What a finder scope does
A telescope shows a tiny patch of sky at useful magnifications, which makes aiming surprisingly hard. A finder is a small sighting device mounted alongside the main tube. It gives a wider view at low or no magnification so you can point at a target, center it, and then look through the main eyepiece. A finder helps only if it is aligned with the telescope; see how to use a finderscope.
Main types of finders
Red dot finders
A red dot finder projects a small red dot onto a window of glass. You look through it with both eyes open and place the dot on your target. There is no magnification, the sky looks natural and right-way-up, and they are light and simple. They need power (usually a small battery) and show only the stars your eye can see, so faint targets can be harder to confirm.
Reflex finders with rings
Some reflex finders, such as the widely used Telrad, project a set of rings instead of a single dot. These rings correspond to known angles on the sky, which is handy for star-hopping with a chart. They are bulkier than a red dot unit and can catch dew, but many observers like the large, natural view.
Optical finderscopes
An optical finderscope is a small refractor with crosshairs. Common sizes are written like 6x30 or 8x50, meaning magnification times aperture in millimeters. Larger apertures show more stars and fainter objects. Beginners’ telescopes commonly come with a 6x30 finder, which many people find limited, and upgrading to a larger one is a typical improvement.
Most basic optical finders show an inverted image, which can feel confusing when moving the telescope.
Right-angle, erecting-image finders
These finders have an eyepiece angled at 90 degrees and a correct-image (erect) view, often in 8x50 or 9x50 sizes. They are comfortable when the telescope points high overhead, and the image is right-way-up although mirror-reversed in some designs. They are more expensive than simple finders.
Illuminated crosshairs
Some optical finders have illuminated crosshairs, which are easier to see against a dark sky than plain wires. A dim red light is generally best for preserving night vision.
Quick comparison
| Type | Magnification | Image | Strength | Drawback |
|---|---|---|---|---|
| Red dot | None | Natural | Light, easy | Limited faint-star help |
| Ring reflex | None | Natural | Good for star-hopping | Bulkier, dew |
| Straight-through optical | Low (e.g. 6x to 9x) | Usually inverted | Shows fainter stars | Awkward overhead |
| Right-angle erecting | Low | Corrected | Comfortable, easy to use | Cost |
Which one should you choose?
- Brand new to astronomy: a red dot or a ring reflex is usually the easiest start, because there is no inverted image to deal with.
- Using a refractor or small scope: a lightweight red dot is a good fit, because a heavy finder can unbalance it.
- Dobsonian or reflector: many observers use a red dot or reflex finder plus a small optical finder. The eyepiece location on a Newtonian can make some angled finders convenient.
- Dark sites and faint targets: an 8x50 or larger optical finder can show stars you could not otherwise confirm.
Many people combine two: a red dot for the rough aim and an optical finder for confirmation.
Do you need one with a GoTo mount?
A computerized GoTo mount points itself after alignment, but a finder can still help during the alignment stage and for checking that the target is near the middle. Opinions differ on how much you need one, but a simple red dot is cheap and rarely a bad idea.
Mounting and compatibility
Finders attach with a bracket or a base. Look for a sturdy bracket with adjustment screws that hold their position, because a loose finder drifts out of alignment. Check that any replacement uses a compatible base for your telescope, since there are several common shoe styles and some older telescopes use different mounting patterns.
Next steps
Once your finder is fitted, align it, then practice with bright objects. You will use the finder with a low-power eyepiece to start; see the telescope eyepieces guide for choosing one.