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.
Related terms
- Field curvature
A telescope’s sharp image is not flat but slightly curved: when the centre is in focus the edges are not, and the other way round. The eye copes, but a flat sensor shows it. A field flattener corrects it.
- 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.
- Coma corrector
A lens group placed in a Newtonian’s focuser to remove coma at the edge of the field. It is almost essential for imaging at f/5 or faster, and welcome visually with wide-angle eyepieces. It too needs a precise spacing to the sensor.
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