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.
ToolsNewtonian calculator, Collimation
Related terms
- 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.
- 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.
- Collimation
The adjustment that lines up a telescope’s mirrors on a single axis. A Newtonian drifts a little each time it is moved and should be checked before every session, especially if it is fast (f/5 or below). A poorly collimated telescope gives soft images, even with excellent optics.
← The whole glossary