Quote:
Originally Posted by stephend
......I read about coma in the context of someone saying it was a sign that the light wasn't hitting the mirror square. Which would be fixable. But then it was suggested it was native to all parabaloid mirrors.
Light from a star at the edge of the field of view isn't striking the mirror squarely, I suppose.
Funny that I've never heard of it before.
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A bit more reading, perhaps:-
http://www.astrosurf.com/luxorion/reports-coma.htm
http://www.atmsite.org/contrib/Carlin/collimation/ (search coma)
Coma: an aberration which results in a point object being turned into a pear-shape or comet shape at the focal plane, most commonly off-axis. It is caused by unequal magnification in different zones of a lens for obliquely incident rays from an off-axis object. An easier way of putting this may be to say that coma is caused when light enters a lens or mirror from the side, and rays from different parts of the lens intersect the axis of those rays at different distances. In a Newtonian with a paraboloidal primary mirror, coma is an inherent property.
Coma Correctors
In a reflecting telescope, coma makes stars at the edge of the field look like blurry comets whose tails point out radially from the center of the field. All parabolic mirrors used in Newtonian reflecting telescopes are limited in field sharpness due to coma. The faster your telescope's focal ratio, the worse the effect. Even a perfectly made 13" f/4.5 parabola has a diffraction limited field that’s only about 0.1° across. Coma correctors are auxiliary lens systems that reduce the coma at the edges of the field to provide images that are sharp across the entire eyepiece or photographic field. You no longer have to constantly shift your scope to keep objects centered for sharp viewing.