Light-Gathering Area:

The light-gathering area of a telescope is the quantitative measure of its capability to collect light for seeing. It can be defined in centimeters squared (cm²) or meters squared (m²), which is, in terms of the efficient surface area of the objective lens or mirror as measured in a plane perpendicular to its axis. At times it is stated in inches squared (in²).

For the refracting telescope, specified an objective radius of r, the light-gathering area A can be computed according to this formula as follows:

A = πr²

here π is around equivalent to 3.14159. When r is stated in centimeters, then A is in centimeters squared; when r is in meters, subsequently A is in meters squared.

For the reflecting telescope, given an objective radius of r, the light-gathering area A can be computed according to this formula as follows:

A = πr² - B

here B is the area blocked by the secondary mirror assembly. When r is stated in centimeters and B is stated in centimeters squared, then A is in centimeters squared; when r is in meters and B is in meters squared, in that case A is in meters squared.