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A Finderscope is a small auxiliary telescope mounted atop the main astronomical telescope and pointed in the same direction. The finderscope usually has a much smaller magnification than the main telescope can provide and therefore can see more of the sky. This helps in locating the desired astronomical object in the night sky. Some finderscopes have crosshairs to mark exactly where the main telescope is looking.
Finderscopes usually come with a designation of the form AxB, where A is the magnification and B is the aperture of the finderscope's objective lens in millimeters; for example, a 6×30 finderscope means a finderscope with a 30 mm objective and a magnification of 6×. This designation is in the same format used by most binoculars.
A 6×30 finderscope is the minimum useful size for a finderscope on an amateur telescope, and an 8×50 or larger finderscope is preferred. Many inexpensive telescopes come with 5×24 finderscopes or smaller, and in some cases the true aperture is even less due to the presence of an internal aperture stop.
Most finderscopes have one of three viewing orientations:
| Type | Eyepiece mount | Image orientation |
|---|---|---|
| Standard | Straight through | Upside down and reversed (i.e. rotated 180 degrees) |
| Right-angle | 90 degrees | Backwards (mirror-image) |
| RACI (Right-angle correct-image) | 90 degrees | Correct |
Telescopes also use finders based on the reflector (reflex) sight. This type of "finderscope" does not magnify the image at all, but instead puts a reticle generated by collimating optics in the users field of view so that one can locate an object very quickly. The image seen through the finder is exactly the same as what can be seen with the naked eye with an illuminated crosshair seeming to float in space at infinity.
See also
References
- "The Backyard Astronomer's Guide", by Terence Dickinson and Alan Dyer.
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