Calculate the magnification of your telescope and eyepiece combination, and determine the exit pupil size for optimal viewing.
Last updated: March 26, 2026 | By ForgeCalc Engineering
Telescope magnification is the factor by which a telescope increases the apparent size of celestial objects when viewed through an eyepiece. It is determined by dividing the telescope's focal length by the eyepiece's focal length. For example, a 1000mm telescope with a 25mm eyepiece produces 40x magnification.
While higher magnification might seem better, it comes with trade-offs. Higher magnification spreads the available light over a larger area, making the image dimmer. It also magnifies atmospheric turbulence and requires more precise tracking. The maximum useful magnification is typically limited to about 2x per millimeter of aperture (or 50x per inch of aperture).
The exit pupil is the diameter of the beam of light that exits the eyepiece. For optimal viewing, the exit pupil should match your eye's pupil size. If the exit pupil is larger than your dilated pupil (typically 5-7mm for young observers), light is wasted. If it's too small (below 0.5mm), the image appears dim and difficult to focus.
Step-by-step guide:
You have an 8-inch (200mm) Schmidt-Cassegrain telescope with a focal length of 2000mm, and you want to observe Jupiter using a 10mm eyepiece.
This configuration provides excellent magnification for planetary viewing. The 1mm exit pupil is ideal for bright objects like Jupiter, and the magnification is well within the telescope's useful range. The image will show considerable detail in Jupiter's cloud bands and the Galilean moons.
No. High magnification spreads the light over a larger area, making the image dimmer and more susceptible to atmospheric turbulence. Beyond the maximum useful magnification (2x per mm of aperture), you're just magnifying blur.
The exit pupil is the diameter of the beam of light exiting the eyepiece. If it's larger than your eye's pupil (5-7mm), light is wasted. For planetary viewing, smaller exit pupils (0.5-2mm) work well, while deep-sky viewing benefits from larger ones (4-7mm).
Multiply your telescope's aperture in millimeters by 2 (or inches by 50). For example, a 100mm telescope has a maximum useful magnification of about 200x. Beyond this, atmospheric conditions and optical quality limit performance.
Planets are small and bright, so they typically benefit from higher magnification (150x-300x), provided the atmosphere is stable. Start with medium power and increase until the image quality begins to degrade.
Deep-sky objects like nebulae and galaxies are faint and extended, so they benefit from lower magnification (30x-100x) to maximize brightness and field of view. Higher power can reveal detail in planetary nebulae and galaxy cores.
Many cheap telescopes advertise unrealistic 'maximum magnification' as a marketing gimmick. The actual useful magnification is limited by aperture and atmospheric conditions, not by using extremely short eyepieces.
Yes! A Barlow lens is placed between the eyepiece and telescope to multiply magnification (typically 2x or 3x). A 2x Barlow with a 10mm eyepiece acts like a 5mm eyepiece, doubling magnification.
Atmospheric turbulence ('seeing') limits useful magnification. On nights with poor seeing, even 100x may appear blurry. On exceptional nights with steady air, you might push to 300x or more on bright targets.
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