About Finder Scopes

If you had to define the term "exercise in futility," you might consider, "finding a celestial object with a telescope without the use of a finder scope."

A finder scope is a small telescope that rides piggyback on the main telescope. Its purpose is simple: to aid in aiming the main telescope toward objects of interest. By virtue of low magnification and a wide field of view, the finder allows you to see more of the sky than you can through the main telescope.

The idea is that you locate an object in the finder scope and center it on the crosshairs, then look in the main telescope and — voila! — it will be waiting right there for you in the eyepiece, whether the object is a crater on the Moon or a faint galaxy tens of millions of light years distant.

First, Align It
This only works if the finder scope has first been aligned with the main scope, so the two are aiming at exactly the same spot. That's easy to do.

To avoid hassle, align the finder scope while it's still light outside. A good time to do it is after the telescope has been set up but before the Sun goes down.

First, put a low-power (long focal length) eyepiece in the focuser of the main telescope. Looking into the eyepiece, center a distant object in the field of view — the top of a telephone pole, a treetop, or a chimney on a house. The object should be at least a quarter-mile away. Now look through the finder scope and see if the object appears in the center of the finder's field of view (where the crosshairs intersect). If it does not, use the adjustment screws on the finder scope bracket to adjust the aim of the finder until the object is centered. Then, look back into the telescope eyepiece and make sure the object is still centered there as well. If it is, you're done. If it is not, repeat the procedure, being careful not to move the main telescope while you're adjusting the finder scope positioning.

When the distant object is centered in both the main telescope and the finder scope, the finder scope is properly aligned and ready to use. Verifying the finder's alignment should be one of the steps you go through each time you set up for an observing session.

Focusing the Finder Scope
Many finder scopes have an adjustment that allows the image to be focused for your eyesight. Focusing is done either by turning the eyepiece assembly or by moving the objective (front) lens in or out. Consult the instruction manual that came with your telescope or finder scope.

Upside-Down is "Normal"
The basic finder scope included with most telescopes is itself a miniature refracting telescope. Good ones have a two-element (i.e., achromatic) objective lens and most offer modest magnification on the order of 5x to 10x, and a field of view of 5 degrees or more. A 6x30 finder scope is a common one. The 6 is the magnifying power, while the 30 represents the diameter, or aperture, of the main lens, in millimeters.

Beginners are often surprised that the image in a standard finder scope is upside-down (rotated 180 degrees). That's normal (though perhaps not ideal) for any refractor used without a correcting prism. It's not that big of a deal, however, since all you're trying to do is center the object on the crosshairs. Whether it's upside down or not doesn't much matter, though it does make navigation by star-hopping a little trickier.

The Bigger the Finder, the More You'll See
As with any scope, the bigger the diameter of the light-collecting lens or mirror (i.e., the aperture), the more you'll be able to see with it. Greater light-grasp translates into seeing fainter objects and finer detail. So larger, more expensive 7x50 or 9x60 finders garner more light to pull in fainter targets and, more importantly, dimmer stars. Such large-aperture finders are popular add-on accessories for telescopes of 6" and greater apertures.

If the object you're targeting is too faint to be seen in the finder scope, then you have to aim where you think it is and "sweep" the area a little with the main telescope until it appears in the eyepiece.

Using a Finder Scope: Eyes (Both of 'Em) Wide Open
The best way to use a finder scope is to keep both eyes open. Although this sounds odd, and may take some getting used to, it really works.

Before you look into the finder scope, use just your eyes to locate a bright star in the sky that you want to aim at. It may be close to a deep-sky object you want to view, according to your star chart. (If the object is bright enough, you may be able to spot it directly.)

With the telescope free to move about both axes, sight along the finder scope — not through it yet — and aim the scope roughly at the star, which you're looking at with your unaided eyes. Now, when the finder scope is pointed in the general direction of the star, look through the finder with one eye while keeping the other eye locked on the star. This will make it easier to aim the finder scope at the star. Eventually, you should see the same star in the field of view of the finder scope with one eye that you're seeing with your other, unaided eye. This may take a little practice, so do be patient. Once the star is in the field of view of the finder scope, lock the mount's axes, then center star on the on the crosshairs using the mount's slow-motion controls. Keep in mind that the image in the finder scope will be upside-down compared to the naked-eye view.

Reflex Sights, or "Red Dot" Finders
A relatively new type of finder scope is called a reflex sight, or "red dot" scope. It is a nonmagnifying device that displays a red, LED-lit bull's-eye pattern or red dot in the center of the field of view. The red dot appears superimposed on the sky, showing exactly where the telescope is pointed (once properly aligned, of course).

While a reflex finder can only locate objects that are visible to the naked eye, its main advantage is that it does not invert or flip the image like a standard finder scope does. It also has a very wide field of view. So it makes star-hopping very easy and intuitive. Look on a star chart, locate an object's position relative to bright nearby stars. Move the scope to place the reflex sight's red dot on that spot, and, presto, the object is right there in the main telescope's eyepiece.

Because standard finders and reflex sights each have their own merits, many amateur astronomers use them both, mounting them side by side on their telescopes.