Aperture, the diameter of the main lens or mirror, sets how much light a telescope gathers and how fine a detail it can resolve. Magnification is just the telescope's focal length divided by the eyepiece's. The maximum useful magnification is about 50 times per inch of aperture (2 times per millimeter), and steady air rarely allows more than about 250 to 300 times. Ignore boxes that advertise huge magnification.
The number on the box that matters is aperture, not magnification.
Aperture
| Aperture | Light gathered vs the eye (about 7 mm pupil) | Typical views |
|---|---|---|
| 70 mm (2.8 in) | About 100× | Moon, planets, bright clusters |
| 130 mm (5.1 in) | About 340× | More detail on planets, brighter galaxies |
| 200 mm (8 in) | About 800× | Many galaxies and nebulae, fine planetary detail |
| 300 mm (12 in) | About 1,800× | Faint deep-sky objects |
Magnification
Magnification = telescope focal length ÷ eyepiece focal length.
| Telescope focal length | 25 mm eyepiece | 10 mm eyepiece | 6 mm eyepiece |
|---|---|---|---|
| 650 mm | 26× | 65× | 108× |
| 1,200 mm | 48× | 120× | 200× |
Limits
- Maximum useful power ≈ 50× per inch (2× per mm) of aperture.
- Seeing. Atmospheric turbulence usually caps useful power at about 250 to 300×, often less.
- Resolution. Finer with more aperture; roughly 116 ÷ aperture in mm arcseconds (Dawes limit).
Focal ratio
Focal ratio = focal length ÷ aperture. f/4 to f/6 scopes give wide fields for deep sky; f/10 to f/15 scopes suit planets and double stars.
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Common questions
How do I calculate telescope magnification?
Divide the telescope's focal length by the eyepiece's focal length. A 1,200 mm telescope with a 10 mm eyepiece gives 120 times.
What is the maximum useful magnification?
About 50 times per inch of aperture, or 2 times per millimeter, and often less because of atmospheric seeing.
What does focal ratio mean?
Focal length divided by aperture. Low focal ratios give wider fields; high ratios suit higher power views.
Why does my telescope image look blurry at high power?
Usually atmospheric turbulence, an uncooled telescope or poor collimation, not the eyepiece.