Telescope calculator
Magnification, field of view and what you'll actually see
Pick a telescope and an eyepiece. Every other calculator makes you look up your focal length first — we already have it for 91 telescopes, including live prices if you're still choosing.
Telescope
Eyepiece
Apparent field is printed on the eyepiece. Plössl ≈ 52°, wide-angle ≈ 68°, ultra-wide ≈ 82°.
This combination
| Magnification | — |
|---|---|
| True field of view | — |
| Exit pupil | — |
| Focal ratio | — |
What this telescope can do
| Useful magnification | — |
|---|---|
| Resolving power | — |
| Limiting magnitude | — |
| Light gathering | — |
Aperture alone sets these — no eyepiece can add resolution or light grasp that the objective didn't collect.
What you'll see
Rough guidance for a decent night away from city lights. Seeing conditions, light pollution and experience move these more than a few millimetres of aperture do.
The formulas
Magnification = telescope focal length ÷ eyepiece focal length. A 2032mm scope with a 25mm eyepiece gives 81×.
True field of view = eyepiece apparent field ÷ magnification. It is how much sky you actually see — a wider eyepiece at the same magnification shows more of it.
Exit pupil = aperture ÷ magnification. Bigger than about 7mm and the beam is wider than your dark-adapted pupil, so you throw light away; below about 0.5mm the view goes dim and every flaw in the optics shows.
Maximum useful magnification is roughly 2× the aperture in millimetres. Past that you enlarge the blur, not the detail — which is why a "675× !" claim on a 60mm department-store telescope is meaningless.
Dawes limit = 116 ÷ aperture in mm, in arcseconds — the closest double star the optics can split. Limiting magnitude ≈ 7.5 + 5·log₁₀(aperture in cm), the faintest star visible under a dark sky.