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!
is part of the Marketing team at Optics Trade. She is a nature and astronomy enthusiast, that’s why you’ll find most of her articles in these two categories.