Tools

Telescope Resolution Calculator

What This Calculator Tells You

Resolution (or resolving power) is the smallest angular separation your telescope can distinguish between two objects. This determines whether you can split close double stars or see fine planetary detail. The limit depends primarily on your aperture.

Calculator

The diameter of your telescope's primary mirror or lens.

0.54"
Sparrow Limit

Absolute minimum

0.58"
Dawes' Limit

Practical limit

0.69"
Rayleigh Criterion

Conservative estimate

How to Interpret the Result

  • Resolution is in arcseconds (") - Smaller numbers mean finer detail. 1 arcsecond = 1/3600 of a degree.
  • Dawes' limit is the practical standard - Most observers use this for planning double star observations.
  • Atmosphere often limits more than optics - Typical seeing is 2-4", which may be larger than your telescope's resolution.

Understanding the Different Limits

Dawes' Limit (116/D)

Empirically determined by William Dawes for equal-brightness double stars. This is the most commonly used measure for visual observers and represents the separation where two stars appear as a "figure 8" shape.

Rayleigh Criterion (138/D)

The theoretical limit based on diffraction physics, where the center of one Airy disk falls on the first dark ring of another. More conservative than Dawes' limit.

Sparrow Limit (108/D)

The absolute minimum where two points can be distinguished as separate. At this limit, there's no dip between the two light sources - just an elongated blob.

Aperture vs Resolution Examples

ApertureDawes' LimitCan Resolve
60mm (2.4")1.93"Wide doubles, lunar craters >4km
100mm (4")1.16"Most doubles, Jupiter's GRS detail
150mm (6")0.77"Close doubles, planetary detail
200mm (8")0.58"Tight doubles, fine lunar features
300mm (12")0.39"Challenging doubles, subtle planetary

Rules of Thumb

  • Double aperture, halve the resolution limit: A 200mm scope resolves twice as fine as a 100mm scope.
  • Seeing usually limits before optics: On typical nights, 1-2" seeing means even a small scope is atmosphere-limited.
  • Use high magnification to reach resolution: You need about 25x per arcsecond of resolution to see the detail.
  • Color matters: Resolution is wavelength-dependent; it's slightly worse for red light than blue.

Common Mistakes & Misconceptions

  • "Resolution equals magnification" - Resolution is about detail, not size. More magnification doesn't create detail that isn't there.
  • "I can't split that double - my scope is broken" - Check the atmospheric seeing first. Turbulence may be the limit, not your optics.
  • "Bigger is always better" - Large scopes suffer more from atmospheric turbulence. A smaller scope may outperform on poor seeing nights.

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