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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 ?

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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.

enter image description here

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  • $\begingroup$ Sorry, but I didn't understand. What is $D$ in the picture? $\endgroup$ – user183683 Jan 21 at 15:05
  • $\begingroup$ @Aierel I can see no "D" in the picture other than at the beginning of the word"Distance". $\endgroup$ – Farcher Jan 21 at 15:23
  • $\begingroup$ 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. $\endgroup$ – user183683 Jan 21 at 15:36
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    $\begingroup$ @Aierel I have added some labels on the diagram. $\endgroup$ – Farcher Jan 21 at 15:50
  • $\begingroup$ Thanks, I am very new to optics and barely anything. $\endgroup$ – user183683 Jan 21 at 15:54

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