How to Use a Finderscope on a Telescope

What the finderscope does

A finderscope is a small sighting device attached to the main telescope. It has a wider field of view than the telescope at normal observing powers, so it is much easier to aim. The main telescope has the aperture for light gathering and detail; the finder only helps you point. If you are still choosing one, read about finder scope types.

Before you start

• Set the telescope on a stable mount, ideally outdoors in daylight.
• Choose your lowest-power eyepiece, meaning the one with the largest millimeter number.
• Make sure the finder’s mounting screws are snug but not forced.
• Remove any lens caps from both the finder and the main telescope. Never point a telescope at the Sun.

Aligning the finder in daylight

1. Pick a target. Choose a distant, stationary object such as a chimney, a pole or a sign. A few hundred meters away is generally enough.
2. Center it in the telescope. Use the low-power eyepiece and focus, then center the target in the eyepiece.
3. Lock the mount so the telescope no longer moves.
4. Look through the finder. The target will probably be off to one side.
5. Use the adjustment screws. Most finders have three or more screws or a spring-loaded design. Turn them a little at a time until the target is on the crosshair or red dot.
6. Check back in the eyepiece. The target should still be centered. Repeat if needed.
7. Tighten gently so the finder does not drift.

Fine-tuning at night

Alignment done by day may be slightly off. At night, center a bright star or the Moon in the telescope at low power, then nudge the finder adjustments until it sits on the same spot. This usually takes a minute and makes a noticeable difference.

Finding objects in the sky

1. Locate a bright star near your target using a star chart or app.
2. Aim the finder at it and check that it appears in your main eyepiece.
3. Move step by step from one star pattern to the next, a technique called star-hopping, until you reach the target.
4. Center the target in the finder, then look through the eyepiece at low power.
5. Increase magnification only after the object is centered.

A red dot or ring finder works the same way: place the dot or ring on the star pattern, rather than looking at an enlarged field.

Why is the finderscope image upside down?

Many optical finders use simple lenses, which flip the image upside down and sometimes left-to-right. This is normal and not a fault. It takes some practice. Finders with erecting prisms show the image the right way up, and red dot finders show the real sky with no inversion.

Why can’t I see anything through the eyepiece?

Common reasons:

• Focus is off. Turn the focuser slowly in each direction. Start in daylight with a distant object.
• The finder is not aligned, so you are pointing at empty sky.
• A cap is still on, or a diagonal or eyepiece is not seated properly.
• The eyepiece is too powerful. Start with the longest focal length. See the eyepiece magnification chart.
• The target is too faint or too low. Begin with the Moon or a bright planet.

Telescope versus finderscope

The telescope is the main instrument: larger aperture, higher magnification, narrower field. The finder is a small aid: wide field, low power, used for aiming. Do not worry if the view through the finder is dim; its job is only to point you.

Troubleshooting

• If the finder keeps drifting out of alignment, check the bracket for loose parts.
• If the crosshair is hard to see, try a dim red light or illuminated model.
• If your neck hurts when pointing high, consider an angled finder.
• If you often struggle, try a different eyepiece first: see the telescope eyepieces guide.