Astronomy quiz question: Celestron Newtonian reflector magnifying power

Astronomy 210 Quiz 3, spring semester 2016
Cuesta College, San Luis Obispo, CA

The magnifying power for the Celestron® Newtonian focus reflector shown at right depends on:
(A) the width of the tube.
(B) the length of the tube.
(C) twice the length of the tube.
(D) three times the length of the tube.

Correct answer (highlight to unhide): (B)

Light enters the tube and is then reflected off of the primary mirror at the back, coming to a focus as it travels back up the tube, then deflected out the side of the tube by the secondary mirror at the front of the lens, such that focal length of the mirror is approximately the length of the tube.

The magnifying power of the telescope depends on the primary mirror focal length f_primary (which is approximately the length of the tube) divided by the eyepiece focal length f_eyepiece (which is not given):

Magnifying power = f_primary/f_primary.

Section 30674
Exam code: quiz03n0V4
(A) : 5 student
(B) : 11 students
(C) : 3 students
(D) : 1 student
(No response: 1 student

Success level: 54% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67