Do you know someone who would like to take up astronomy? Why not get them a telescope this Christmas? Before you whip out the credit card, though, you'll need to familiarize yourself with the various types and sizes of telescopes used for different kinds of viewing.
The key statistic about any telescope is its aperture: this tells you the amount of light the scope allows in to its lenses or mirrors. Aperture controls how bright the objects viewed will appear, and the resolving power of the telescope (how much fine detail the viewer will be able to see). A larger aperture means more light, so a brighter and more detailed view. The limitations on aperture are weight, bulk, and cost; for a beginner, don't buy a 25-inch, 300 pound telescope that requires a truck and a team of burly men to move it!
The least important number in buying a telescope is the magnification. A telescope can have a wide range of magnification powers, depending on the eyepiece used- and the useful maximum magnification is limited by the aperture. Don't be fooled into buying a cheapo scope that advertises "500x power!"; small telescopes (4 inches and less) have to be used at about 100-150x magnification for fine detail to appear. If a user pushes the magnification past the limit, the objects in view will dissolve into smudges of light. The normal rule-of-thumb for the maximum is 50x the aperture in inches. Above that, the view dissolves.
A few more preliminary considerations: would your new astronomer want to have a computerized star-finder, or would he or she prefer manual controls? This can impact the type of telescope you choose. One very important component is the mount; make sure that it is sturdy enough to provide stable viewing, but not so sturdy that the user will need to hire a team of Sherpas to carry it around! Finally, think about ease of handling and set-up; particularly if the new astronomer is a child, make sure to get a light-weight scope that doesn't require a lot of assembly.
So, those are the basics. Now, what about the different kinds of telescopes? There are reflecting, refracting, Dobsonian, and other kinds of telescopes.
Refractors are the original type of telescopes, and use lenses to magnify celestial bodies, rather than having mirrors to gather starlight. For observing the planets or double stars, refractors are best because they provide the crispest, highest-contrast images. If your gift recipient is fascinated by the planets, and will probably spend more time looking at the ice caps of Mars or Jupiter's Great Red Spot than the stars, consider getting them the Meade RB-70 Refracting Telescope. Low-maintenance and not too fragile, this 2.8 inch scope weighs only 10 pounds. It's a good choice for child astronomers, and a bargain at $99.00.
Reflectors use aluminum-coated mirrors to collect light and direct it to the eyepiece. They are often less expensive than refractors, and can be made much larger. A mid-sized (4.5 inches or larger) reflecting telescope is the best value for money, if you think your recipient is likely to continue this hobby long-term. These scopes have the capability to see fairly faint objects in the deep sky; they are perfect for finding nebulae, globular clusters, and maybe even a new comet! One disadvantage of reflectors is that the mirrors need to be adjusted ("collimated") from time to time; the mirrors also have to be cleaned. Also, the eyepieces can be quite high off the ground, depending on the scope, so they may be difficult for children to use and maintain. For the serious beginner, a 6-inch Celestron C6 N-HD Newtonian Reflector will set you back about $550, but it's a lot of telescope for the money. If that's too expensive, another decent option is a Meade 114E-AST Model 4.5-inch telescope, at about $200.
Dobsonian telescopes are a subset of reflectors, perfect for old-school observers who want to do all the focusing manually, and see objects millions of light years away from Earth. Dobsonian telescopes have two different housings for the mirrors which bolt together, reducing the scopes' weight, bulk, and cost. For the new astronomer who eschews computerized telescopes, a ten-inch Dobsonian is the gift that will never need upgraded! (Note, however, that this design is not really suitable for astrophotography, since there is no computer to "track" the subject as the Earth rotates.) A 10-inch Meade Lightbridge costs about $800; an 8-inch version is about $600.
BUILD YOUR OWN TELESCOPE KITS
If your friend of relative loves tinkering, or wants to really get into astronomy from the ground level up, consider buying them a telescope-building kit.
For kids as young as 8, Edmunds Scientifics offers a kit for a simple 3x refractor telescope. This magnification power can reveal Jupiter's moons, our moon's craters, and stars that aren't visible to the naked eye.
For a more serious build-it-yourself, the Astronomical Society of Las Cruces, New Mexico offers a complete kit for constructing a 4.5 inch reflector, $85, with complete instructions at http://aslc-nm.org/TMW%20spring%2006.pdf.
Whatever your gift recipient's interests and needs, you are sure to find a telescope to suit them. There are so many out there from which to choose! Happy shopping.