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| Another type of finder commonly found on amateur telescopes is known as a ]. This type of "finderscope" is non-magnifying, and puts a reticle generated by ] in the users field of view so that one can point the telescope at 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 or dot seeming to float in space at ]. These crosshairs are generally illuminated by a small LED. Reflex sights are most useful when the target has a high ] (such as stars or ] visible to the naked eye); since the sight is not magnified, it becomes more difficult to accurately locate dimmer objects, such as ], especially in observing locations with moderate to severe ]. | |||
| ==See also== | ==See also== | ||
Revision as of 15:53, 15 November 2013
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 |
Another type of finder commonly found on amateur telescopes is known as a reflector (reflex) sight. This type of "finderscope" is non-magnifying, and puts a reticle generated by collimating optics in the users field of view so that one can point the telescope at 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 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 or planets visible to the naked eye); since the sight is not magnified, it becomes more difficult to accurately locate dimmer objects, such as nebulae, especially in observing locations with moderate to severe light pollution.
See also
References
- "The Backyard Astronomer's Guide", by Terence Dickinson and Alan Dyer.
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