# Homework Help: Magnification formula for a Galilean telescope

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1. Oct 22, 2016

### Cocoleia

1. The problem statement, all variables and given/known data
I am asked to draw the ray diagram
for an incident plane wave whose rays are at an angle α with respect to the optical axis of the telescope. I have done this, but I need to find an expression for the angle between the outgoing rays and the optical axis, in terms of f1, f2, and α . f1 is the focal length for the converging lens and f2 is the focal length for the diverging lens

2. Relevant equations

3. The attempt at a solution
I know that generally the magnification would be (-) f1/f2. I am wondering how the angle α will affect this. As to the actual question at hand, I am unsure how to proceed. I believe that the rays are not outgoing at an angle of α, but otherwise I am unsure.

2. Oct 23, 2016

### haruspex

As I understand it, parallel rays entering the telescope emerge still parallel. If opposite edges of some distant object subtend an angle θ at the telescope, and the telescope has magnification m, what angle must the rays from one edge and rays from the other edge form on exit from the telescope?

3. Oct 24, 2016

### andrevdh

The magnification that you mention Cocoleia is the angular magnification.
It is the ratio of the angle subtended by the object at the eye when the object is viewed with telescope, or the angle of the image formed by the telescope, divided by the angle of the object when viewed without the telescope.