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A finderscope is an accessory sighting device used in
astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...
and stargazing, typically a small auxiliary
refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...
/
monocular A monocular is a compact refracting telescope used to magnify images of distant objects, typically using an optical prism to ensure an erect image, instead of using relay lenses like most telescopic sights. The volume and weight of a monocula ...
mounted parallelly on a larger astronomical telescope along the same
line of sight The line of sight, also known as visual axis or sightline (also sight line), is an imaginary line between a viewer/ observer/ spectator's eye(s) and a subject of interest, or their relative direction. The subject may be any definable object taken ...
. The finderscope usually has a much smaller
magnification Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a size ratio called optical magnification. When this number is less than one, it refers to a reduction in size, so ...
than the main telescope, thus providing a larger
field of view The field of view (FOV) is the angle, angular extent of the observable world that is visual perception, seen at any given moment. In the case of optical instruments or sensors, it is a solid angle through which a detector is sensitive to elec ...
, 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
reticle A reticle or reticule, also known as a graticule or crosshair, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the electronic v ...
s 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
magnification Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a size ratio called optical magnification. When this number is less than one, it refers to a reduction in size, so ...
and B is the
aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...
of the finderscope's
objective lens In optical engineering, an objective is an optical element that gathers light from an object being observed and focuses the light rays from it to produce a real image of the object. Objectives can be a single lens or mirror, or combinations of ...
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 Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
. A 6×30 finderscope is typically considered the minimum useful size for a magnifying finderscope on an amateur telescope. An 8×50 or larger finderscope is preferred, which allows sighting of fainter objects. Most finderscopes have one of three viewing orientations: {, class="wikitable" , - ! Type !
Eyepiece An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as Optical telescope, telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks thro ...
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 reflector (reflex) sight. This non-magnifying
sight Visual perception is the ability to detect light and use it to form an image of the surrounding Biophysical environment, environment. Photodetection without image formation is classified as ''light sensing''. In most vertebrates, visual percept ...
(technically not a "scope") uses a type of
beam splitter A beam splitter or beamsplitter is an optical instrument, optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as Interferometry, int ...
to "reflect" a reticle generated by collimating optics into the users field of view. The view of the sky 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 Infinity is something which is boundless, endless, or larger than any natural number. It is denoted by \infty, called the infinity symbol. From the time of the Ancient Greek mathematics, ancient Greeks, the Infinity (philosophy), philosophic ...
. These crosshairs are generally illuminated by a small LED. Reflector sights are useful for locating bright objects visible to the naked eye such as stars and
planets A planet is a large, rounded astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets by the most restrictive definition of the te ...
. 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.Rod Mollise, The Urban Astronomer's Guide: A Walking Tour of the Cosmos for City Sky Watchers, Springer Science & Business Media - 2006, page 43


See also

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List of telescope parts and construction Hardware Accessories * Finderscope *Iron sight * Reflector (reflex) sight * Cheshire collimator: A simple tool to collimate a telescope Control * Clock drive * GoTo Mechanical construction * Mirror support cell * Serrurier truss *Silvering Moun ...
*
Viewfinder In photography, a viewfinder is a device on a camera that a photographer uses to determine exactly where the camera is pointed, and approximately how much of that view will be photographed. A viewfinder can be mechanical (indicating only direct ...


References

Optical telescopes