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Telescopes Telescopes Reflectors
Refractors
Primary aperture
Focal length
Advertised maximum power or magnification
Resolving power
Limiting visual magnitude
Altazimuth
Equatorial
Dobsonian
Reflectors
Most consumer-grade reflector telescopes use a Newtonian design, where light enters one end of a large, straight tube, bounces off a concave focusing mirror at the other end of the tube, then travels back up the tube, where the concentrated beam is reflected off a secondary mirror and down through the telescope's eyepiece for final viewing. Since the secondary mirror cannot be suspended in the middle of the tube, it's held in place by spokes, which block some of the light coming into the tube. Despite the spokes, a reflector can produce crisp, clear images, and if the mirrors are properly aligned, few people will notice the difference between them and theoretically superior refractor designs. Refractors
Refractors are standard, tapered telescopes with a large objective lens on one end and a small eyepiece on the other. Light passes through the lens, which focuses it into a narrow beam that strikes the eyepiece, where final focusing can be adjusted to produce clear images. Primary aperture
Determines the telescope's ability to gather light. Usually measured in millimeters (but sometimes in inches), aperture is the diameter of the telescope's main optical element, be it a lens or a mirror. The more light a telescope can gather, the better its performance will be, and, since telescopes are circular, small increases in aperture lead to enormous gains in light-gathering ability. For example, a 70mm telescope can take in 36 percent more light than a 60mm model. Focal length
Focal length is the distance the path of light takes from the front of the telescope until a focused image arrives in the eyepiece. Don't mistake the length of the tube for the focal length, as certain designs reflect light inside the tube to artificially increase the true focal length of the telescope. Advertised maximum power or magnification
The magnification a telescope is theoretically capable of providing. This is mostly an irrelevant number; if you switch eyepieces, a telescope can be made to magnify at almost any power, but there is an absolute limit to the resolution possible for each aperture. As the magnification is pushed beyond this limit, the image will stop revealing any additional detail. Resolving power
A number amount indicating how well (in theory) a telescope can separate a closely spaced pair of binary stars. Measured in arcseconds; smaller numbers are better. Limiting visual magnitude
A way to specify the light-gathering power of a telescope by indicating the faintest star the telescope is capable of showing, though this is really determined by the size of the aperture. This is registered in "magnitudes"a 1st magnitude star is the brightest, while a 15th magnitude star can only be seen with a powerful telescope. Often, the limiting visual magnitude figures stated by manufacturers refer to the generally accepted figures for any telescope at that aperture. Altazimuth
A mount allowing a telescope to move freely both horizontally and vertically. This is a common mounting, though tracking objects requires moving the scope on both axes, which can be difficult. The freedom of this mount makes the design ideal for viewing ground objects though, and a solid altazimuth mount with slow-motion controls (or electronic controls) works well for beginners. Equatorial
A telescope mount that allows tracking by merely moving the telescope right or left once a celestial object is centered on. Equatorial mounts are calibrated to follow the polar axis, eliminating the need for vertical adjustments when tracking objects. They must be pointed at Polaris (the North Star) to work, but even beginners should have no trouble becoming familiar with the design after a few hours of use. Dobsonian
Dobsonian mounts sit flat on the ground upon a swiveling base, with two "arms" extending upward. Pads on the side of the telescope fit into half-circle cutouts at the top of the arms, and the telescope is then free to rotate up, down, left, and right. Dobsonian mounts are incredibly stable, have few moving parts, and can really cut down on the overall cost of a new telescope.
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