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.
ToolsNewtonian calculator
Related terms
- Coma
A flaw of parabolic mirrors: away from the centre of the field, stars take the shape of a little comet, tail pointing outwards. It grows quickly as the focal ratio drops: faint at f/8, obvious at f/4. Poor collimation shifts it and makes it visible in the centre.
- Newtonian telescope
The telescope invented by Isaac Newton in 1668: a concave primary mirror at the bottom of the tube and a small flat mirror at 45° that sends the image out to the side, near the top. With no lenses it is free of chromatic aberration and offers a lot of aperture for its simplicity. It needs regular collimation.
- 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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