- A 12 inch F/5 has a 60 inch focal length should be an easy sit down scope. My 16 inch F/4.4 which has a 70.9 inch focal length is a sit down scope with a catsperch Pro chair.
- Coma is real. At F/5 the diffraction limited circle is 2.75 mm. In a 12 inch, thats 0.1 degrees. If you're letting objects drift across the field, at 200x, 21 degrees AFoV is diffraction limited. Your perfect mirror has a Strehl of 0.8 or more inside that circle.
I use a Paracorr 2 with my 13.1 F/5.5. with type 6 Naglers, the planets are clean and sharp across the entire field, without correcting the coma, the sweet spot is small..
My 12.5 inch is F/4.06, it's the reason I have a Paracorr. I've had it for nearly 20 years. We've been friends a long time.. it has its advantages and disadvantages but it I were starting from scratch, I'd go F/5. I'd still be using a Paracorr...
Regarding Vtornado's questions about coma correctors and spacing: The Paracorr 2 has this nailed. The Tuneable Top is like a very fast smooth, very tight helical focuser. You can focus with the Tuneable Top, fine focus with the Focuser.
The 12.5 inch F/4.06 alongside the 13.1 inch F/5.5 (72 inch FL) person is 5'10".