The 4.25" Edmund f/10 reflector I used to own gave superb images, even on the moon and planets. This was with its original rectangular secondary mirror, which was not ideal.
Same is true for the Criterion 4" f/10 Newtonian I owned as a kid. Gave high contrast views of the moon and planets and double stars were resolved to the limit of its aperture with perfect Airy discs.
Edit. It's unfortunate that small 4"-5" slow Newts are not available. Even a spherical mirror at f/10 -12 will have a wave rating of better than 1/5 wave. They are easy to produce and the tube assembly can be made with all the latest technology for a perfectly ergonomic, CA-free and yes, coma-free (or nearly so) optic.
A 4-1/2" f/10 Newt would be no longer and no heavier than a 4" f/10 ED and should cost much less and be just as good for deep sky and planetary observations.
All the lens technology that is required for that same performance is really overkill if one is interested only in refractor-like views for visual observations.
I like refractors, but I would also like a 5" f/10 Newt built to the same standards as a modern ED, which, unfortunately is nowhere to be found.
Edited by daquad, 22 March 2020 - 06:59 PM.