What Can I See? Aperture Calculator
The first question every beginner asks is the right one: what will this telescope actually show me? Aperture — the diameter of the main lens or mirror — is the number that answers it. Bigger aperture gathers more light and resolves finer detail, which matters far more than the headline magnification on the box. Enter your aperture below and this calculator works out, from the physics, what you can expect to see.
Light pollution only affects the limiting magnitude (how faint you can go). The optical limits below are fixed by aperture.
How this is calculated
- Limiting magnitude (faintest star visible) ≈
7.7 + 5 × log₁₀(aperture in cm), the standard visual estimate for a dark site. We then subtract for light pollution by Bortle class. Fainter objects have higher magnitude numbers, so a bigger aperture (and darker sky) reaches a higher number. - Maximum useful magnification ≈
2 × aperture in mm— the same thing as the old "50× per inch of aperture" rule of thumb. Push past it and the image just gets bigger and blurrier ("empty magnification"), which is exactly the trap behind department-store "600×" boxes. - Resolving power (Dawes limit) =
116 ÷ aperture in mmarc-seconds — W. R. Dawes' 1867 empirical limit for splitting a close double star. Smaller is sharper; it's why aperture, not magnification, sets how much fine detail you can see. - Light-gathering vs the naked eye =
(aperture in mm ÷ 7)², taking a dark-adapted human pupil as about 7 mm. It's the ratio of the collecting areas.
These figures are physics-derived from aperture alone — not hands-on test results. Seeing conditions, optical quality, collimation and your own eyes all affect the real view on any given night.
Found an aperture that shows what you want to see? See how the real scopes compare on aperture, type and mount in our best telescopes for beginners guide. Once you've a scope in mind, the magnification & true-FOV matcher pairs it with the right eyepiece.
Affiliate note: our guides earn from qualifying Amazon purchases at no cost to you. Full disclosure. Specs are verified against manufacturer details and current listings — no hands-on testing claims.