Astronomy midterm question: "good beginner telescope" size

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

[20 points.] An astronomy question on an online discussion board(*) was asked and answered:
ahriik: What is a good telescope for a beginner? I want to get into astronomy, so I believe a telescope is essential.
eri:[F]or a good beginner telescope; the most important factor is the size of the primary mirror... There's not much point in investing in a telescope if the primary mirror diameter is less than 4.5", and the bigger the better...
Discuss reasons why "there's not much point in investing" in a small diameter telescope. Support your answer using the properties of telescope parameters and telescope powers.

*Source: http://answers.yahoo.com/question/index;_ylt=ArVtg7r0DwT2ZpjF5GpF2_sjzKIX;_ylv=3?qid=20100520162052AASAINW.

Solution and grading rubric:
  • p = 20/20:
    Correct. Discusses how diameter of primary lens/mirror determines the light-gathering power and resolving power, such that a large diameter lens/mirror would result in bright, finely resolved images worth magnifying.
  • r = 16/20:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Discusses how diameter affects either light-gathering power or resolving power, other diameter/power discussion incomplete.
  • t = 12/20:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. Problematic discussion of how diameter is related to either light-gathering power or resolving power.
  • 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:
Grading distribution:
Section 30674
Exam code: midterm01neE7
p: 18 students
r: 14 students
t: 0 students
v: 1 student
x: 0 students
y: 0 students
z: 0 students

Section 30676
Exam code: midterm01s0Be
p: 21 students
r: 15 students
t: 1 student
v: 3 students
x: 1 student
y: 1 student
z: 0 students

A sample "p" response (from student 4490):
Another sample "p" response (from student 0010):

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