A diverging lens placed before the eyepiece that multiplies the telescope’s focal length, usually by 2 or 3. A 10 mm eyepiece with a 2× Barlow on a 1,000 mm telescope gives 200× instead of 100×. It doubles your range of eyepieces while keeping their eye relief.
ToolsEyepieces, Useful formulas
Related terms
- Magnification
How many times the instrument enlarges the object: telescope focal length divided by eyepiece focal length. A 1,000 mm telescope with a 10 mm eyepiece magnifies 100 times (100×). More is not always better: past a limit the image becomes dim and blurred.
- Focal length
The distance between the main lens (or mirror) and the point where it brings a star to focus, in millimetres. A long focal length gives a larger image and a narrower field. A 1,200 mm telescope magnifies 60 times with a 20 mm eyepiece.
- Focal reducer
The opposite of a Barlow: a lens that shortens the focal length, for example ×0.63 on a Schmidt-Cassegrain, taking it from f/10 to f/6.3. The field widens and photo exposures get shorter (2.5 times here). Many reducers also flatten the field.
← The whole glossary