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A finderscope is an aiming device used in astronomy, typically a small auxiliary telescope mounted on the main astronomical telescope along the same line of sight. The finderscope usually has a smaller magnification than the main telescope, providing a much larger field of view, useful for manually aiming (also called slewing) a telescope and locating a desired astronomical object. Some finderscopes have crosshairs to aid in accurately pointing the telescope system at a target.

Function and Design

In order to work, a finderscope must first be properly aligned with the main telescopes line of sight. Accomplishing this alignment varies based on the design of the finderscope and its mount, but the basic method involves pointing the main telescope at an object sufficiently far enough from the finderscope and main telescope so that parallax is not an issue during the day (or a bright celestial object such as a star at night), centering the object in the telescope eyepieces field of view, and adjusting the finderscope until the same object is also centered in its field of view (or the crosshairs are directly over the object).

Finderscopes usually come with a designation of the form A×B, 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 typically considered the minimum useful size for a magnifying finderscope on an amateur telescope, and an 8×50 or larger finderscope is preferred for more accurate aiming.

Most finderscopes have one of three viewing orientations:

Another type of finder commonly found on amateur telescopes is known as a reflector (reflex) sight. This non-magnifying sight (technically not a "scope") uses a type of beam splitter to "reflect" a reticle generated by collimating optics in the users field of view so that one can point the telescope at an object very quickly. Therefore the image seen through the sight is just what can be seen with the naked eye with an illuminated crosshair or dot seeming to float in space at infinity. These crosshairs are generally illuminated by a small LED. Reflex sights are most useful when the target has a high apparent magnitude (such as stars, planets, or nebulae visible to the naked eye). Since the sight uses a beam splitter "window", instead of an optical telescope with the ability to gather light, objects dimmer than the naked eye limit can not be seen through it. The lack of an optical telescope's magnification can make it harder to discern the difference between objects, such as stars and small angular size nebulae.

See also

• List of telescope parts and construction
• Viewfinder

References

1. "About Finder Scopes". Orion Telescopes and Binoculars. Retrieved 15 November 2013.
2. "How to Select the Right Binocular". Meade Instruments Corporation. Retrieved 15 November 2013.
3. "The Backyard Astronomer's Guide", by Terence Dickinson and Alan Dyer.

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