Telescope, CCD and Eyepiece Calculator - Deep Sky Watch
Cassegrain Formulas and Tips by Mike Lockwood
Telescope Equations
A Galilean telescope measures 9 cm from the objective to the eye-piece. The focal length of the objective is 15 cm. Its magnifying power is A. $2.5$ B. $\\dfrac{2}{5}$ C. $\\dfrac{5}{3}$ D. $\\dfrac{3}{5}$
Telescope Equations: Minimum Magnification
Simple Formulas for the Telescope Owner - Sky & Telescope - Sky & Telescope