By 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.

An all-sky constellation map (such as the foldout evening sky map in Sky & Telescope every month) will get you started. So will a planisphere. Think of your all-sky map as like a map of the world; if you don't know where Japan or England are, you need to learn.

But 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. In addition to a wide-scale constellation map, a telescope user needs a more detailed, magnified sky atlas in order to locate specific points of interest.

The standard atlas for serious telescope users is Sky Atlas 2000.0 by Wil Tirion and Roger W. Sinnott. 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.

Such maps may look terribly complex at first. But step back for a minute, squint your eyes, and look at only the brighter stars. These form the same, familiar constellation patterns as are on your naked-eye all-sky map.

Directions and Distances

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. 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. 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.

Directions on a sky map take a little getting used to. East is left of north on a sky map, not to the right like on a map of the ground. The reason is simple: You look down at the ground but up at the sky. (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.)

One trick for keeping east and west straight on a celestial map is remembering that right ascension increases to the east. If hours of right ascension are printed on the map's top or bottom, they'll set you straight.

The next step is to learn the map's scale. You have to know how much of the scene on paper appears in your finderscope's eyepiece before you can compare the map to what you see! As a rough guideline, a typical finderscope shows a field 5° wide, and a typical 50x telescope eyepiece shows a field 1° across. To see how big these sizes are on your star-atlas map, compare with the degree scale (the declination scale) along the map’s left and right edges.

Knowing the size of your finderscope’s field, and your lowest-power eyepiece field, is so important that you should make an effort to get it right. 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). Then see how many degrees apart these stars are on the map, by referring to the scale of degrees along the sides.

Now do the same to find the field diameter of the main telescope’s lowest-power eyepiece. It may be hard to identify a good star pair on your map to measure a field so small. So here’s another way. 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. Center the star. 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. The time in seconds, divided by 120, equals the field diameter in degrees.

Next, using the scale on the margin of the charts, make little rings out of wire as shown above — or draw circles on clear plastic — corresponding to your field sizes. By sliding these circles across the charts, you can see exactly what star patterns will pass through your field of view when you sweep across the sky.

Beginners are always surprised at how tiny the view really is. Keep these little tools with the charts; you'll need them whenever you observe.

Now we're ready to go on our first deep-sky hunt.