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Hi Malcolm. Good list.
I have an older f4 GSO 8 inch and from experience with that, would add the following:
1. Focus is touchy at f4, since the diffraction limited focal depth is only about 40 microns. Seeing effects will give you a bit more latitude, but you will need a good stepper motor focuser with automatic focusing software or you will be forever refocusing to try to track variations in temperature etc.
2. Mechanical alignment is critical at f4 - everything must be as near as possible to perfect or you will get distorted stars somewhere in the FofV. Expect to spend some time tracking down minor flexures or misalignments in the tube/focuser/MPCC/mirror supports/camera etc.
3. A 12 inch Newt OTA will act like a sail in even the slightest wind, so you will need a very strong mount - you will probably need something much heavier than an EQ6.
4. You will not be able to use adaptive optics at this focal ratio.
5. Optically mine is very good - able to resolve much better than seeing will allow, but an MPCC or similar is essential, since coma is a killer at f4. No problems with contrast at all.
6. At f4 you can use much shorter subs, so can possibly get by with slightly more differential flexure than you would with a higher FR and consequent longer subs.
7. Balancing can sometimes be a problem with a big lump of camera hanging way off the tube axis.
Scopes like these are not black box refractors where light goes in one end and a near perfect image comes out the other. You will probably have to spend a lot of time fine tuning and understanding the scope's idiosyncrasies, but when you do get it right, the results can be very satisfying and take only a short time - and the scopes you are looking at are a lot of capability for the money. I am certainly going to keep my f4, even though it can be frustrating at times.
Regards Ray
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