Astronomy quiz question: Orion® Dobsonian-mount Newtonian focus reflector primary focal length

Astronomy 210 Quiz 3, spring semester 2017
Cuesta College, San Luis Obispo, CA

The primary focal length for the Orion® Dobsonian-mount Newtonian focus reflector shown at right is:
(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 by the secondary mirror at the front of the tube to exit out the side; thus traveling once along the length of the tube while coming to a focus.

Section 30674
Exam code: quiz03n5By
(A) : 2 students
(B) : 9 students
(C) : 10 students
(D) : 2 student

Success level: 45% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0

Section 30676
Exam code: quiz03s4adD
(A) : 4 students
(B) : 20 students
(C) : 15 students
(D) : 2 students

Success level: 54% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.27