Free Telescope Light Gathering Calculator
Compare the light-gathering power of a telescope to the naked eye: LGP = (D / d_eye)^2.
mm
mm
Light Gathering Power
816.3 ×
Collecting Area314.2 cm²
Light Gathering Power vs Telescope Aperture
Light Gathering Power
LGP = (D_telescope / D_eye)²
Light collection scales with the area of the aperture. A 200 mm scope collects (200/7)² ≈ 816 times more light than the dark-adapted eye (7 mm pupil).
This is why larger telescopes see fainter objects.
Example Calculation
A 200 mm telescope vs 7 mm pupil.
- 01LGP = (200/7)² = 28.57² = 816×
- 02The scope gathers 816 times as much light.
- 03Area = π × (200/20)² = π × 100 = 314 cm²