Finder Scopes: An Astronomer’s Best Friend

On top of your main telescope, you may have a smaller wide-field telescope. This is your finder… an essential tool to help you see and magnify faint stars and deep sky objects. But finder scopes can be a little confusing at first. If you’re frustrated by your finder, here are some tips to help you get aligned and find your way around the sky.

The Basics

• Most finder scopes magnify 6-9x with an objective lens of 30-50 mm. A 7×50 finder, for example, magnifies an image 7 times and has a 50 mm objective. Try to get a finder with an objective of at least 40 mm. Any smaller and the image will be too faint.

• Most finders come pre-focused. If you need to adjust the focus, it’s not as easy as focusing your telescope. You may need to rotate a ring near the eyepiece. Or in some cases rotate the main objective lens of the finder.

• You need to know what kind of image you see in your finder. If you look straight through, you may see an inverted image. That means up is down and left is right. It takes some practice to get used to this, but you’ll figure it out pretty quickly with a bit of practice. More modern finders give a corrected image, just like you see with your eye. A merciful feature, to be sure. If you don’t know what you’ve got, look through your finder at a terrestrial object to see if the image is right-side up.

A magnifying optical finder with a right-angle prism

A Deeper Look

• Your finder is aligned when it points in the same direction, precisely, as your telescope. To align your finder, get a bright object in your telescope’s field of view by sighting an object down the length of the telescope tube. A bright star or an image of a daytime terrestrial object works best. Then, with the adjustment screws, align your finder until you can see the same object in the crosshairs as you see through your telescope’s eyepiece.

• This takes a little practice, especially if you’re a beginner, but it’s the most important step. If you get frustrated, have a cup of tea before you get back to it. There is no rush.

• Next, you want to get a good feel for your finder’s field of view, that is, how much sky you can see at one time. Check out two or three stars that lie close to each other. You’ll get a good idea of how much you can see… probably 4-6 degrees of sky at one time. That’s still fairly narrow, but much wider than the 0.5 degree or smaller field of view with your telescope.

Good To Know

If you’re lucky enough to live under dark sky, you may find it hard to see the reticle (or the cross-hairs) in your finder’s eyepiece. If you can, get a finder with an illuminated reticle. It’s much easier to see against the dark background sky. Battery required, however.

Personal View

In the last few years, I’ve discovered the wonders of non-magnifying finders (which we’ll cover in an upcoming issue). But I still need my trusty 8×50 to find dimmer objects, especially the galaxies in Virgo and Coma Berenices which are in star-sparse regions of the sky. And some extended star clusters like the Pleiades actually look better in my finder than in my telescope.