While refractors have come and gone in my collection I have always had Newtonian.
So it pains me to say this, but the ideal planet scope is a refractor.
Newts can be very very good ... but never ideal. Taking 7% of the signal (Airy Disk) and dumping into noise (diffraction rings) by adding an obstruction just can not be undone. And that is before the issues of scatter and thermal performance.
But, playing along with the topic at hand ...
Permanent set-up is good. From there you go straight to accessories (and that's ok), but let's stay on the scope itself for a moment. I would suggest these additional features:
Getting back to accessories, the one I would add for sure is an autocollimator. I really like the CatsEye with dual pupils.
- Solid Tube with minimum 1.5" air gap between tube wall and mirror, regardless of what that implies for secondary mirror size.
- Wire spider, minimum thermal effects, diffraction is least, and spikes minimal (though perhaps more noticeable than curved spiders).
- Push-pull locking collimation on primary mirror cell - f/8 has a wide collimation envelope, but why explore the edges?
- Tracking mount.
- Cooling fan(s) of course. Scrubber fan if mechanically feasible.
- Rear baffle against ground reflections back up the tube.
- Nose extension (stray light concerns again), especially for a low-profile focuser.
- Ground surrounding the mount (or dome) is grass covered. Else, light colored materials.
- If the scope is housed in an observatory, air conditioning that can be activated six hours before intended use.
Refractor?! Heathen! Obviously the short comings of a refractor are costs and physical size. While you're right about the obstruction, the Newt lets me go to 8" for $700 including the mount. I have a custom 5" f/12, and while it's really sharp and contrasty, it looses brightness with magnification quickly.
Yup, I'll set it up on grass.
Yes, the tube is ventilated which is simply a must-have. Nothing will kill an image faster than thermals. Same with the fan.
I already have all those collimation tools for my 12.5" f/5 DOB and my 8" f/6 Dob. The 8" f/6 is a sonotube scope with 9" tube. I have some black foam weatherstripping on the mirror end as a baffle. It's visual images are astonishingly sharp.
I don't know how the collimation screws are, but I'm sure they work. Also the secondary vane are the typical flat variety. Yeah, wire would be better, but probably beyond my technical ability. I'm pretty sure the tube is long enough not to need an extension. It comes with a Meade RG mount, so it will have tracking.
I'm having Terry Ostahowski check the mirrors and recoat them,