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Aperture Help:

It can be argued that aperture is one of the most important aspects in evaluating and buying a telescope. Aperture is defined as been the width of the objective (front) lens of a telescope or the diameter of the primary mirror if the scope is a reflector.
A common misconception is that a 8 inch scope gathers twice as much light as a 4 inch scope, this simply isn't the case.

Using the 2 charts below, you can compare scopes of different aperture, the first chart has scope aperture along its horizontal axis while the vertical axis shows the scopes light gathering power in square inches.

So you can see for the above graph that the light gathering power of a 8 inch scope is 50, while a 16 inch scope is 200, so a 16 inch scope gathers 4 times more light then a 8 inch telescope.
The below graph is refined for smaller scopes, these scopes apertures tend to be measured in millimeters, the light gathering power is still in square inches so easily comparisons between scopes can be made.