Astronomy midterm question: telescope comparison

Astronomy 210 Midterm 1, spring semester 2011
Cuesta College, San Luis Obispo, CA

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

Emma B: Which one of these telescopes [is] the better telescope?


Celestron™ 21061 AstroMaster® 70 AZ Refractor
      [70 mm diameter lens, 900 mm long tube, f = 20 mm eyepiece]

Orion™ SpaceProbe® 3 Altazimuth Reflector

      [76 mm diameter mirror, 700 mm long tube, f = 25 mm eyepiece]
George N: The second one...
--Adapted from http://answers.yahoo.com/question/index?qid=20101214213555AA1VrSt

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.

Solution and grading rubric:

• p = 20/20:
  Correct. Discusses how diameter of primary lens/mirror determines the light-gathering power and resolving power, while the (focal) length determines the magnifying power, the latter of which is the least important due to priority given to collecting light that will result in bright, resolvable images worth magnifying. Thus the Orion(TM) telescope will have better light-gathering power and resolving power, while having less magnifying power, as purchased, but another eyepiece could be used to give it greater magnifying power. Argument may point out nearly comparable LGP and RP (within 8%), while magnification is 60% greater for Celestron(TM) telescope.
  
• r = 16/20:
  Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Two of three telescope powers discussed correctly, third is problematic/missing.
  
• t = 12/20:
  Contains right ideas, but discussion is unclear/incomplete or contains major errors. Only one telescope power discussed correctly, other two problematic.
  
• v = 8/20:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Only one telescope power discussed correctly with other two missing, or all three problematic.
  
• x = 4/20:
  Implementation/application of ideas, but credit given for effort rather than merit. Primarily makes a decision without substantive discussion of relevant telescope powers.
  
• y = 2/20:
  Irrelevant discussion/effectively blank.
  
• z = 0/20:
  Blank.
  

Grading distribution:
Section 30674
Exam code: midterm01n0M3
p: 10 students
r: 12 students
t: 7 students
v: 8 students
x: 3 students
y: 1 student
z: 0 students

Grading distribution:
Section 30676
Exam code: midterm01sl7k
p: 19 students
r: 10 students
t: 5 students
v: 10 students
x: 5 students
y: 0 students
z: 0 students

A sample "p" response (from student 0523):

A sample "p" response (from student 1994), disagreeing with George N.'s decision: