Astronomy quiz question: primary mirror focal length

Astronomy 210 Quiz 3, Spring Semester 2009
Cuesta College, San Luis Obispo, CA

What power of an optical telescope is affected by the focal length of the primary mirror?
(A) Light-gathering power.
(B) Resolving power.
(C) Magnifying power.
(D) (None of the above choices.)

Section 30676
(A) : 12 students
(B) : 19 students
(C) : 31 students
(D) : 0 students

Correct answer: (C)

The magnification of an optical telescope depends on the ratio of the focal lengths of the primary lens (or mirror) and the eyepiece. The light-gathering power and the resolving power of an optical telescope depend only on the properties of the primary lens (or mirror), specifically the area and diameter, respectively.

"Difficulty level": 54% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66