Telescope

A {Keplerian, refractive} telescope is described below: The unaided angle is given by the below:

$${\alpha }_{\text{unaided}}=\frac{-y_1}{f_{\text{obj}}}$$

Core idea is that the object is off-axis, so this calculation is valid. Because rays going through the optical centre of the objective lens have the same entering/exiting angle.

Its magnifying power is given by:

$${\text{MP}}_{s_i=\infty }=-\frac{f_o}{f_e}$$