Finder Scopes

A magnifying finder scope mounted to a telescope.

A magnifying finder scope mounted to a telescope.

When you look through a telescope, you look at a very small slice of sky. That makes it almost impossible to point your telescope directly at your target. Sometimes, it’s even hard to find the Moon directly with the main tube of a telescope, even at lowest magnification. That’s why most telescopes come with a finder, a small telescope or range finder to help you easily locate objects in the night sky. There are two main types of finders: magnifying finders and non-magnifying finders.

Magnifying Finder Scopes

A magnifying finder is simply a small, low-power wide-field telescope mounted on top of the optical tube of the main scope.  The eyepiece of the finder has some sort of reticle or cross-hair to help you line precisely locate an object in the finder’s field of view. Because you can see more sky in a finder than with your telescope, typically about 5º to 7°, compared to less than 1° with your telescope, a finder makes it easier to locate celestial objects.

Most magnifying finder scopes magnify 6-9x with an objective lens of 30-50 mm. A 9×50 finder, for example, magnifies an image 9 times and has a 50 mm objective.  Many telescopes come with underpowered magnifying finders.  Even a good SCT telescope costing $1,000 or more comes with a 6×30 finder, which is barely adequate.  But in most cases, you can upgrade your magnifying finder.  Try to get one with an objective lens of at least 40 mm to get a brighter image.  A good 50 mm finder cost about $100, but you will appreciate the difference over a stock 6×30 mm finder.  Before you upgrade your finder, consult with the vendor to make sure a larger finder will fit on your optical tube. You may need to buy an extra bracket.

A 9x50 finder scope with right-angle prism. This scope gives a “correct image”

A 9×50 finder scope with right-angle prism. This scope gives a “correct image”

Most magnifying 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 to loosen the eyepiece or the objective lens, then turn the eyepiece or objective, then tighten the locking ring again.

You will also need to know what kind of image you see in your finder. Many magnifying finders, especially those you look straight through, show an inverted image, which 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. The most convenient finders have a right-angle prism as in the image above. Such finders are a blessing because you don’t have to bend your neck and lay your head along the side of the telescope tube to look through the finder. Some modern finders, usually those with right-angle eyepieces, are engineered to give a corrected image, just like you see with your eye or a pair of binoculars. This is a merciful feature and makes it more intuitive to finder objects with your scope. A finder than has a right-angle eyepiece and a corrected (left-to-right) image is called a RACI finder.

Even the fanciest finder isn’t of much use unless it points in exactly the same direction as your telescope. To align your finder with your scope, 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 just after sunset, so you have enough light to work with, or an image of a daytime terrestrial object such as the tip of a flagpole or telephone pole works best.  Then, with the adjustment screws of the finder, align your finder mount 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 an essential step.

A red-dot finder mounted on a small refractor telescope.

A red-dot finder mounted on a small refractor telescope.

Non-Magnifying Finders

Non-magnifying or “unit-power” finders don’t magnify an image and so are much more intuitive for beginners and in many cases much easier to use, especially in dark sky with lots of brighter stars nearby the objects you’re trying to find. Non-magnifying finders simply consist of a small ground glass screen onto which is projected a red dot or a circular reticle from an LED.

A red dot finder is the simplest and least expensive non-magnifying finder. It has a flat glass window that shows a single 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.  A new red-dot finder costs about $30-$50.

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

The Telrad is a more sophisticated non-magnifying finder. 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 your telescope 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 using the finder.

Telrads are fairly large… about 21 cm long, which makes it hard to mount onto small telescopes. And they’re heavy… about 300 grams. So you might have to rebalance your telescope so it doesn’t tip over when you mount a Telrad.  A Telrad costs about $50.

A Telrad finder and its reticle pattern

A Telrad finder and its reticle pattern

How To Use a Finder

You usually use a finder to locate an object in one of three ways. For objects like bright stars and planets, you simply sight the object in center of the field of view of the finder scope or at the center of the reticle of a unit-power finder, and, if the finder is aligned with your telescope, you will see the object in the much narrower field of view of the eyepiece of your telescope.  You can still use non-magnifying finders to locate fainter objects that are invisible to the unaided eye if you know where they are in relation to a brighter object.  For example, the Ring Nebula in Lyra is too faint to see in a red-dot or Telrad, but it is near the mid-point of the bright stars β and γ Lyrae.  So use a low-power eyepiece in your telescope to get the maximum field of view, aim your red-dot between these two stars, and the Ring Nebula should be somewhere in the field of view of your eyepiece.

A Telrad finder aimed skyward, it's concentric circles projected onto a glass screen.

A Telrad finder aimed skyward, it’s concentric circles projected onto a glass screen.

For fainter objects just beyond the limit of perception with your unaided eye and down to magnitude 7 or 8, you can use a 40-50 mm optical finder scope to brighten and magnify the image of the object, just as you would use a pair of binoculars. Then you center the faint image in the finder, and you’ll see it in the telescope. Non-magnifying finders cannot help with such objects.  For example, the galaxy M81 in Ursa Major is not near any bright stars, but it is bright enough to see directly in a 50 mm finder.  Line it up directly in your finder, and you are all set (again, if your finder and telescope are aligned with each other),

For even fainter objects that are not visible in the finder, you can use either a magnifying or non-magnifying finder to hop from star to star across several fields of view to find the object you’re looking for. This method is called star hopping. It takes a little practice, a good star chart, and accurate knowledge of the field of view of the finder, but it’s a foolproof way to find anything in the sky, especially if you don’t have a Go-To telescope mount.  Magnifying finders and Telrads are great for star hopping in regions with few bright objects such as in the Virgo Galaxy cluster.

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