Finderscope

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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 6x30 finderscope means a finderscope with a 30 mm objective and a magnification of 6x. This designation is in the same format used by most binoculars.

A 6x30 finderscope is the minimum useful size for a findescope on an amateur telescope, and an 8x50 or larger finderscope is preferred.^[1] Many inexpensive telescopes come with 5x24 finderscopes or smaller, and in some cases the true aperture is even less due to the presence of an internal aperture stop.

Finderscopes most commonly come with three viewing orientations:

Type	Eyepiece mount	Image orientation
Standard	Straight through	Upside down and reversed (i.e. rotated 90 degrees)
Right-angle	90 degrees	Backwards (mirror-image)
Correct orientation	90 degrees	Correct

[edit] References

1. ^ "The Backyard Astronomer's Guide", by Terence Dickinson and Alan Dyer.