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.
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- Barlow lens
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.
- Focal ratio
The focal length divided by the aperture, written f/5: a 200 mm telescope with a 1,000 mm focal length works at f/5. A “fast” instrument (f/4 to f/6) gives wide fields and short photo exposures; a “slow” one (f/10 to f/15) suits the planets. For the eye, at the same magnification, it does not change how bright the image looks.
- Field flattener
A lens placed in front of the camera that corrects a refractor’s field curvature, so stars stay sharp to the edge of the frame. It must sit at the right distance from the sensor (often 55 mm). At the eyepiece the eye compensates and it is not needed.
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