The amount of distortion due to seeing varies, depending upon the behavior of air currents in the upper atmosphere, and to a lesser extent upon the altitude and topography of the observing site. But on an average night, at an average site, air turbulence will limit useful magnification to 250x or 300x, and prevent telescopes larger than about 8" or 10" aperture from achieving their full potential for high-powered viewing. Telescopes larger than 10" are most often chosen by observers who want to gather as much light as possible for viewing dim galaxies, nebulas, and star clusters. These "deep sky" objects, affectionately called "faint fuzzies," are most often viewed at much lower power than the planets, so seeing is less of a problem.

Altaz mounts are generally lighter and simpler to use, and are preferred if the telescope is to be used both for both astronomy and daytime observing (or for daytime observing only). The better ones offer slow-motion controls to aid in moving the scope by small increments, and are useful for powers up to about 150x. The Dobsonian mount is a variation on the altaz mount. It employs unconventional (for telescopes) materials like plywood and Teflon in a compact mounting that moves easily, is extremely stable, and can adequately support large telescopes at a very low cost. Though there are no mechanical slow-motions or electric drives on a Dobsonian, a well-made example glides so smoothly on the Teflon bearings that with a little practice it is quite easy to track objects manually at 200x or more!

Equatorial mounts are designed specially for astronomy, and are not recommended for terrestrial viewing. Their advantage is that they allow easier tracking of the stars across the sky. This motion can be achieved with either a single manual slow-motion control or an electric motor drive (or clock drive). The easier viewing they provide at high power makes equatorials preferred by observers who are most interested in the Moon and planets. Also, you’ll need an equatorial mount if you want to do astrophotography.

A 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 rupees, 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 reflectors 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 amateur telescopes 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 telescopes, 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.

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!

The best telescope 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).

The advice is to select a well-made telescope, of a design matched as well as possible to your primary observing interest and most frequent observing site. Make sure it’s a size that can be handled easily (by your standards and no one else’s) and used often, and you will enjoy a lifetime of awe and wonder under the stars!

Of course, should you need more assistance in choosing a telescope or help just send us a note on nileshv@vsnl.com or feel free to visit at http://www.atiindia.com