AS A ROOKIE stargazer, how do you know when you're ready to buy a telescope? One way to find out is by bundling up and attending a few fall or winter star parties. If you're still keen on the hobby after those cold-weather skywatching sessions, you're probably ready to get your own scope.

Experienced amateurs offer loads of buying advice, and though some of their specific recommendations differ, they agree on these basic points:

You must pay $400 or more for a decent instrument. Geoff Chester, a staff member of the Einstein Planetarium, puts the minimum at $500, but Marty Cohen of Company Seven in Laurel swears by the portable Astroscan model he sells for $380 plus tax. If that sounds high, keep in mind that telescopes last for decades when properly cared for, and tend to hold their resale value well.

Be leery if a retailer's first question is "How much are you willing to spend?" A good telescope salesperson should seem more interested in educating you than in making a sale, says Chester. Ask the retailer how often he stargazes. If he says he merely sells the instruments, he won't be much help when you ask about maintaining or using the telescope. Also, ask if the instrument has been inspected since it arrived from the manufacturer; good dealers always run a telescope through a series of tests before selling it.

Focus on portability, not power. The bigger and more complex the telescope, the less inclined you'll be to use it. Be realistic about your viewing opportunities before you shell out major bucks for a fancy but cumbersome instrument.

Making your own telescope is a reasonable idea. Amateurs who've done it say it's a time-consuming but rewarding project. Some clubs in this area offer telescope-building courses, and Chester makes available a set of plans for an eight-inch reflecting telescope he has built. (To request a free copy, write him at NASM 3356E, Smithsonian Institution, Washington, DC 20560.)