[mental4astro (Alexander)]

My 8" dob is one of these GSO f/4 Newtonians. Being "optimised" for imaging, it just means that the primary mirror is set closer to the secondary mirror, putting the focus further out the side of the scope. Technically, the secondary mirror should be a little larger to deal with the larger cone of light section that the secondary mirror intersects (though I really wonder if this is the case with these scopes).

Conversely, with the focus set further out, for it to be used as a visual instrument, all I do is use extension tubes. Not a big deal. With binoviewers, this could be an advantage having the focus so far out as some need more in-focus travel than some scopes can give.

Coma doesn't bother me. I can't say I really notice it. Astigmatism in eyepieces is much more significant, and an f/4 scope really punishes and shows very quickly any weakness in an EP being used. For me, what is much more important than coma is making sure that the EPs being used with such a fast Newtonian for visual use is matching the EP to the Newtonian design. The field curvature (the shape of the focal plane that the scope produces, in this case it is concave) needs to be matched as best as possible. I've had my 8" f/4 dob for around three years now, and I've used many different EP designs in it to find what best works in it, and in turn which don't. This is what I've found:

Good in a fast Newtonian:
* Vixen LVW range.
* Meade series 5000 SWA & UWA range
* Explore Scientific 68deg range
* TV Panoptic range
* TMB Planetary Type II
* Plossl EPs do surprisingly well!
* 5mm Baader Hyperion is the only Hyperion I can put here. All the other Hyperions are best suited to SCT's and Mak's.

These are just the EPs that I've tried that perform well in a fast Newtonian. Remember, I don't use a coma corrector, and it is not a concern of mine. But a coma corrector won't help much, if at all, with field curvature that other EP designs just can't deal with in a fast Newt. There may well be other EPs that perform just as well in a fast Newt as the ones I've mentioned only because I just haven't used them, and I'll leave that to others to add to the offering.

Fast Newtonians make for very good visual use. Just be aware that at f/4, they are punishing on EPs, and will throw up a poor image if the EP and scope are not a good match. My 8" dobbie is capable of giving me a 2.5deg TFOV, which is flaming huge! Yeah, some coma, but it is not really objectionable, and it certainly is not the same as trying to look through a brick. If it does bother you, by all means use a coma corrector. But it is not essential unless you are doing photography.

Mental.