How a telescope magnifies
A telescope’s main lens or mirror forms an image of the sky at its focal point. The eyepiece then works like a magnifying glass on that image. The magnification depends only on the two focal lengths:
magnification = telescope focal length ÷ eyepiece focal length
A Barlow lens placed before the eyepiece multiplies the telescope’s effective focal length, usually by 2 or 3.
What else the calculator shows
- True field of view = apparent field of the eyepiece ÷ magnification: how much sky you see.
- Exit pupil = aperture ÷ magnification: the width of the light beam reaching your eye.
- Focal ratio = focal length ÷ aperture.
- Maximum useful magnification, about 2× per millimeter of aperture.
- Lowest useful magnification, when the exit pupil reaches 7 mm, about the widest a young dark-adapted eye opens.
- Dawes limit, 116 ÷ aperture in mm, the closest double star the telescope can split, in arcseconds.
- Limiting magnitude, roughly 2 + 5 × log₁₀(aperture in mm) under dark skies.
Worked example
A popular 130 mm f/5 reflector (650 mm focal length) with a 10 mm Plössl eyepiece (52° apparent field):
| Result | Value |
|---|---|
| Magnification | 65× |
| True field of view | 0.80°, enough to frame the full Moon |
| Exit pupil | 2.0 mm |
| Maximum useful magnification | 260× |
| Lowest useful magnification | about 19× |
| Dawes limit | 0.89 arcsec |
| Limiting magnitude | about 12.6 |
| Light gathered vs the eye | about 345× |
Add a 2× Barlow and you get 130× with a 0.40° field and a 1 mm exit pupil, good for planets. Swap in a 25 mm eyepiece instead and you get 26× with a 2° field and a 5 mm exit pupil, ideal for star clusters and nebulae.
Choosing eyepieces
A useful set covers three ranges:
| Use | Exit pupil | Example on a 130 mm f/5 |
|---|---|---|
| Wide views, faint galaxies and nebulae | 4 to 7 mm | 25 to 32 mm eyepiece |
| General viewing, clusters | 2 to 4 mm | 10 to 20 mm |
| Moon, planets, double stars | 0.5 to 1.5 mm | 3 to 7 mm, or a Barlow |
Tips
- On most nights, the atmosphere limits sharp views to about 200× to 250×, whatever the telescope’s size.
- Start at low power to find your target, then increase the magnification.
- Wide-field eyepieces (68° or 82°) show more sky at the same magnification, which makes tracking easier on a manual mount.
- Advertised magnifications of 500× or more on small telescopes are a marketing figure, not a useful one.