The time in seconds, divided by 120, equals the field diameter in degrees. Then turn off the telescope’s clock drive (if any), and time how long the star takes to drift from the center of the field to the edge. Aim at any star in the general area of the celestial equator - in Orion’s Belt, for example, or the Circlet of Pisces, or Procyon, Spica, or Altair. It may be hard to find a good star pair on your map to measure a field so small. Now do the same to find the field diameter of the telescope’s lowest-power eyepiece. Then see how many degrees apart these stars are on the map, by referring to the scale of degrees along the sides. To determine the size of your finderscope’s field, locate two stars at night that just fit into its edges (try pairs in the Big Dipper or Cassiopeia). Knowing the size of your finderscope’s field, and your lowest-power eyepiece's field, is so important that you should make an effort to get it right. To see how big these sizes are on your star atlas, compare with the degree scale (the declination scale) along the star chart’s left and right edges. You have to know how much of the scene on paper appears in your finderscope's eyepiece before you can compare the star chart to what you see! As a rough guideline, a typical finderscope shows a field 5° wide, typical binoculars 6° or 7°, and a typical 50x telescope eyepiece shows a field only 1° wide. The next step is to learn the star chart's scale. ![]() If hours of right ascension are printed along the map's top or bottom, that'll set you straight. One trick for keeping east and west straight on a celestial map is remembering that right ascension increases to the east (it rhymes). (If you looked up through the bottom of a land map of, say, the United States - as if you were at the center of a transparent Earth - it too would have east left when north was up.) The reason is simple: You look down at the ground but up at the sky. This is usually tilted at an angle.Įast on a star chart is left of north, not to the right like on a map of the ground. North (up) on a star chart is always the direction toward Polaris in the sky, no matter what part of the sky you're looking at. To keep the familiar naked-eye patterns in perspective, some people draw in the constellation stick figures with a pencil, as we've done here.ĭirections on a star chart take a little getting used to. ![]() The same stars of Gemini appear on Chart 5 of Sky Atlas 2000.0 - but at a much larger scale and almost lost in a wealth of detail, as shown above. Suppose, for instance, you've learned Gemini as it's drawn on the monthly Sky & Telescope charts, where the stars are connected to form two stick figures holding hands. Examining the sky at 50 power is almost like examining the chart with a microscope! Note the tiny size of the telescopic field, even on a large-scale chart like Sky Atlas 2000.0. The small ring shows the 1° field of an average 50x eyepiece’s view. The large wire ring shows the 5° field of a typical finderscope. How much of a star chart appears in your eyepiece? You’ll be lost until you know. These form the same, familiar constellation patterns as on a naked-eye map. But step back for a minute, squint your eyes, and look at only the brighter stars. ![]() The smaller Pocket Atlas, with stars to magnitude 7.6, is an excellent low-cost starter and all you'll ever need for binoculars or a telescope of 3-inch aperture or less.ĭetailed, zoomed-in star charts may look terribly complex at first. It covers the celestial sphere in 26 big charts that plot a total of 81,000 stars (to as faint as magnitude 8.5) and 2,700 other objects. Using A Star Chart at the TelescopeĪ standard atlas for serious telescope users is Sky Atlas 2000.0 by Wil Tirion and Roger W. In addition to a wide-scale constellation star chart, a telescope user needs a more detailed, magnified sky atlas in order to locate specific points of interest. ![]() If you don't know where Japan or England are, you need to learn.īut once you've found England on a world map, it's not much good for getting you to a particular street address in Tunbridge Wells. Think of your all-sky star chart as like a map of the world, and the constellations as countries. An all-sky constellation star chart (such as the evening chart in the center of Sky & Telescope every month) will get you started. Click for larger view.īy the time you set out into the night with a telescope, you should know the constellations well enough to find your way around the sky. Amid swarms of details on Sky Atlas 2000.0, the naked-eye stars of Gemini are connected here to form their familiar stick figures.
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