Telescope Magnification Calculator
Telescope magnification determines how much larger celestial objects appear compared to the naked eye. It is calculated by dividing the focal length of the objective (primary lens or mirror) by the focal length of the eyepiece. This calculator also computes exit pupil, estimated field of view, and the approximate limiting stellar magnitude.
Magnification (M) equals the ratio of the objective focal length to the eyepiece focal length: M = fobjective / feyepiece. A 1200 mm objective paired with a 25 mm eyepiece yields 48x magnification. Higher magnification narrows the field of view and dims the image.
The exit pupil is the diameter of the light beam leaving the eyepiece, calculated as D / M (aperture divided by magnification). For comfortable viewing, the exit pupil should not exceed 7 mm (the dark-adapted human pupil). An exit pupil of 2-5 mm is ideal for most astronomical observation.
The limiting magnitude estimates the faintest star visible through the telescope under good conditions. It depends primarily on aperture, following the formula mlimit ≈ 2 + 5 log10(D), where D is in millimeters. A 200 mm telescope can theoretically reach about magnitude 13.5, revealing galaxies and faint nebulae invisible to the naked eye.