Museo Galileo
Virtual Museum

Accessory telescope mounted in parallel with the main telescope for easier aiming. Typically a small, short-focus refractor with a wide field (3-6° or more). The eyepiece is usually fitted with a cross-hair for exact centering of the object to be observed. Attached to main instrument by means of one or two supports fitted with a screw adjustment system to achieve perfect parallelism with the main telescope's optical axis. In small commercial instruments, the finder may be replaced by a plain sight similar to those of firearms. As a rule, finder dimensions are proportional to those of the main instrument: the Soviet telescope with a 6-m aperture is fitted with a finder 70 cm in diameter! On large professional instruments, two or even three finders of different sizes are not uncommon.