Telescope Resolution Calculator: How Fine Can You See?

Resolution is the finest detail a telescope can separate, set almost entirely by aperture, not by magnification. Enter the diameter of your main lens or mirror and this works out the theoretical limit in arcseconds, the tightest double star you could split, and how much magnification you need to actually see it. The catch worth knowing up front: British skies rarely let you use the full figure, because the atmosphere usually blurs detail to around 1 to 2 arcseconds anyway.

Two standard figures describe the same limit slightly differently. The Dawes limit comes from what a sharp-eyed observer can just split on a pair of equally bright stars; the Rayleigh limit is the stricter, more formal optical figure. Both scale the same way: double the aperture and you halve the smallest detail you can see. Aperture is the one number that sets this, which is why observers say aperture wins.

Why you rarely get the full figure

The numbers above assume perfect optics and perfectly still air. In reality the atmosphere shimmers, and on a typical UK night that blurs fine detail to roughly 1 to 2 arcseconds no matter how large your telescope is. On the steadiest nights, and for close double stars caught in brief moments of calm, apertures above about 150mm can still show more than that. You also need enough magnification: the eye cannot perceive detail at the resolving limit until you are at roughly the aperture in millimetres as a magnification, and double-star observers often push to about twice that.