A finderscope is a small, low powered telescope mounted on another more powerful telescope. The finderscope allows the viewer to more easily target a specific area for study. Many are equipped with crosshairs to help the user accurately target distant objects.
Powerful telescopes allow observers to view distant objects with amazing clarity, but that highly magnified vision typically covers only a small area. High magnification can give a very narrow field of vision. Targeting desired objects becomes difficult when the field of vision is too narrow to find any landmarks.
Mounting a finderscope on the main telescope allows the observer to target more easily. The lower magnification gives a view of a significantly larger area, allowing the observer to locate the desired object. Once that object it identified and centered in the scope, it can be studied in detail through the main telescope.
Two factors, magnification and aperture, are measured to gauge the power of a finderscope. Magnification is measured in a multiple, for instance 8x, meaning that the image is 8 times larger than the actual unmagnified image. Aperture describes the size of the finderscope’s objective lens measured in millimetres, with a larger aperture giving a greater field of vision. Amateur stargazers will most likely need a finderscope of at least 6x30, with stronger scopes of 8x50 or higher allowing the viewer to spot fainter stars for closer study with the main telescope.
These low powered telescopes are mounted on a more powerful telescope with one of three orientations. Standard orientation uses a straight scope that flips and reverses the image, so that it appears upside down. Right-angle scopes place the eyepiece and the aperture at 90°, and gives the viewer a mirror image. Correct orientation scopes are also mounted at 90°, but the magnified image matches the real objects in view.
Crosshairs are often found on finderscopes to assist with aiming, allowing the viewer to easily focus on the desired object. These crosshairs are only useful if they are clearly visible against the observed area. A powerful telescope which requires a finderscope is most likely to be used on the night sky, making illuminated crosshairs especially valuable.
Other types of finderscope use projected dots to target objects. Reflex finderscopes do not offer any magnification, but project a small red dot into the center of the image. Finderscopes combining features of reflex scopes and magnifying scopes are also available. Users of illuminated and reflex scopes should bear in mind that these devices do require electricity and they will need keep spare batteries on hand.