Telescope Limiting Magnitude Diameter mm

Telescope limiting magnitude

The limiting magnitude is the faintest star a telescope can show you under dark skies. The magnitude scale is logarithmic and runs backwards, so bigger numbers mean fainter stars. This calculator leans on the classic formula m = 2.7 + 5 × log₁₀(D), with D being the aperture in millimetres. Plug in a 150 mm telescope and you get roughly 2.7 + 5 × log₁₀(150) ≈ 13.6.

One step in magnitude is a brightness ratio of about 2.512×, and a full 5-magnitude span comes out to exactly 100× in brightness. Light-gathering follows the aperture area (D²), so doubling the aperture quadruples the light collected and drives the limit roughly 1.5 magnitudes fainter. Your naked eye under dark skies tops out around magnitude 6.

Applications

It tells you whether a target galaxy, comet or asteroid is bright enough for your scope, helps you plan deep-sky sessions, and lets you weigh one aperture against another when shopping for a telescope. Match it against a catalogue magnitude (from a star atlas or planetarium app, say) to predict what you’ll actually see.

FAQ

Why is a higher magnitude fainter? The scale is inverted. Bright Vega sits near 0, while faint stars carry large positive numbers. A higher limiting magnitude therefore means your scope digs out fainter objects.

Will light pollution change this? A lot, yes. The formula assumes dark, transparent skies. From a city, light pollution can knock the practical limit down by several magnitudes.

Does magnification help see fainter stars? With point-like stars it can. More magnification darkens the sky background and may pull out stars a touch fainter than the formula predicts. Extended objects, though, get little out of it.