Telescope Comparison Calculator

This calculator lines up two telescopes side by side on the three specs that actually determine what you'll see through each one. F/ratio (focal length divided by aperture) describes how the light cone converges — a low f/ratio scope (f/5 or faster) gathers a wide field of view and suits short exposures for imaging faint, extended objects, while a high f/ratio (f/10 or slower) delivers higher magnification per eyepiece and tends to be more forgiving of eyepiece quality, favoring planetary and lunar detail. Light-gathering power scales with aperture area, not aperture diameter, so it's computed as the ratio of the two apertures squared — an 8-inch scope doesn't gather twice the light of a 4-inch scope, it gathers four times as much, since a circle's area grows with the square of its diameter, not in direct proportion to it.

Resolution uses Dawes' limit (116 divided by aperture in millimeters, in arcseconds) — an empirical resolving-power criterion the English astronomer William Rutter Dawes derived in 1867 from his own observations of close double stars, distinct from the theoretical Rayleigh criterion but very close to it in practice — which estimates the smallest angular separation a telescope can theoretically resolve between two close point sources like a tight double star, where a smaller arcsecond value means finer resolving power. Notice that focal length has no bearing on either light-gathering power or resolution — both are purely functions of aperture — so a telescope with a longer focal length isn't inherently more powerful for either metric; focal length only interacts with eyepiece choice to determine magnification and field of view. Dawes' limit is also a theoretical ceiling: atmospheric seeing, optical quality, and collimation all typically prevent real-world telescopes from reaching it on most nights.