[ausastronomer (John Bambury)]

John,

I will also try to re phrase what Geoff is correctly pointing out.

In newtonians Coma is an aberration caused directly by the parabolic mirror. It has nothing to do with the eyepiece and the eyepiece does not correct for it. Geoff correctly explains by saying it is caused by the angle of incidence becoming steeper as you go further from the central axis of the mirror, this causes a change in the lateral magnification at different points on the mirror. A good way to experience this is to focus the suns rays on concrete with a magnifying glass. When the magnifying glass is at 90% to the suns rays the light focuses to a point, angle the magnifying glass and the light focuses to a point with a fan-like tail, this is coma and what a paraccor is designed to correct.

A cheap eyepiece in a fast scope will suffer from numerous aberrations because the eyepiece is not properly corrected to work with such a steep light cone. These aberrations will mainly be astigmatism, field curvature and barrel and pincushion distortion. These aberrations will generally mask any actual coma that you would normally notice eminating from the fast parabolic mirror itself, as these eyepiece aberrations also get worse as you go off axis. A paraccor will not correct for these aberrations but it "may" clean them up a little, partially because the paraccor effectively increases the focal ratio of the scope by 15%. Another way to "clean up" the images when using cheap eyepieces in a fast scope is to use a longer focal length eyepiece with a decent barlow. ie a 30mm eyepiece with a 2X barlow will give a nicer image than a 15mm native eyepiece. This occurs because a normal barlow (2X) works by effectively doubling the focal length of the scope hence the focal ratio of the scope is also doubled. ie an f5 scope becomes an F10 scope when a 2X barlow is used.

A paraccor is not the answer with cheap eyepieces IMO, its expensive in its own right. If your going to use a paraccor you would generally do so with scopes faster than F4.7 or so and using good eyepieces so the paraccor is only correcting what its designed to correct and thats coma. Thats a personal thing however, as some use a paraccor even in an F5 or slower scopes, its up to the individual. I have no issues with coma in F5 scopes. I actually used an F3.8 scope (GOTH was his name) at SPSP without a paraccor and didn't find the coma intollerable.

I hope this explains it a little better for you.

CS-John B