Astronomy quiz question: comparing telescope magnifying powers

Astronomy 210 Quiz 3, spring semester 2013
Cuesta College, San Luis Obispo, CA

Consider two Orion™ reflector telescopes, which have mirrors with different diameters and focal lengths. The StarBlast®(*) telescope has a larger diameter and longer focal length mirror than the SkyScanner®(**) telescope. If a 20 mm focal length eyepiece is used with both telescopes, the __________ telescope will have more magnifying power.
(A) StarBlast®.
(B) SkyScanner®.
(C) (There is a tie.)
(D) (More information is needed to determine this.)

(*) "Orion™ StarBlast® 4.5 Astro Reflector Telescope, 114 mm aperture, 450 mm focal length," bit.ly/XXpRpM.
(**) "Orion™ SkyScanner® 100 mm TableTop Reflector Telescope, 100 mm aperture, 400 mm focal length," bit.ly/YyqFA1.

Correct answer: (A)

The magnifying power of a telescope is the focal length of the primary mirror (or lens) divided by the focal length of the eyepiece. Since the same eyepiece is used by both telescopes, the telescope with the longer primary mirror focal length will have more magnifying power.

Section 30674
Exam code: quiz03nPmP
(A) : 18 students
(B) : 8 students
(C) : 4 students
(D) : 0 students

Success level: 63% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.58

Section 30676
Exam code: quiz03sPmP
(A) : 18 students
(B) : 10 students
(C) : 14 students
(D) : 2 students

Success level: 45% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.18