🔭Telescope Magnification Calculator
Calculate magnification, exit pupil, true field of view, and limiting magnitude for your telescope and eyepiece combination.
Telescope magnification calculator. Compute exit pupil, true field of view, limiting magnitude, and Dawes' limit. Free for all telescopes.
How to Use Telescope Magnification Calculator — Step by Step
- 1Enter your telescope's focal length and aperture in mm
- 2Enter your eyepiece focal length and apparent field of view
- 3Click 'Calculate Optics'
- 4View magnification, exit pupil, true field of view, and limiting magnitude
- 5See warnings if magnification is outside the useful range
Why Use ToolNest for Telescope Magnification Calculator?
- 🔭Magnification, exit pupil, true FOV, and limiting magnitude in one tool
- ⚠️Smart warnings if magnification is outside the useful range (D/6 to 2D)
- 👁️Dawes' limit resolution calculation — minimum separation of double stars
- 📊Multi-eyepiece comparison table for side-by-side analysis
- 🆓Works for any telescope: refractor, reflector, or catadioptric
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Frequently Asked Questions
What magnification is best for planetary viewing?
For planetary viewing, use 100x–200x under typical seeing conditions. Maximum useful magnification is 2× per mm of aperture (so 200x for a 100mm telescope). Higher magnification just makes the image bigger and blurrier.
What is exit pupil and why does it matter?
Exit pupil is the width of the beam of light leaving the eyepiece (in mm). At night, your dark-adapted pupil is 5–7mm. An exit pupil larger than 7mm wastes light; smaller than 0.5mm makes the image too dark to see well. 2–4mm is ideal.
What is the Dawes' Limit?
Dawes' Limit is the theoretical minimum angular separation between two stars that your telescope can resolve: θ = 116 / D (arcseconds), where D is aperture in mm. A 100mm telescope can theoretically separate stars 1.16 arcseconds apart.
What is apparent vs true field of view?
Apparent field of view (AFOV) is an intrinsic property of the eyepiece — the angle of the image circle you see (typically 50°–82°). True FOV is the actual slice of sky visible: TFOV = AFOV / magnification. A wider TFOV helps find and track objects.
What magnification do I need to see Saturn's rings?
Saturn's rings are visible at around 25x–30x. For detail in the rings (Cassini Division), use 100x–150x. For serious study, 200x+ in good seeing conditions.