Quote:
Originally Posted by astro744
You're going to get off axis coma in both telescopes but it will be more obvious at f5. Coma is a linear aberration and a function of a paraboloidal mirror and can be eliminated/reduced with a coma corrector. It will be more noticeable with the lower power eyepieces as they have a larger field stop.
However, off axis coma may not bother you and to actually see pure coma and nothing else you need a highly corrected eyepiece such as a Nagler. Other less well corrected eyepieces will likely exhibit coma, astigmatism, field curvature all mixed in together.
Now before others step in and advise against recommending such expensive eyepieces I would like to say that I am merely letting you know what a Nagler will do for you, (and not do i.e. eliminate coma).
You are likely not going to be bothered by the off axis coma at f5 and if you can afford the larger aperture then go for it. You can always add a coma corrector later if it really bothers you.
I actually like seeing pure coma through a Nagler at the edge of field and there are no other aberrations and no to very minimal field curvature depending on the telescope used. What I don't like is significant field curvature in the outer 30% of field and I hear it said so many times about low cost eyepieces that "this eyepiece is great but the outer 30% is out of focus". You may as well buy a Plossl with a smaller apparent field than have a wide angle with 30% out of focus. If stars are blurry due to field curvature you have no chance of spotting a galaxy off axis.
Collimation will be a little more sensitive at f5 but once you learn the right technique it is easy to do.
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Thanks very much for this advice. So you think there won't be that much difference between the 8" and 10" when it comes to using the eyepieces that come with it, the GSO plossls? I'm actually looking forward to the challenge of collimation, having read a lot of stuff. As a side question, I've been thinking a cheshire collimator is all I need? Some people say the laser collimator often causes more trouble? I guess the extra 2" aperture is valuable in the long term - I just wish it wasn't an extra $200. Thanks again.