Question

find the focal length of telescope

Answer 1

Focal Ratio (f/stop) For example, a telescope with a focal length of 1000mm and a 100mm (4") clear aperture has a focal ratio of f/10 (1000/100=10). An example calculation of focal ratio and magnification for a scope with a 25 mm focal length, eyepiece, 102 mm aperture, and 900 mm focal length, telescope