Optics - Diameter of moon viewed through telescope

1. Sep 24, 2007

elizabeth9681

1. The problem statement, all variables and given/known data

I've been stuck on this problem for a while now, and have tried it several different ways, but with no results. I'm sure I'm missing something obvious or simply approaching it from the wrong direction.

The moon subtends an angle of 0.5 degrees at the objective lens of a terrestrial telescope. The focal lengths of the objective and ocular lenses are 20 cm and 5 cm, respectively. Find the diameter of the image of the moon viewed through the telescope at near point of 25 cm.

2. Relevant equations

M = - fo/fe where fo = focal point of objective and fe = focal point of eyepiece