LearnInstruments & observingSome maths
Aperture and resolving power
The finest detail a telescope can separate, set by diffraction and therefore by the diameter of its objective alone.
Why it matters
Magnification is free and meaningless; resolution is bought with aperture and nothing else. This is the single most misunderstood fact in amateur astronomy.
The maths, in layers
1 · Intuition
Bigger openings diffract less, so they resolve finer detail.
2 · The equation
θ [arcsec] ≈ 116 / D [mm]
- D
- aperture diameter in millimetres
3 · Where it comes from
This is the Dawes limit, an empirical fit for splitting equal double stars. The Rayleigh criterion, 138/D, is the stricter theoretical version.
What people usually get wrong
A telescope's power is its magnification.
Magnification is just eyepiece choice. Past about 2× the aperture in millimetres you are enlarging a blur, which is why '600×' on a box is a warning rather than a feature.
In practice
Every acquisition panel on this site prints the Dawes limit beside the image scale, because sampling finer than the optics can resolve buys nothing but noise.
Rests on
Nothing. This is a starting point — you can begin here.