About this tool
The Telescope Resolving Power Calculator estimates the finest detail a telescope can separate, based on nothing more than its aperture. It reports both the classic Dawes limit and the slightly more conservative Rayleigh criterion, the faintest star the scope should reach, and how much more light it collects than the naked eye.
Resolution is inversely proportional to aperture. Dawes limit = 116 ÷ aperture(mm) arcseconds and Rayleigh limit = 138 ÷ aperture(mm) arcseconds, so a 100 mm scope resolves about 1.16″ by Dawes. Limiting magnitude ≈ 2 + 5 × log₁₀(aperture in mm), and light-gathering power = (aperture ÷ 7)², comparing the aperture to a 7 mm dark-adapted human pupil.
Enter the aperture in millimetres or inches (inches are converted to millimetres first). These are theoretical, diffraction-limited figures — real performance also depends on atmospheric seeing, optical quality, collimation and light pollution. All numbers are computed in your browser with no data leaving your device.
Frequently asked questions
What is the difference between the Dawes and Rayleigh limits?
Both give a telescope's smallest resolvable angle. Dawes (116 ÷ mm) is an empirical limit for splitting equal double stars; Rayleigh (138 ÷ mm) is the stricter diffraction criterion, so it gives a slightly larger, more conservative angle.
Why does a bigger aperture resolve finer detail?
Diffraction spreads light into a disk whose size shrinks as aperture grows. Because resolution is inversely proportional to aperture, doubling the aperture halves the resolvable angle and reveals twice the fine detail.
What is limiting magnitude?
The faintest star a telescope can show, estimated here as 2 + 5 × log₁₀(aperture in mm). Larger apertures gather more light and reach fainter stars, though sky brightness and your eyesight also matter.
Will I actually reach these numbers?
Rarely at the eyepiece. These are ideal, diffraction-limited values. Atmospheric turbulence (seeing), optical quality, collimation, cool-down and light pollution usually make real resolution and limiting magnitude a little worse.
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