Hi Rex
You seem to have it mostly figured out.
- A slow F12 refractor is “relatively” cheap as the objective lens is relatively simple to design, manufacture and assemble.
- A fast F6 refractor requires a more complex lens design, the use of exotic glass to minimise chromatic aberration and is therefore more difficult to design, manufacture and assemble.
- Similarly, a slow F8 Newtonian is “relatively” cheap as the parabaloid mirror is relatively simple to design, manufacture and assemble.
- A fast F4 Newtonian requires a lot of grinding and figuring to minimise aberrations and is therefore more difficult to design, manufacture and assemble.
- A budget F4 camera lens is relatively cheap as the lens is relatively simple to design, manufacture and assemble and the user knows that using it at F4 will produce images that have false colour, are soft and distorted at the edges. Stopping the lens down to F8 will greatly reduce these unwanted artifacts.
- An expensive F2.8 camera lens requires a more complex lens design, the use of exotic glass to minimise chromatic aberration and is therefore more difficult to design, manufacture and assemble. Usually you can use these wide open, at F2.8, and still get a nice, sharp image over most of the frame.
- An F12 cassegrain has a relatively large secondary mirror, or central obstruction, in the range 30 to 35% which can reduce contrast in an image.
- I suspect that an F6 cassegrain would have a monstrous secondary which would degrade the image to such an extent that it would not be worth building one. It would require a complex design and lot of glass pushing to manufacture the primary and secondary mirrors. Collimation would likely be very sensitive too.
My Tokina 500mm F8 mirror lens is a Maksutov design and it has a whopping central obstruction and the images it produces are consequently a little soft.
Cheers
Dennis