Refracting telescopes are built with the objective lens and eyepiece separated by a distance equal to the sum of their focal lengths, fo + fe. Ray tracing is done as follows:
1) Incoming ray i goes through the center of the objective lens and continues in a straight line as ray ii.
2) Ray ii leaves the objective and hits the image plane a distance fo away from the objective.
3) After passing through the image plane ray ii becomes ray iii travelling parallel to the optic axis, and strikes the eyepiece.
4) Ray iii becomes ray iv at the eyepiece and emerges heading for the focal point fe on the optic axis.

If ao is the angle of the light ray entering the telescope's objective lens, and ae is the angle of light ray coming out of the eyepiece, then the Angular Magnification is :    A_mag = ( ae/ao) .

Exercise:

Using a straight edge, draw this diagram out, using steps 1) through 4) above assuming that fo= 4", and fe = 1".
Draw in ray i with an angle ao = 5 degrees. Find the angle ae and determine the angular magnification.