6. What determines the "light gathering power" of a telescope? The amount of light which is gathered by the telescope
will be proportional (in the ideal case) to the square of the diameter of
the objective lens or mirror (this is simply because the light collected
will depend on the area of the objective). For example, small department
store telescopes often have an objective about 5 cm in diameter. A modest
amateur astronomer telescope is often about 20 cm diameter. The latter instrument
would collect 4 x 4 or 16 times as much light. This corresponds, on the
astronomical magnitude scale, to a difference of about 3 magnitudes ( |