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finderscope

n. (context astronomy English) A small telescope, or other sighting device, mounted on top of a main telescope to enable approximate directional positioning.

Wikipedia
Finderscope

A finderscope is an aiming device used in astronomy, typically a small auxiliary telescope mounted on the main astronomical telescope along the same line of sight. The finderscope usually has a smaller magnification than the main telescope, providing a much larger field of view, useful for manually aiming (also called "slewing") a telescope and locating a desired astronomical object. Some finderscopes have crosshairs to aid in accurately pointing the telescope system at a target.