In this equation, A represents the larger telescope and B the smaller telescope or human eye. The diameter of the objective lens or mirror for each telescope is represented by D. Solving this equation yields how much greater the light gathering power (LGP) of the bigger telescope is over the smaller one. For example, if the diameter of the large telescope is 100 cm and the smaller telescope is 10 cm, the light gathering power of the larger telescope will be 100 times greater than that of the smaller scope.

Light gathering power is an important measure of the potential performance of a telescope. If an astronomer is studying faint objects, the telescope used must have a sufficient light gathering power to collect enough light to make those objects visible. Even with the very largest telescopes, some distant space objects appear so faint that the only way they become visible is through long-exposure photography or by using CCDs. A photographic plate at the focus of a telescope may require several hours of exposure before enough light collects to form an image for an astronomer to study. Unfortunately, very large ground-based telescopes also detect extremely faint atmospheric glow, which interferes with the image. Not having to look through the atmosphere to see faint objects is one of the advantages space-based telescopes have over ground-based instruments.

Teacher Notes:

  • In this activity younger students can use larger objects such as pennies, washers, or poker chips in place of the paper punchouts. Enlarge the black circles accordingly. Discs can be eliminated entirely by using graph paper and a
    compass. Draw several circles on the graph paper and count the squares to estimate light gathering power of different sized lenses and mirrors.
  • If students notice that the punchouts do not entirely cover the black circles, ask them what they should do to compensate for the leftover black space.
For Further Research:
  • Compare the light gathering power of the various lenses you collected with the human eye. Have students measure the diameters, in centimeters, of each lens. Hold a small plastic ruler in front of each student's eye in the class and derive an average pupil diameter for all students. Be careful not to touch eyes with the ruler. If you have an astronomical telescope, determine its light gathering power over the unaided human eye.
  • Does the light gathering power formula work for mirror type telescopes?
  • Use the following formulas to determine other important measures of telescope performance:

F0 is the focal length of the objective lens or mirror.
Fe is the focal length of the eyepiece lens.

Resolving power is the ability of a telescope to distinguish between two objects.

a is the resolving power in arc seconds
D is the diameter of the objective lens or mirror in centimeters.

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Last modified prior to September, 2000 by the Windows Team

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