# What is prime focus of a telescope

Telescope here refers to a system of two convex lenses. In one of the problems I wanted to solve, it is given that "a Barlow lens of focal length $$f_{b}$$ is placed at a distance $$D < f_{b}$$, from the prime focus (and toward the objective lens) ." What is prime focus of a telescope ?

• Sorry, but I didn't understand. What is $D$ in the picture? – user183683 Jan 21 '19 at 15:05