Coma corrector doubts. How does it work ?

Hi,

I have some doubts about the coma corrector as I haven't used one or even seen one in Brazil.
Using the coma corrector is the same thing as using a scope with slower f ratio? How can I compare this? Is a f4.7 with such device compared to a f8 or f12 in terms of ep performance regarding coma?
Does cheap wide angle eps such the BO SWA 32mm performs like in f12 scope if I use a coma corrector on a f4.7 scope?
I am confused about it.

Thanks,
Andre

Andre,

A coma corrector reduces coma. Unfortunately coma is just one of the aberrations that leads to poor performance of eyepieces in fast telescopes. The Tele Vue Paracorr will also reduce field curvature (when the edge is out of focus even though the center is in focus), but other aberrations are not corrected. Unfortunately, the problem with cheap eyepieces in fast scopes is not coma, rather astigmatism, and a coma corrector will not fix that. For example, in your BO SWA 32mm in an f/4.7 Newtonian there will be coma, field curvature, and astigmatism at the edge of field. Using a Paracorr will reduce coma and field curvature, but it won't help with astigmatism. Using an f/8 or f/12 scope instead will reduce all three aberrations. Where coma correctors make the most sense are with expensive eyepieces that have very little astigmatism even in fast scopes. Then, the Paracorr makes a big difference because it is correcting all the dominant aberrations at the edge of field.
--
Mike