On the diagram above,

(i) label with the letter F the two focal points of the eyepiece lens.

(ii) draw rays to determine the location of the final image of the Moon.

The diameter of the Moon subtends an angle of 8.7×10^–3 rad at the unaided eye.

(i) Determine the diameter of the image of the Moon formed by the objective lens.

(ii) The focal length of the eyepiece is 30 cm. Calculate the angle that the final image of the Moon subtends at the eyepiece.

(i) F at P and second F at the same distance to the right of the eyepiece; (judge by eye)

(ii) 

first construction line or ray; (judge by eye)
second construction line or ray;
extension of these to the left as parallel lines;

(i) d=ƒ₀ tanθ≈ƒ₀θ ;
d=90×tan(8.7×10^-3)=0.78cm;

(ii) angular magnification is \(M = \left( { - \frac{{{f_0}}}{{{f_{\rm{e}}}}} = } \right)\left(  -  \right)3\);

hence \(\theta ' =  - 3\theta  = \left(  -  \right)0.026{\rm{rad}}\);

or

\(\theta ' = \frac{{0.78}}{{30}}\);
\(\theta ' = \left(  -  \right)0.026{\rm{rad}}\);