The finder is the “gun sight” that allows you to point your telescope at, or close to, the object you want to observe. Some kind of finder is always included with beginner-level telescopes (while with advanced telescopes, it is usually omitted so the astronomer can make that choice separately).
Because it is included in the purchase, many beginners don’t even realize there is a choice of types of finder – they just consider it “part of the telescope”. Unfortunately, the finders included with beginner-class telescopes are sometimes inadequate, chosen more to keep the total cost down than as an appropriate tool. This can contribute to frustration and disappointment.
A good finder will not guarantee you find the objects you are looking for, but a poor finder will guarantee you do not.
Two Kinds of Finders
Unit-power finders have many advantages, including:
- It is very easy to point your telescope to things you can see, or to general areas of the sky, using a unit power finder because the sky remains in view to provide context.
- Since you are looking directly at the sky, not via mirrors or lenses, the image is upright and is not reversed, making it easier to move the telescope.
- Unit power finders are usually very easy to align, with simple up/down and left/right adjustments.
- Basic units can be quite inexpensive, allowing low-end telescopes to come with an affordable yet effective finder.
However, they also have certain shortcomings, including:
- Since they rely on an illuminated pattern being seen against the sky, they can’t be used in bright daylight. This means you can’t use them for terrestrial viewing, and also means you cannot easily align them in daylight – you must wait until dusk or dark.
- Since they don’t magnify, you can only point to things you can see, or to general areas of the sky. You can’t point to a dim object which is invisible to the naked eye.
- Since you can only point to things you can see, they may be of limited use in very light-polluted areas, where few bright stars are visible.
- They require batteries. You will forget to turn the illuminator off, killing the batteries, so you will need to have spares with you.
- On many finders, the red pattern is too bright, and “swamps” visibility of the dim objects you may be trying to point to.
I list here a few common examples of unit power finders, to give you an idea of the variety available.
The Telrad is the original, classic unit-power finder. It is rugged and easy to use, it’s inexpensive, it mounts on a dovetail pad for easy removal and storage, it is easy to align precisely, and it uses inexpensive and easily-replaceable AA batteries. The variable intensity works well, and it can be dimmed enough to be unobtrusive.
It projects a series of concentric circles of a known size, and many star charts are available with templates that show the Telrad circles over the maps, to aid in finding things. Since there is no centre dot, it doesn’t obscure the dim object you are trying to centre.
The only real disadvantages are that it is rather large, and can overwhelm, or be difficult to mount on, small refractors, and that its large window is quite susceptible to dew.
Finders that magnify the view are simply small telescopes, mounted on the main body of your large telescope. They are usually fixed at a low power (6x, 8x, and 9x are common) and have a wide field of view. Fine wires or engravings in the eyepiece project cross-hairs in the view, and you sight on the intersection of these lines.
Although the above properties are common to all magnifying finders, there are also several variations:
- The finder may be used “straight-through”, or there may be a mirror or prism diagonal allowing it to be used from a right angle. (The right angle is much more comfortable.)
- On the simplest finders, the image will be seen upside-down (if straight-through) or reversed (if right-angle). On a RACI (Right Angle Correct Image) finder, a prism is used to keep the image properly oriented.
- The black cross hairs in simple finders can be difficult to see against a dark sky. Illuminated Reticle finders have battery-powered illuminators that make the black cross-hairs glow red for better visibility.
- Most basic magnifying finders have built-in fixed eyepieces, but on some finders, any standard 1.25″ eyepiece can be used. This gives more magnification options and even allows the finder to be used as a small telescope.
- As telescopes, finders need to be focused. On some this is simple, by rotating a ring near the eyepiece. On others, focusing is more difficult, possibly involving rotating the main objective lens.
This type of finder has certain advantages:
- Being a small telescope, it magnifies and brightens the image, so you may be able to find things in the finder that you can’t see with the naked eye. This enables you to find dimmer objects, or to see more in light-polluted skies.
- The black cross-hair reticle is visible in daylight, so the scope can be used for terrestrial observing, and can be aligned in daytime.
Some of the disadvantages of this type of finder include:
- Being magnified, the view in the finder is different from your naked-eye view of the sky. This can make it harder to interpret what you see and orient yourself. Especially since:
- The image in these finders is often reversed or upside-down (except for RACI versions), so orienting yourself and moving the telescope while looking through the finder takes some practice.
- Being magnified, these finders are more sensitive to misalignment and, unfortunately, they are usually harder to align because they will be mounted in a 3-point or 6-point ring system.
- In non-illuminated versions, the black cross hairs are invisible in the dark.
Some finders come with what is considered an optional feature: a mounting mechanism that allows them to be quickly removed from the telescope, and quickly-reattached later at close to the correct alignment. Since you usually need to remove the finder to store or transport the telescope, you will quickly come to regard this feature as essential, and it’s well worth a small extra investment.
Ideally, you should have two finders – a unit-power finder for quickly finding easy objects and a magnifying finder for hunting for difficult objects. Since they need not be expensive, and most telescopes have ample room to mount both, this is certainly feasible.
For the beginner who only wants to invest in one finder, what would I recommend? Each has advantages, but my advice would be:
For a child or an adult absolute beginner, a simple red-dot unit power finder is easier to use and adequate for the types of objects a beginner will observe, and would be the best start.
For a more serious amateur, or as an upgrade once a beginner gains some experience, a magnifying scope-style finder is useful in more circumstances, especially hunting for dim objects, but can still be used for all the cases where a unit power finder works. My personal preference is for the RACI (Right Angle Correct Image) style for comfort and simple movement.
Your finder needs to be aligned so that it points exactly where the main telescope is pointing. This article explains how to align a finder.