Know Your Equipment

Naturally, everyone first tries out a new telescope in the daytime. This is when to become familiar with its motions, pointing, focusing, different eyepieces, and magnifying powers, so you can then do everything in the dark. Here are some things you'll want to get straight in daylight before taking your telescope out under the night:

The Finder. Most telescopes have a finderscope attached to the side to help aim it. You need a finder because the main telescope has such a tiny field of view — that is, it shows such a tiny piece of sky — that you can't tell exactly where it's pointed just by looking. The higher the power of your main telescope, the smaller the field of view. For example, at 50 power you're looking at a magnified piece of sky about as small as your little fingernail covers when held at arm's length. Pretty tiny!

An 8× finderscope, by contrast, displays about as much sky as a golf ball covers at arm's length. This is big enough so you can aim at something you see with the naked eye and get it in the finderscope's view. Once it's there, you center it in the finder's crosshairs. That should be a precise enough aim for the object to appear in the view of the main telescope.

First things first: you'll need to adjust the finder's alignment screws so it gets aimed parallel to the main telescope. In daylight, point the main scope at something at least several hundred feet away using the lowest-power eyepiece. (But not the Sun! Never look through a telescope that might get aimed at the Sun or you could blind yourself.) A distant treetop is ideal. Center it in your view. Never mind if it appears upside down.

Now look in the finder. See the treetop? Is it centered in the crosshairs? Adjust the screws holding the finder until the crosshairs line up on the target. Now check back in the main telescope to make sure it hasn't moved. Then switch to a high-power eyepiece in the main telescope, and repeat the operation until the finder is locked in position with perfect aim.

And why, you ask, is the treetop upside down or oriented at some other weird angle? The answer is that this is an astronomical telescope, and after all, there's no up or down in space. So it doesn't matter how the field is oriented. Turning the image right-side up would require extra optical parts, adding to the instrument's expense and possibly degrading its performance. ("Image erecting" lenses are best used in terrestrial telescopes: those intended for looking at things on Earth in daytime, when there's no shortage of light.)

The Mount. As noted in the article "Choosing Your First Telescope," telescope mounts come in two basic types: equatorial and altazimuth.

An equatorial mount allows the telescope to swing only in the directions of celestial north-south and east-west. An altazimuth goes up-down (moving in altitude) and side to side (azimuth). The altazimuth mount has the virtue of simplicity. An equatorial mount is ultimately more helpful, but it takes some getting used to.

If you have an equatorial mount, find its polar axis (the rotating part that's closer to the telescope's base; it may have a setting circle showing right ascension). Outdoors, place the telescope so the polar axis points roughly to where you know Polaris, the North Star, will be located after dark. The telescope's motion around this axis now traces the paths taken by celestial bodies across the sky as the Earth turns.

Sweep the telescope around its polar axis from the eastern horizon across the sky to the west to visualize the paths the stars will follow over the course of a night. The mount's motions may seem awkward and unpredictable at first. But remember that no matter where the telescope is pointed, it will move only toward or away from Polaris (celestial north-south) and at right angles to this direction (celestial east-west). The orientation of these two directions varies in different parts of the sky, but with some practice swinging the telescope around in daytime you'll get used to them.