Astronomy midterm question: better telescope choice

Astronomy 210 Midterm 1, fall semester 2012
Cuesta College, San Luis Obispo, CA

[20 points.] An astronomy question on an online discussion board(*) was asked and answered:

Mr. Dingo: Which [telescope] has [the brightest image], best detail, [and] magnification?

     FunScope™ [Tabletop] Reflector

          [76 mm diameter mirror, 300 mm long tube, f = 10 mm eyepiece]
     Orion™ SkyScanner® Reflector

          [100 mm diameter mirror, 400 mm long tube, f = 20 mm eyepiece]
Mark H: ...the answer is the [SkyScanner®].

Discuss whether you agree or disagree with this answer, and the criteria used in your decision. Support your answer using the properties of telescopes and telescope powers.

*Source: http://answers.yahoo.com/question/index?qid=20100516131920AAPZR2i.

Solution and grading rubric:

• p = 20/20:
  Correct. Discusses how diameter of primary mirror determines the light-gathering power and resolving power, such that the larger diameter mirror of the Orion™ SkyScanner® would result in brighter, more finely resolved images. Also discusses how the primary focal length and eyepiece supplied with this telescope would result in lower magnification than the primary focal length and eyepiece supplied with the FunScope™, although this is a less important consideration in purchasing a telescope than the light-gathering and resolving power.
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. As (p), but two of three telescope powers is correct, while discussion of third telescope power is problematic.
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Problematic discussion of two telescope powers, while third is complete/correct; or complete/correct discussion of two telescope powers, while discussion of third telescope power is omitted.
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least discusses some connection between telescope parameters and telescope power.
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit. Does not discuss connection between telescope parameters and telescope powers.
• y = 2/20:
  Irrelevant discussion/effectively blank.
• z = 0/20:
  Blank.

Grading distribution:
Section 70158
Exam code: midterm01sLa6
p: 6 students
r: 13 students
t: 16 students
v: 2 students
x: 2 students
y: 0 students
z: 0 students

Grading distribution:
Section 70160
Exam code: midterm01n4rN
p: 9 students
r: 7 students
t: 0 students
v: 11 students
x: 3 students
y: 0 students
z: 0 students

A sample "p" response (from student 8669):