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 is parallel to the optic axis and hits the objective lens.
2) Ray ii leaves the objective and passes through the focal point a distance fo away from the objective.
3) After passing through the focal point ray ii becomes ray iii and strikes the eyepiece.
4) Ray iii becomes ray iv at the eyepiece and emerges parallel to the optic axis.

If do is the diameter of the bundle of light entering the telescope's objective lens, and de is the diameter of the bundle of light ray coming out of the eyepiece, then the Brightness Magnification is :   B_mag = ( do/de)² .

Exercise:

Using a straight edge, draw this diagram out, using steps 1) through 4) above assuming that fo= 4", and fe = 1".
From your measured values of de and do determine the brightness magnification.