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:
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F0 is the focal length of the objective
lens or mirror.
Resolving power is the ability of a telescope to distinguish between two objects.
a is the resolving power in arc
seconds |