What Does a Finderscope Do?

The powerful magnification of a telescope can sometimes work against it. Being able to see a patch of sky in very clear detail comes at the price of only being able to see a small patch of sky in clear detail. This is a problem when there is the need to search large swathes of the sky for a certain phenomena, because the small area visible and the tendency to get disoriented when moving the visible area too quickly conspire against the astronomer in a hurry. Adjusting the telescope’s focus is a possible solution, but often, the focus and/or magnification controls are designed for fine manipulation and are not suitable for hasty searching. In addition, the sight picture might be disrupted while adjusting the telescope’s magnification, which is counterproductive.

The solution is thus to use a smaller, auxiliary telescope which is attached alongside the main telescope. This smaller telescope has a wider field of vision, which sacrifices in-depth inspection ability for the ability to cover bigger areas of the sky in search for desired phenomena. These smaller telescopes are called “finder scopes”, and they are named because they help find interesting items for the main telescope to focus on. Most finder scopes, apart from being mounted co-axial to the telescope, are also sighted so that there is a crosshair or reticle depicting the area of sky that the main telescope would be focused on if the user were to switch back to the main telescope, which is certainly a handy function.

Finder scopes are usually mounted on the main telescope in such a way as to place the finder scope’s eyepiece next to that of the main telescope. This facilitates the switching from the finder scope to the main scope. In addition, finder scopes usually have three viewing modes: standard (produces an image tilted 90 degrees), wide-angle (produces an image which is inverted), and correct (produces an image which is the right way up. Changing the orientation of the eyepiece changes the viewing modes.

The power of a finder scope is usually expressed in an AxB format, similar to that used in most binoculars. A represents the magnification and B represents the aperture of the lens. In general, only finder scopes with an absolute minimum of a 6x30 lens are good enough for being used on even an amateur’s telescope, and most serious hobbyists will want to invest in a finder scope of 8x50 or better.

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