2 posts • Page 1 of 1
Divide the focal length of the telescope optical system by the focal length of the eyepiece. The result is magnification (x). With a refracting telescope, the objective lens focal length alone may be used for this calculation. In some reflecting telescope designs, the net focal length may be greater than that of the primary mirror alone, as the secondary mirror may add to the system focal length. This combined number, the "system focal length", usually appears on the ring locking the front lens in place at the front of the optical tube.