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 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.

Newtonian reflectors will have either a spherical shaped mirror, which is less expensive to produce, or a higher quality parabola, which does not result in spherical aberration. Cassegrain telescopes routinely use spheres in addition to other lenses in the optical path to correct for residual spherical aberration. Refractors use a series of lenses to provide a clear image. Designs range from a standard air-spaced doublet (two lenses in a row) to exotic designs such as oil-spaced triplets and 4-element multi group lenses.

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.

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 thought 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.