Telescope Resolving Power Diameter mm

Telescope resolving power (Dawes limit)

Resolving power tells you the smallest angular gap between two stars that your telescope can still render as two separate points rather than a single blur. The calculator here leans on the empirical Dawes limit: R = 116 / D, where R is the resolution in arcseconds and D is the aperture (objective or mirror diameter) in millimetres. Open up the aperture and R drops, which means you pick out finer detail. Run a 150 mm telescope through the formula and you get roughly 116 / 150 ≈ 0.77″.

W. R. Dawes worked the limit out from his own visual study of double stars. You hit it when the Airy disk of one star lands right on the first diffraction ring of the other, leaving the pair only just split. There is a related theoretical figure too, the Rayleigh criterion, R = 138 / D, which errs a bit on the cautious side. Out under the sky, though, what you actually resolve is also held back by atmospheric seeing, which often hovers around 1″–2″ even on a good night.

Applications

The figure helps you check whether your scope can split a known double star, gauge the finest planetary or lunar detail it might reveal, and weigh one instrument against another before you spend money. Match the resolving power to a sensible magnification, somewhere around 25×–50× per inch of aperture, and you can actually see the detail the optics are capable of.

FAQ

Why does smaller R mean better resolution? R is an angle, the tightest separation you can still split. A smaller angle means the telescope can tell apart objects sitting closer together, so smaller wins.

Does focal length matter? No. Resolving power hangs on aperture alone (and wavelength). Focal length changes your magnification and field of view, but it leaves the diffraction-limited resolution untouched.

Will I always reach this limit? Rarely. Atmospheric turbulence (seeing) plus optical quality, collimation and thermal currents tend to keep you sitting above the theoretical value, and that gets worse once apertures climb past 200 mm.