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 ?

It is the focal plane of the objective lens where a real intermediate image is usually formed.
Often the intermediate image is then viewed by a second lens (system).

In your example the Barlow (diverging) lens (system) can be thought of as producing an objective with effectively a larger focal length than the objective alone.
A real intermediate image due to the objective is not formed rather it acts as a virtual object for the Barlow lens which produces a real image as shown below in a new focal plane. • Sorry, but I didn't understand. What is $D$ in the picture? – user183683 Jan 21 at 15:05
• @Aierel I can see no "D" in the picture other than at the beginning of the word"Distance". – Farcher Jan 21 at 15:23
• Yes, there's no D, in the picture. I mentioned D in the question as the distance from the prime focus to the Barlow lens. Is it the distance from the new focal plane (or the original focal length ) to the Barlow lens. – user183683 Jan 21 at 15:36
• @Aierel I have added some labels on the diagram. – Farcher Jan 21 at 15:50
• Thanks, I am very new to optics and barely anything. – user183683 Jan 21 at 15:54