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Mark
On the vibrations, do you mean vibrations from wind buffeting or vibrations from accidently knocking the telescope, focussing etc? I'm not necessarily going to do a solid tube, I haven't decided yet.
I'm assuming you mean F/26 at the RoC; At that distance the mirror surface would appear just over 2 degrees across so the size it appears should be ok. Air currents will likely be a problem at a 5 metre RoC but I'll see how it goes. Is it possible to test at the focus using a star or very distant artificial light as the light source, meaning that the mirror would go grey all at once for a parabola, not a sphere or would that actually increase the light path from the distant source and worsen the problem?
For me for the eyepieces it's a question of eye relief as the eye relief of plossls scales with the focal length. An F/13 would have 13/8 of the eye relief at the same magnification using plossls as an F/8. I own two kellner eyepeices, a 15mm and a 9mm which is close to the 13/8 ratio. I love the 15mm, in fact it's one of my main eyepieces but the 9mm requires me to jam my eye up against the lens which detracts from the comfort of the view. I'm obviously not going to be able to sit down while observing unless I go to F/5, possibly F/6 and I don't mind standing on a chair to observe. A barlow would be a partial get-around but no matter how good it is, adding more glass to the optical train will scatter more light.
For Les, I've attached images of the mount, sorry they're blurry but it shows the general form of the mount.
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