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interferometer light-gathering power magnifying power
resolving power


  1. Light-Gathering Power = pi ×(diameter of objective)2/4.
  2. Resolving Power thetaR = 252,000×(observation wavelength/diameter of objective). Better resolving power has smaller thetaR.
  3. Magnifying Power = (objective focal length) / (eyepiece focal length).

Review Questions

  1. Of the three powers of the telescope (light-gathering power, resolving power, magnification) which is least important? Which depend on the size of the objective mirror or lens?
  2. How many times brighter will a 60-centimeter telescope make a 10-second exposure image than a 12-centimeter telescope?
  3. How many times better resolution does a 48-centimeter telescope have than a 12-centimeter telescope?
  4. Will a shorter or longer wavelength enable us to see smaller details?
  5. Why do radio telescopes have to be so large?
  6. How can an interferometer be used to improve resolution?
  7. What is the maximum magnification that should be used with a 20-centimeter telescope?
  8. Would a 30-power telescope with lens 4 centimeters across be better for observing a faint, faraway object than a 60-power telescope with lens 3 centimeters across? Why or why not?

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last updated: March 4, 2014

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Author of original content: Nick Strobel