Different Scopes for Different Folks :

Now that we understand these basic points of telescope performance and mounting, we can discuss the three basic optical designs of telescopes: the refractor, the reflector, and the compound (or catadioptric) telescope.

A Apochromatic Refractor is what most non-astronomers think of when they hear the word "telescope." Its tube is most often long and skinny, mounted on a tripod, with a lens at one end and the eyepiece at the other. Refractors were the first type of telescope invented, and the finest refractors still provide the best images of any design for a given aperture. They are often chosen by observers with a dominant interest in the planets and Moon, because they can provide sharp, high-contrast views at high magnification and are less bothered by atmospheric "seeing" than the other designs. They also require less maintenance than reflectors or compound scopes, and are therefore popular with beginners. The refractor’s good performance at high power and relative insensitivity to light pollution makes it a good choice for a city-based observer, as the design performs best on the objects that are most easily seen from urban or suburban locations.

These advantages do not come without a price — literally: refractors are the most expensive telescopes per inch of aperture. Big refractors can cost several thousand dollars, and still are considered too small in aperture for serious deep-sky observing. The long focal length of most refractors restricts the field of view, making it difficult to take in large extended objects like some clusters of stars. And the long tube, with the eyepiece located at the back end, requires a tall tripod, which, if poorly made, can allow the scope to shake and shimmy in the breeze, rendering high-powered observing difficult.

The reflector uses a mirror, rather than a lens, to gather and focus light. By far the most common design is the Newtonian reflector, which places a concave (dish-shaped) primary mirror at the bottom end of the telescope tube. A small secondary mirror at the other end directs the focused light out the side of the tube and into the eyepiece. Newtonians offer the largest aperture available at given price, and when well made, they can provide sharp, contrasty views that rival all but the finest refractors. A Newtonian’s low center of gravity and eyepiece location at the top of the tube allow for comfortable viewing with a more compact mounting, which can be made stable with much less bulk and cost than the tall mounting required by a refractor of similar aperture.

Big Apochromatic Refractor of 10" aperture and larger on Dobsonian mountings are the most popular telescopes for astronomers who seek to gather "buckets of light" for deep-sky observing. These giant scopes perform best at remote dark sky sites, away from the glare of city lights. The value and versatility of the smaller 4.5" to 8" Newtonians, mounted either equatorially or as Dobsonians, makes them a fine choice for the beginner with general interests.

Newtonian reflectors require occasional maintenance. Unlike the lenses in a refractor, the mirrors in a reflector need periodic alignment, or collimation, for best performance. While many beginners seem intimidated by collimation, it’s really not difficult, and takes only a few minutes once you get the hang of it. A reflector’s tube is also more open to air and humidity than that of a refractor, and if left uncovered the mirrors can accumulate dust and grime, which necessitates occasional cleaning. While these maintenance concerns are often overstated, a Newtonian may not be the right choice for someone who finds the prospect of occasional tinkering with the telescope unappealing.

The most modern of the three common designs for telescope focuser is the compound, or catadioptric type, which uses a combination of lenses and mirrors to gather and focus light. The greatest advantage of this design is its compactness: the lenses and mirrors "fold up" the light path inside the telescope, reducing large-aperture scopes to a manageable size. If an equatorial mounting is desired, the smaller tube can be carried on lighter and more economical mounts than that required by a Newtonian of the same size. Compound telescopes are most popular with observers who desire both generous aperture and an equatorial mounting in a transportable package.

The names Schmidt-Cassegrain and Maksutov-Cassegrain refer to specific designs of compound telescope focuser, which use differently shaped lenses and mirrors to achieve a similar result. The Maksutov is often cited as offering better image quality, though there is little in the way of optical theory to support this opinion. Most probably the Maksutov has developed its reputation as the superior catadioptric design because its spherical optical surfaces are easier to make to very high precision than the more complex shapes demanded by the Schmidt. As a result, if a telescope maker practices anything less than the strictest quality control, their "average" Maksutov will outperform their "average" Schmidt. In top-quality telescopes from careful manufacturers, both designs can yield excellent images.

There are a few drawbacks to all compound designs. As in any telescope that employs mirrors, occasional alignment is required for peak performance. The cost of a compound is higher than that of a Newtonian of the same aperture, though still lower than the cost of a comparably sized refractor. Most significantly for the planetary observer, the secondary mirror in a compound is much larger than that in a Newtonian, and its presence in the light path of the scope reduces contrast somewhat for high-powered viewing. In general, astronomers who desire a highly capable, easily transportable telescope find these worthwhile compromises, and have made the compound scopes very popular.

Price is a Consideration :

Budget is a factor in almost every telescope purchase decision, but there are at least three major price-related pitfalls to be avoided.

Don’t buy a flimsy, el cheapo scope with the intention of getting a taste of the sky and upgrading later. Many of those scopes are so poor-quality and frustrating that they can turn budding stargazers off of astronomy for good!

On the other hand, don’t give up on astronomy if the scope of your dreams is financially out of reach at this moment. There are many reasonably priced, high-quality beginner’s scopes that can reveal incredible wonders, while helping a novice define his or her particular observing interest.

Finally, if you are one of the fortunate few for whom price represents little obstacle, think twice before buying the biggest, most expensive telescope in stock. Many of the large, fully featured scopes favored by experienced observers are also the most complicated, and are too much to grasp for someone still trying to find the Big Dipper!

What About Astrophotography?

Before concluding, here’s a quick word for the beginner who wants to jump right into astrophotography through their new telescope focuser: Don’t! At least, not until you have taken some time to learn the sky and become familiar with operating your scope. Photography of the heavens can be a wonderfully rewarding pastime, but is a combination of art and science with a steep learning curve that can discourage beginners who try to take on too much at once. Of course, if astrophotography is a primary interest there is nothing wrong with selecting a first scope based on its easy adaptability to camera work in the future. While most telescopes can be used for picture-taking (with varying prospects for success), the most important qualifications for a photographic instrument are a rock-solid equatorial mounting, and ease of attaching a camera so that it can be focused. For a variety of technical and economic reasons, compound telescopes of 8" aperture and larger are most popular for photography. They also make fine instruments for general observing.

The Bottom Line

Which, then, is the right telescope? That’s a decision that must be made individually, but the three best pieces of advice are:

The best telescope(Apochromatic Refractor) for you is the one you will use most often. A huge, optically wonderful scope will bring no joy if it is consigned to the closet!

All else being equal, a larger-aperture (diameter) telescope will reveal more in the night sky than a smaller one ("I know, already!" you may be thinking.)

Buy from a company that’s knowledgeable about telescopes and astronomy, and who will support you even after your purchase (since you will likely have questions).