The telescope magnification calculator computes magnification, true field of view, and exit pupil for any combination of telescope and eyepiece. Enter your scope's focal length and aperture, plus the eyepiece focal length, to get complete optical performance data.
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How to Use the Telescope Magnification Calculator
This telescope magnification calculator uses three inputs — telescope focal length, aperture, and eyepiece focal length — to compute the four key optical performance metrics for any telescope/eyepiece combination.
Understanding the Four Metrics
Magnification is the ratio of telescope focal length to eyepiece focal length. A 1000mm telescope with a 25mm eyepiece gives 40× — objects appear 40 times larger. Increasing magnification (shorter eyepiece) reduces brightness and field of view.
True Field of View is how large a piece of sky you see through the eyepiece. At 50× with a 50° apparent FOV eyepiece, you see 1° of sky — wide enough for open star clusters and comets. At 200× the field narrows to about 0.25°, focusing on planets or double stars.
Exit Pupil determines how bright the image appears. A 7mm exit pupil is optimal for dark-sky viewing because it matches a dark-adapted eye. For moon or planetary viewing, 1–2mm is fine. A very small exit pupil (below 0.5mm) will reveal dust on your eyepiece and dim the image significantly.
Maximum Useful Magnification is approximately 2× the aperture in millimeters. Beyond this limit, you're magnifying the blur caused by diffraction limits and atmospheric turbulence — more magnification makes things larger but not clearer.
Choosing Eyepieces for Different Targets
For deep-sky objects (nebulae, galaxies, open clusters): use low magnification (30–80×) with wide FOV. For the Moon and planets: use higher magnification (100–200×) when atmospheric seeing permits. For double stars: push to maximum useful magnification. A typical complete eyepiece set includes 5mm, 10mm, 20mm, and 32mm focal lengths covering the full range for most telescopes.
FAQ
How do I calculate telescope magnification?
Magnification = Telescope Focal Length ÷ Eyepiece Focal Length. For example, a 1000mm telescope with a 10mm eyepiece gives 100× magnification. This is the fundamental telescope magnification formula used by all astronomers.
What is exit pupil and why does it matter?
Exit pupil = Telescope Aperture ÷ Magnification. It represents the diameter of the light beam entering your eye. An exit pupil of 7mm is ideal for dark sky viewing (matching a fully dilated human eye). For high magnification planetary work, 1–2mm is acceptable. Below 0.5mm the image dims significantly.
What is the maximum useful magnification for a telescope?
The practical rule is 2× the aperture in millimeters. A 100mm telescope has a maximum useful magnification of about 200×. Beyond this, the image becomes blurry from diffraction and atmospheric turbulence. On nights of exceptional seeing, some telescopes can push slightly beyond this limit.
What is true field of view?
True Field of View = Apparent Field of View ÷ Magnification. Eyepieces typically have apparent FOVs of 50° (Plössl) to 100° (wide-angle). A 20mm Plössl (50°) at 50× magnification gives a 1° true field — enough to fit about two full moons side by side.
Is this tool free?
Yes, completely free with no signup or download required.