Magnification of Astronomical Telescope

The magnification $m$ of an astronomical telescope is the ratio of the angle $\beta$ the final image subtends at the eye to the angle $\alpha$ the object itself subtends at the (unaided) eye or the objective, $m=\beta/\alpha$; using $\alpha\approx h/f_o$ and $\beta\approx h/f_e$ for the same intermediate image height $h$ gives $\boxed{m=f_o/f_e}$. The approximate overall length of the telescope, since the objective's second focal point an …