Coma Correctors and how they work?

[kirkster501]

Hello,

Not sure i understand these devices. What do they do in the context of Newtonian reflectors please?

Rgds, Steve

[andrew s]

Steve - Put simply coma is an off axis aberration which results from using a parabolic mirror to focus the light. Also the surface of best focus is curved. A well designed coma corrector adds the opposite amount of coma and field curvature to remove the coma and flatten the field. It principle this will only be perfect at one focal ratio but in practice works for a range of focal ratios because it can reduce the coma below the airy disk.

Regards Andrew

[Moonshane]

or put another way, makes the coma of a f4 scope behave a bit like an f8 scope. http://www.oldham-optical.co.uk/Photo.htm

[Willard]

Does this solve the problem or create a bigger one ?