|
Selecting the telescope for purchase/Buying tips Summary Your best choice in a telescope isn't buying the most powerful one you can afford, nor the biggest, nor the one with the most features. The best telescope is the one you will use most often. It will be far better than any telescope that idly is kept in the closed box because it is too heavy or too cumbersome. Its use depends on - Its being light (to be easily carried and set wherever you want) Discard any telescope that is promoted for its high magnification. The stunning and beautiful images are seen between 12.5 to 25x per inch of aperture - like for 4", from 50 to 100x... An ugly telescope (lookwise) can outperform a pretty one; quality of optics, firmness of mount, and ease of use are all that matter You don't just look through a telescope. You have to store it and carry it. You have to set it up and take it down at the end of a long day when most people are ready for bed. How good an astronomer you become, depends not on the aperture or how good telescope you own but how much time you spend observing. The following features are also hallmarks of a quality telescope: If possible, star-test a telescope before buying. Use a well corrected eyepiece (Nagler, Radian, Plossl or Ortho) and don't use a diagonal or barlow in your tests (unless it is known without question, to be of very high quality). But never expect your telescope will star test perfect, as it will not. The test is just too sensitive to wavefront aberrations. You can also do the "snap" test, which is a subjective way to test optics, but still a good general test. If the star (or planet) snaps into focus without any ambiguity of focus, the optic is probably a good one. The same holds for high power viewing. A good optic can stand high magnification on a night of good seeing, and a poor one will get mushy at high powers. If you haven't specialized and don't intend to, an all-purpose mid-range telescope should serve best. From 4.5" to 6" is beginner's simple reflector telescope. 7" onwards Cassegrain or reflectors/refractors may be regarded for serious astronomers. Details APERTURE WINS, and wins big. Power, or magnification, is not something to consider Even among telescopes with the same aperture, The most optical performance per unit of clear aperture For doing 35mm film photography or comfortable visual use The most optical performance per unit of cost comes from Having narrowed your choices get all the manufacturers' catalogs and compare details, paying careful attention to size, weight and optical quality. If possible, star-test a telescope before buying. Use a well corrected eyepiece (Nagler, Radian, Plossl or Ortho). Don't use diagonal or barlow in your tests (unless it is known without question, to be of very high quality). Give the scope a full two hours of cool-down time (or more if the scope has a long thermal equilibrium time or you live in a cold area), and make sure it is a night of good seeing, say 7 or better on a scale of 10. Focus on a 1st or 2nd manitude star using very high power (50x per inch) But the most important thing to remember about star testing a telescope is
to What you want to see is a well defined, round Airy disk, with no more than 2 or 3 diffraction rings around it. This is the real meat and potatoes of optical performance, that is, how dim are the first and second diffraction rings at best focus? If they are dim (a perfect optic would have 7% of the total energy in the first ring, 3% in the second), then you know the optics are good. Poor atmospheric "seeing" -- the quivering and blurring caused by the Earth's unsteady atmosphere -- may make this test difficult. You can also do the "snap" test, which is a subjective way to test optics, but still a good general test. If the star (or planet) snaps into focus without any ambiguity of focus, the optic is probably a good one. The same holds for high power viewing. A good optic can stand high magnification on a night of good seeing, and a poor one will get mushy at high powers. The following advice will help you juggle all factors to make the best decision. The best telescope is worthless if it is on a poor mounting. Long Refractor, For focal ratios longer than about f/12, the Dall-Kirkham design would be the better choice due to cost. If you wanted a short tube with a focal ratio between f/8 and f/12 or so, then the Classical Cassegrain would be the better choice. Should a long tube and very high cost for a moderate aperture be acceptable, then a 6" to 8" refractor would be suitable Very faint objects like galaxies and nebulae need big aperture, more aperture. A big reflector is the logical choice if this will be your specialty. Photographic use of a telescope falls into 3 categories;CCD, 35mm, and large format. Should CCD work be your main interest, then any design would work
exceptionally well. For those who want to do only large format photography, with the high cost of camera equipment, then the Rithey-Cretien is the only viable option. Astrophotography is in your future, then consider nothing less than an
7/8-inch Cassegrain On bad nights, with twinkling, 30x is more appropriate. Of course you can
decide for yourself what magnification you want to use; I am only explaining
how to get satisfying images where you see all the detail you can, without
blurriness Choosing a spaceview telescope also read - |