Telescope Limiting Magnitude Calculator

About this calculator

A telescope's limiting magnitude is the faintest star it can detect — and since the astronomical magnitude scale runs backwards (lower or negative numbers are brighter), a bigger limiting magnitude number means the scope reaches fainter, more distant stars. This calculator estimates it from aperture alone using the standard approximation LM = 7.5 + 5·log₁₀(D), where D is the aperture in centimeters.

A 200mm (8-inch) scope reaches roughly magnitude 13.5 under good conditions — about 3,600 times fainter than the magnitude-6 limit of the naked eye. The calculator also reports light grasp, the ratio of the telescope's light-collecting area to the human eye's pupil, since a wider aperture gathers more photons per second and that's the real physical driver behind reaching fainter magnitudes.

Amateur astronomers use this when choosing a telescope or planning what deep-sky objects — faint galaxies, nebulae, dim moons — will actually be visible with the aperture they own.