Which of the following best describes what we mean by the focal plane of a telescope?
A) It is the upper surface of the telescope's primary lens or mirror.
B) It is the place where, if we mounted film or an electronic detector, we could get a clear (not blurry) image of an object viewed through the telescope.
C) It is the lower surface of the telescope's primary lens or mirror.
D) It is the surface of the lens on the eyepiece, through which you would look to see objects in the telescope's field of view.
Answer: B