Thanks Marty, I'm not very good with the various online calculators. Hence why I search search engines and the various astronomy forums to see if I can find the answers in plain English.
However compared to the other online Newtonian calcs, Stellafane is brilliant thanks.
The spot diagrams show virtually no difference between the different central obstruction sizes. This stands to reason as the size of the airy disk is exactly the same at f11 at all obstruction sizes
All light is contained within the airy disk out to 0.5 field width. Therefore no coma, curvature of field nor astigmatism. Ie near perfect resolution across within a 0.5 degree field. Ie near perfect P-V at all your choices of C.O.
The question is how much contrast and brightness is affected by the size of obstruction (of your near perfect optics)
Increasing the size of the central obstruction not only dims the image but reduces contrast Crucial consideration for planetary observing!
The MTF (Modulation Transfer Function) diagrams show how contrast decreases as the C.O.increases. The graph lines droop leftward with increase C.O. size. The zero C.O. is more straight
Others here may be able to confirm if the difference is detectable in practice.
A curious caveat. Increasing the size of the C.O. increases resolution a bit because some of the light from the centre of the airy disk is transferredn to brightening in the airy ring(s) making the centre of the airy disk smaller and thus sharpening the image. This is similar to how "sharpening" works in image processing. It's just that the wave nature of light does it for you with C.O.Check this.
As far as I know planetary observers go for the smallest C.O. for best image contrast and brightness
Too small C.O. may present practical construction problems as the focal plane is much closer to the secondary with eyepiece possibly intruding into the tube too far and thus becoming and obstruction issue itself!
Hope this helps