Magnification is not the main power of a telescope - aperture is. Using too much magnification is the most common reason for disappointing views.
Working out magnification
Magnification = telescope focal length / eyepiece focal length.
For a telescope with a 750 mm focal length:
| Eyepiece | Magnification |
|---|---|
| 25 mm | 30x |
| 23 mm | about 33x |
| 10 mm | 75x |
| 6 mm | 125x |
A 2x Barlow lens doubles the magnification of any eyepiece: a 10 mm eyepiece with a 2x Barlow gives 150x on the same telescope.
How much is useful?
- The practical upper limit is roughly twice the aperture in millimetres (about 300x for a 150 mm telescope), and only on nights of very steady air.
- On most nights, especially over large cities, the air limits sharp views to around 150-200x. Beyond that the image becomes larger but softer and dimmer.
- Claims of "600x" on a small telescope describe an image that is far too dim and blurred to use.
Which eyepiece when
- Low (25 mm): finding objects, the whole Moon, star clusters, nebulae, galaxies.
- Medium (10 mm): planets on ordinary nights, lunar craters.
- High (6 mm or 10 mm with a Barlow): planets and lunar detail on steady nights.
Focusing
Start at low magnification and turn the focuser slowly through its whole range until stars are points. When you change eyepieces, refocus. On a hazy night the image may never look perfectly crisp - that is the air, not the telescope.