The light gathering power (also light grasp) is an ability of a telescope to collect more light than the human eye.
It is expressed as the ratio of the objective area to the human eye pupil area.
- For the eye pupil, the value ‘7 mm’ is most frequently used.
The larger the size of the primary mirror/objective lens (aperture), the more light the telescope can collect (brighter image)!
Star cluster M13. On the left image, viewed with small aperture telescope - on the right image, viewed with large aperture telescope (Source: Griffithobservatory.org)
Light gathering power compared to the human eye
Area is proportional to the square of the diameter.
D = objective diameter (mm)
d = eye pupil diameter (mm)
Example:
d=7mm
D=125mm
Telescope with the objective diameter 125mm gathers 319x more light than the human eye.
This calculation doesn't account for loses!
