October 28, 2008

In Today's One Minute Astronomer...

Ready, Aim, Enjoy: Unit-Power Finders

Last time we went through the basics of magnifying optical finders. Today, we cover “red-dot” and “Telrad” finders. These devices, also called unit-power finders because they don't magnify an image, are much more intuitive for beginners and in many cases much easier to use. Especially in dark sky.

The Basics

• A red dot finder is the simplest and least expensive non-magnifying finder. It has a flat glass window onto which is projected a red dot from a small LED.

• To use the finder, you look through the glass window towards the sky. The red dot looks like it's projected on the sky. When the finder is aligned with your telescope, the dot should point to whatever's in the middle of the field of view of your eyepiece. Simple and very intuitive.

• Red-dot finders are much simpler to align than magnifying finders. There is one adjustment screw for up-down in one adjustment screw for left-right. That's it.

• Red-dot finders are also fairly inexpensive: only $30-$60 or so.

A simple red-dot finder

• The disadvantage of red-dot finders? You can only point to things you can see, so it's not directly useful for finding things to faint to be seen with the naked eye. This is a big disadvantage in light polluted city skies where it's hard to see faint objects.

A Deeper Look

• A more sophisticated non-magnifying finder is the Telrad. These devices project a series of concentric circles of diameter 0.5, 2.0, and 4.0 degrees onto the sky. There is no center dot. You can use the concentric circles to navigate from place to place in the sky by a known number of degrees. Also, many star maps have Telrad-sized circles to help you hop from object to object in the sky. Very nice.

• The biggest disadvantage of the Telrad is its size. It's about 21 cm long, which makes it hard to mount onto small telescopes. And it's heavy… about 300 grams or so. So you might have to rebalance your telescope so it doesn't tip over when you mount a Telrad.

A Telrad finder

Concentric circles of a Telrad finder reticle

Good To Know

Unit power finders use a battery to power the red LED, which means you have to turn them on… and turn them off. It's pretty much guaranteed you'll forget to turn it off one night. So carry an extra battery in your field kit.

Personal View

Though I was skeptical at first, I now love my red-dot finder, which I use on a short-focal length refractor. To be honest, if your telescope is large enough, you should have a magnifying and non-magnifying finder. Learn how to use them well and you'll be able to find anything in the sky.

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