Finderscope
A finderscope is an accessory
line of sight. The finderscope usually has a much smaller magnification than the main telescope, thus providing a larger field of view, useful for manually pointing (a.k.a. "slewing") the main telescope into a roughly correct direction that can easily place a desired astronomical object in view when zooming in. Some finderscopes have sophisticated reticles to more accurately aim the main telescope and/or even perform stadiametric measurements
.
Function and Design
Finderscopes contain mechanisms to properly align them with the main telescope's line of sight. Accomplishing this alignment varies based on the design of the finderscope and its mount: usually on amateur telescopes it is done by three or six adjustment screws.
Finderscopes usually come with a designation of the form A×B, where A is the
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.[1]
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.[2]
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 |
Reflex Sights
Another type of finder commonly found on amateur telescopes is known as a
planets. 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. Finding dim objects with a reflector sight is accomplished by using the object's known position relative to brighter objects as a reference and then slewing a known angular distance (or "star hopping") from the bright object to the desired object. Many reflector sights have circles with a given angular dimension in order to facilitate this. Reflector sights are less useful in light polluted areas since the stars used to guide the observer to a dim object may also be invisible.[3]
See also
References
- ^ "How to Select the Right Binocular". Meade Instruments Corporation. Archived from the original on 13 March 2014. Retrieved 15 November 2013.
- ^ "The Backyard Astronomer's Guide", by Terence Dickinson and Alan Dyer.
- ^ Rod Mollise, The Urban Astronomer's Guide: A Walking Tour of the Cosmos for City Sky Watchers, Springer Science & Business Media - 2006, page 43