Telescope Magnification Calculator
Calculate telescope magnification using magnification = focal length of scope ÷ focal length of eyepiece. Also computes true field of view (FOV = apparent FOV ÷ magnification), exit pupil (aperture ÷ magnification), Dawes resolution limit (116 / aperture), and light gathering power relative to the naked eye.
Telescope Magnification Calculator
Calculate magnification, true field of view, exit pupil, and more for any telescope and eyepiece combination.
Frequently Asked Questions
How do I calculate telescope magnification?
Telescope magnification = focal length of telescope (mm) ÷ focal length of eyepiece (mm). For example, a 1200mm telescope with a 25mm eyepiece gives 1200 ÷ 25 = 48× magnification. You can change the magnification by swapping eyepieces.
What is a good magnification for viewing planets?
For planets, 150×–300× is typically useful depending on atmospheric conditions and telescope aperture. Mars, Jupiter, and Saturn show significant detail at 150–200×. The theoretical maximum useful magnification is approximately 2× the aperture in mm, but atmospheric turbulence usually limits practical views to 200–250× for most nights.
What is the exit pupil and why does it matter?
The exit pupil is the diameter of the light beam leaving the eyepiece (aperture ÷ magnification). Ideally, it should match your dark-adapted pupil size (5–7mm). An exit pupil larger than 7mm wastes light; smaller than 0.5mm produces very dim images. An exit pupil of 4–7mm is ideal for wide-field viewing.
What is true field of view?
True FOV = eyepiece apparent FOV ÷ magnification. It represents the actual area of sky visible through the telescope in degrees. A larger true FOV makes it easier to locate and observe extended objects like the Orion Nebula. Low magnification gives wide fields; high magnification narrows the view.
What is the Dawes limit?
The Dawes limit is the theoretical resolution limit of a telescope: 116 / aperture (mm) in arcseconds. A 200mm telescope can theoretically resolve stars separated by 0.58 arcseconds. This is a theoretical limit — atmospheric seeing typically limits practical resolution to 1–2 arcseconds.
What magnification is too much for a telescope?
The maximum useful magnification is approximately 2× the aperture in mm (or 50× per inch of aperture). Beyond this limit, images appear blurry and dim due to diffraction. A 200mm telescope's maximum useful magnification is ~400×. In practice, atmospheric conditions rarely allow more than 200–300× on typical nights.
How does aperture affect light gathering?
Light gathering power = (aperture / 7mm)². A 70mm telescope gathers 100× more light than the naked eye (7mm pupil). A 200mm telescope gathers (200/7)² ≈ 816× more light. More light means you can see fainter objects and finer detail.
What eyepiece focal length should I buy?
Most astronomers recommend starting with three eyepieces: a low-power eyepiece (25–40mm) for finding objects and wide fields, a medium eyepiece (10–15mm) for general viewing, and a high-power eyepiece (4–8mm) for planets and double stars. A 2× Barlow lens doubles your collection by effectively halving each eyepiece focal length.
About the Telescope Magnification Calculator
About This Calculator
This telescope calculator computes the key optical parameters for any telescope and eyepiece combination. Understanding magnification, field of view, exit pupil, and resolution limits helps you choose the right eyepiece for observing different celestial objects.
Magnification Formula
Magnification = Focal Length of Telescope (mm) / Focal Length of Eyepiece (mm)
Example: 1200mm telescope with 25mm eyepiece → 1200 / 25 = 48×
Maximum useful magnification is approximately 2× the aperture in mm (or 50× per inch of aperture). Beyond this limit, images appear blurry due to diffraction. Atmospheric turbulence (seeing) usually limits practical magnification to 200–300× even with large apertures.
True Field of View
True FOV (°) = Apparent FOV of Eyepiece (°) / Magnification
The apparent FOV is a property of the eyepiece design (e.g., Plössl ≈ 50°, wide-angle ≈ 68–82°, ultra-wide ≈ 100°+). A wider true FOV makes it easier to locate and track objects.
| Object Type | Recommended True FOV | Typical Magnification |
|---|---|---|
| Large nebulae | 1°+ | 20–50× |
| Open clusters | 0.5–2° | 30–80× |
| Globular clusters | 0.1–0.5° | 100–200× |
| Planets | 0.05–0.2° | 100–300× |
| Double stars | 0.01–0.1° | 200–400× |
Exit Pupil
Exit Pupil (mm) = Aperture (mm) / Magnification
The exit pupil is the diameter of the light beam exiting the eyepiece. For comfortable viewing, it should match or be smaller than your eye's dark-adapted pupil diameter (5–7mm for adults).
- Exit pupil > 7mm — Light is wasted; your pupil can't admit all the light
- Exit pupil 4–7mm — Ideal for wide-field, low-power views
- Exit pupil 2–4mm — Good general-purpose range
- Exit pupil 0.5–2mm — High-power planetary/lunar viewing
- Exit pupil < 0.5mm — Very dim; use larger aperture telescope
Dawes Limit and Resolution
Dawes Limit (arcseconds) = 116 / Aperture (mm)
The Dawes limit is the theoretical resolution limit of a telescope — the smallest angular separation between two stars that can be distinguished as separate objects. A 200mm telescope can resolve double stars separated by 116/200 = 0.58 arcseconds. Atmospheric seeing typically limits practical resolution to 1–2 arcseconds for ground-based telescopes.
How to Choose the Right Eyepiece
- Start low — begin at the lowest magnification to locate the object in the wider field
- Match exit pupil to conditions — darker skies support larger exit pupils; light-polluted skies benefit from smaller exit pupils
- Consider the object — use low power for extended objects (nebulae, galaxies) and high power for planets and double stars
- Respect the atmosphere — rarely does magnification above 300× improve views due to atmospheric turbulence
- Test Barlow lenses — a 2× Barlow doubles your magnification with any eyepiece