name is commonly used to differentiate it from other reflector types. Light enters the optical tube, strikes the concave (spherical or paraboloidial) primary mirror and is reflected back toward the flat, diagonally mounted secondary mirror. The secondary then reflects the converging light into the eyepiece (or ocular), where the image comes to a focus and is magnified by the ocular.

The Newtonian is popular for many reasons, including good performance, relatively low weight and cost per inch of aperture. Its chief disadvantages are that since it's open to outside air, it's susceptible to tube (air) currents, and requires more frequent alignment (collimation) than some other configurations. Also, all optical surfaces are open to contaminants. However, if properly designed, built and maintained, a good Newtonian can optically challenge any other configuration, generally at a fraction of the cost. The vast majority of "Dobsonian" telescopes are of the Newtonian optical design, including the Obsession line, Synta and Guan Sheng low-cost "Dobs" and countless others.

Similar to the Newtonian is the "Hyperbolic astrograph" - To the eye it looks the same, but it uses a hyperboloidial primary mirror (i.e., flatter in the middle as compared to a sphere), whereas a Newtonian primary is usually paraboloidial (deeper in the middle) except in long f-ratios. Optimized for photography, they are prized by astroimagers for their flat, well-corrected fields.

Schmidt-Cassegrain

Arguably the first or second most popular telescope type among amateurs, the Schmidt-Cassegrain is a catadioptric (or "CAT") design, meaning it employs both reflective and refractive optical elements. Whereas the conventional Cassegrain is an open-tube design, the SCT places a corrector plate across the end of the tube, with the secondary mirror mounted on this element. The corrector plate is only weakly shaped, but it's enough to do the job of compensating for the spherical primary mirror. Tom Johnson at Celestron pioneered the vacuum-distortion process that made commercially produced SCTs viable. With this process, the slight but complex curve of the corrector can be mass-produced and only simple spherical surfaces are needed on the other optical elements.

As with the other Cassegrain designs, the SCT is very compact and portable for any given aperture. When properly designed and built, nicely-corrected images can be produced, making them well adapted for astrophotography. However, achieving good collimation can sometimes be elusive, as some commercially produced instruments suffer from a lack of comprehensive adjustment capabilities. Also, the "moving primary mirror" method of focusing found on these scopes sometimes introduces a bit of image shift when focusing as well as "mirror flop" when the scope tracks over the meridian. Both can result in slight to severe misalignment. However, in all fairness this is not the issue it once was, as manufacturing quality continues to improve. The reputation for poor optical quality from Halley-era SCTs has been virtually erased by the excellent scopes available today. These days a well-made SCT can rival most other designs - It just remains a question of what design and feature-set is the best fit with your requirements.

SCTs continue to enjoy widespread popularity, due in part (no doubt) to the vast array of accessories and add-on products available for them. The market leaders in this area, Celestron and Meade offer numerous SCT models, but other importers are adding more other SCT options under their own brand labels.

Without taking sides in a currently hot topic, there are other configurations (such as the Meade RCX) which appear similar to an SCT, but differ in several technical aspects.

Refractor

A refractor is what many people instantly picture upon hearing the word "telescope". There's an objective lens at one end, an eyepiece at the other and you just look through it.

Inch for inch, refractors are among some of the more expensive telescopes on the market. Then why are they popular? In a word (or two): Image Quality. With no central obstruction, image contrast can be higher. However, there are some drawbacks to refractors. Since the objective is supported around the edge only, there's a limit to how big it can be before it sags unacceptably... yes, glass is a fluid and will sag. Fortunately, that limit is somewhere between 30 and 40 inches, well above what any individual can afford. However, there are other disadvantages.

Since a lens doesn't focus all colors of light to exactly the same point as a mirror does, all refractors exhibit at least some degree of chromacity, or false color. For example, when viewing the moon through a refractor often produces a violet fringe and this effect becomes more and more pronouced with aperture size. However, great strides have been made in the past few years toward virtually eliminating chromatic (and other) abberations (and the best are indeed impressive), but that, too, comes at a price.

In smaller sizes, usually below seven inches, modern refractors have become popular with discriminating astronomers intent on obtaining that ultimate image. Premium refractors include those from Tele Vue, Astro-Physics, Takahashi, TMB and others.

Maksutov

Not as common as the Schmidt Cassegrain, the Maksutov is another catadioptric variant of the basic Cassegrain system. Unlike the SCT's corrector plate however, the Maksutov's "meniscus corrector" is thick and deeply curved (which can lead to increased cooldown times). Instead of a separate secondary mirror, an appropriately sized area of the back side of the meniscus corrector is simply aluminized (except in the case of some Russian-manufactured Maks, which have a separate secondary that can be collimated).

Maksutovs are praised for their high image quality, although for some reason, they weren't seen among amateurs as often as other designs until relatively recently. Various commercial Maks have lately appeared on the market, typically OEM'ed and marketed by Meade and Orion as well as other major vendors under their own brands. For some observers, Maks have become a worthy "refractor replacement" for high(er) power and planetary observing.

Cassegrain

Utilizing a (usually) perforated primary mirror, the Cassegrain configuration was initially developed in 1672 by Guillaume Cassegrain. Although he designed the system, it's unlikely that any actual Cassegrain telescopes were built, since the aspheric (not spherical) surfaces required by the design were yet to be achieved by opticians.

A Cassegrain can be thought of as a Newtonian where, instead of exiting the tube at a right angle, the light path is directly folded back through the hole in the center of the primary mirror. In a "folded Cassegrain" design, there is yet another mirror (a diagonal flat) that diverts the light path out at a right angle instead of through the primary.

The classical Cassegrain system uses a paraboliodial primary and a hyperboloidial secondary. Primary advantages of this design include a flat field and (other than coma), good image correction. Plus, with the "collapsed" optical path, the physical length of the telescope can be remarkably short, even for larger, long focal length instruments.

These days, the most popular form of the Cassegrain configuration is the Schmidt-Cassegrain, which is a catadioptric specialization of the basic Cassegrain design..

The Bottom line?

Every design is a combination of compromises and trade-offs, at least to some degree. Some are more specialized, some more general. As long as the observer's expectations are reasonable and appropriate, and the hardware is properly made, there's really no such thing as a "bad" telescope. The trick is to match the design to the requirements - i.e., don't buy a "short-focus" wide field scope and expect great planetary performance from it.