A cassegrain telescope is built with the mirrors 20 mm apart. If the radius of curvature of the larger mirror is 220 mm and the small mirror is 140 mm. where will the final image of an object at infinity be ?

Explanation

The image formed by the larger (concave) mirror acts as virtual object for the smaller (convex) mirror. Parallel rays coming from the object at infinity will focus at a distance of 110 mm from the larger mirror. The distance of virtual object for the smaller mirror `= (110 -20) = 90mm`. The focal length of smaller mirror is 70 mm. Using the mirror formula, image is formed at 315 mm from the smaller mirror.