Light Gathering Power

The light gathering power (LGP) of a telescope is a measure of its ability to gather light compared to the dark adapted pupil of the average human eye. It is found by dividing the diameter of the telescope by the size of the dark adapted pupil (most commonly 7 mm), and squaring that value.

Example:
Using a 130 mm diameter telescope,

LGP = [ ( 130 mm ) ÷ ( 7 mm ) ]² ≈ 345x

This value means that the telescope gathers 345 times the amount of light as the dark adapted pupil of the average human eye.

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