Astronomy quiz question: primary mirror focal length

Astronomy 210 Quiz 3, fall semester 2009
Cuesta College, San Luis Obispo, CA

The primary focal length for the Orion™ Dobsonian-mount Newtonian focus reflector shown at right is approximately:
(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 near the front of the tube (where it is deflected by a secondary mirror to exit out the side to the eyepiece lens).

Section 70160
(A) : 2 students
(B) : 16 students
(C) : 26 students
(D) : 0 students

Success level: 43% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.82