Telescopes can have multiple uses depending on their design. Refractors and Cassegrains can provide a "correct-image" view, so they can be used for daytime land viewing as well as astronomy. Reflectors render the image upside down so they are not recommended for daytime viewing. This is not an issue for astronomy, however, because there's no "right-side up" in space.

Beginner - Telescopes suited for beginners are easy to use and maintain. Very good optical and mechanical quality. A great first telescope that will show you the moon and planets.

Intermediate -Telescopes for the intermediate user are more robust in features and performance. Excellent quality in optics and mechanics, and more complex in use. An intermediate telescope will allow an enthusiast to "grow" in the hobby.

Advanced - Advanced telescopes are high performance, with exceptional quality. They may require more commitment to use in set-up time and technical savvy. Some advanced telescopes are easy-to-use but large and heavy. An advanced telescope is a purchase for a lifetime.

Expert - Expert telescopes offer uncompromising optical and mechanical quality for the most demanding amateur astronomer. They may be technically involved or designed for specialized use, such as astrophotography or detailed deep sky observation. They carry a premium price; but are designed to provide the ultimate performance in the field.

A small, quality achromatic refractor of 60mm to 80mm aperture makes a fine starter scope for observing the Moon and major planets. They're affordable ($100 to $350), portable, and maintenance-free, all desirable factors if you're just "testing the waters" of Astronomy. If nebulas and galaxies are your main interest, a Newtonian reflector or Schmidt-Cassegrain is the way to go. Moving up to a 90mm or 100mm refractor will snare more objects and provide better performance, for a higher price. Renowned for crisp, sharp images, refractors are the priciest per inch of aperture of all telescope types. A refractor is the scope of choice if you will be doing most of your stargazing from city or suburbs, where the night skies are moderately light-polluted. Since viewing is restricted mostly to the Moon and planets a big scope would only amplify the skyglow, yielding poor washed out images. Reflectors Newtonian reflectors are great all-around scopes, offering generous apertures at affordable prices. They excel for both planetary and deep-sky viewing. Of course, the larger the aperture, the more you'll see. Smaller, 3" and 4.5" equatorially mounted Newtonians will provide a nice "survey" of celestial luminaries, and they maintain their portability. Six-inch and 8" Newts have enough aperture to deliver captivating images of fainter star-clusters, galaxies, and nebulas-especially in a reasonably dark sky. Although the 6" to 8" Newts are bulkier and weigh more, with a Dobsonian mount they are easily manageable by one person making them a wonderful beginner scopes. Dobsonian-mounted reflectors have lower price tags than their equatorial counterparts, starting in the mid-$300s for a 6" Dob. Schmidt-Cassegrains If portability is important to you, you might want to consider a "catadioptric" scope such as a Schmidt-Cassegrain or Maksutov-Cassegrain. They pack a hefty aperture into a very compact tube. An 8" Schmidt-Cassegrain provides excellent views of the Moon, planets, and deep-sky objects, and is well suited for astrophotography. Schmidt-Cassegrains should be considered as an investment priced over $1000 for the most basic 8" models (and hundreds more to outfit it for astrophotography). The Bottom Line Now that you've received the crash course on telescopes, here's some parting advice for aspiring astronomers: Get as much aperture as you can reasonably handle. Big aperture is desirable, however you don't want to end up with a scope that is too big or complicated to conveniently set up, haul around-and use! Also, avoid those gee-whiz, techno-toy scopes with the hefty price tags that are showing up in the big chain stores. For a first telescope, we recommend a basic refractor of 90mm aperture or smaller, or a Newtonian reflector of 6" aperture or less, unless you're really committed. After you've learned the basics of observing and developed an appreciation for the hobby, then you can move up to a bigger, fancier scope.

The optical diameter (also known as aperture), is the size of the telescope's main light collecting lens or mirror, measured in millimeters or inches. As the diameter increases, more light is collected and the resolution increases.

The distance from the center of a curved mirror or lens at which parallel light rays converge to a single point. The focal length is an inherent specification of a mirror or lens and is one of the factors in determining resultant magnification for a telescope (along with the focal length of the eyepiece being used).

The focal ratio of an optical system is the ratio of a telescope's focal length to its aperture. Short focal ratios (f/5, f/4.5) produce wide fields of view and small image scales, while long focal lengths produce narrower fields of views and larger image scales.

Refractors use glass lenses to focus the light, and the glass material plays an important role in the quality of the resulting image. Standard achromatic refractors routinely use Crown and Flint for the two elements, but more expensive apochromatic refractors can use ED (extra low dispersion) glass for one or more of the lenses. Reflector mirrors are made from glass with different levels of thermal expansion. Standard mirrors are made from material such as Soda-Lime Plate glass and BK-7 glass. Glass with Pyrex or other low thermal expansion material will not change shape as dramatically during the cool-down period, resulting in more stable images during this period.

The theoretical resolving power of a telescope can be calculated with the following formula: Resolving power (in arc seconds) = 4.56 divided by aperture of telescope (in inches). In metric units, this is: Resolving power (in arc seconds) = 116 divided by aperture of telescope (in millimeters). Note that the formula is independent of the telescope type or model, and is based only upon the aperture of the telescope. So the larger the telescope's aperture, the more it is capable of resolving. This is important to keep in mind when observing astronomical objects which require high resolution for best viewing, such as planets and double stars. However, it is usually atmospheric seeing conditions (not the telescope) which limits the actual resolving power on a given night; rarely is resolution less than one arc-second possible from even the best viewing locations on Earth.

Lowest useful magnification is the power at which the exit pupil of the telescope (the beam of light exiting the eyepiece) becomes 7mm in diameter. Powers below this can still be reached with the telescope to give wider fields of view, but the image no longer becomes brighter at a lower power. This is due to the fact that the exit pupil is now larger than the average person's dark adapted pupil, therefore the eye cannot absorb any more light.

The highest practical limit is different from the often used "highest theoretical magnification" specification. The "theoretical" limit generally is 50x the aperture of the scope in inches (2x the aperture in mm). So for example, an 80mm refractor is capable of 160x, and a 10" telescope is theoretically capable of 500x magnification. But after approximately 300x, theory breaks down and real world problems take over. The atmosphere above us is constantly in motion, and it will distort the image seen through the telescope. This effect may not be noticeable at lower powers, but at higher powers the atmosphere will dramatically blur the object, reducing the quality of the image. On a good night (a night where the air above is steady and the stars aren't twinkling), the practical upper limit of a large telescope is 300x, even though the theoretical limit may be much higher. This doesn't mean the scope will never be able to reach those higher "theoretical" powers - there will be that rare night where the atmosphere is perfectly still and the scope can be pushed past it's practical limit, but those nights will be few and far between.

The limiting stellar magnitude is a measure of the faintest star you can see through the telescope.

"Diffraction Limited" means that the limits of image detail are determined by the physical properties of light, and not by optical defects in the telescope.