The telescope comparison tool computes key optical performance metrics for up to three telescopes side by side. Enter aperture and focal length to compare light-gathering power, limiting magnitude, resolution, and maximum magnification.
Telescope 1
Telescope 2
Telescope 3 (optional)
Comparison Results (green = best)
| Metric | Scope 1 | Scope 2 | Scope 3 |
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How to Compare Telescopes
This free telescope comparison tool calculates key optical metrics from aperture and focal length, letting you see at a glance which telescope wins on each performance dimension. The green-highlighted cells show the best value for each metric.
What to Compare When Buying a Telescope
Aperture wins for deep-sky: When comparing telescopes for observing nebulae, galaxies, and star clusters, aperture is the single most important factor. A 10-inch Dobsonian will show far more detail in the Orion Nebula than a high-quality 4-inch refractor. The comparison tool's light-gathering ratio makes this difference concrete — the 250mm scope gathers 4× more light than a 125mm scope.
Focal ratio matters for photography: Fast focal ratios (f/4–f/6) capture dim deep-sky objects faster. Slower ratios (f/8–f/12) give higher magnification for the same eyepiece, suiting planetary work.
Limiting magnitude tells you the faintest star visible — a magnitude 14 limit means objects like the Great Globular Cluster in Hercules (M13) will show individual stars, while a magnitude 10 limit will show the cluster as a fuzzy blob.
FAQ
What is light-gathering power and why does it matter?
Light-gathering power is how much more light a telescope collects compared to the naked eye (7mm pupil). It scales with the square of the aperture ratio. A 100mm telescope gathers (100/7)² ≈ 204 times more light than the naked eye — making faint objects visible and increasing image clarity.
What is Dawes limit?
Dawes limit = 116 / aperture(mm) arc-seconds. It describes the minimum angular separation between two stars that a telescope can resolve as two separate points. A 100mm telescope can split stars as close as 1.16 arc-seconds apart — about the separation of a moderately tight double star.
Does more aperture always mean better?
Aperture is the most important factor for faint deep-sky objects — more aperture means seeing fainter galaxies and nebulae. However, for planetary viewing, optical quality (collimation, mirror figure) matters more than aperture once you exceed about 150mm. Large aperture scopes are also heavier and harder to transport.
Is this tool free?
Yes, completely free with no account or download required.