This is very helpful. Thanks. So at what point regarding aperture do diffraction and aberration become practically important? Can I ever match image scales on my FSQ85 or SVR90 to give comparable small detail as my Edge 9.25? Daniel here on CN has posted some impressive images at 650mm focal length using a 2.4 micron pixel ASI 183, which when zoomed in seem to approximate the detail of galaxies taken with my Edge at 2350 FL.
Now of course that does not account for difference skill, sky conditions, etc. which is why I was hoping to find someone out there who can compare with his/her own equipment.
Diffraction as the diameter of the airy disc from first minimum to first minimum in angular terms is:
A = 2.44 * γ / D * 206264.8
γ = wavelength of light in microns (i.e. 0.000555 for green light)
D = diameter of aperture in millimeters
The measure of the diameter of the airy disc is in effect from the edge of the halo to the edge of the halo, or close enough. This is wider measure of a star than FWHM. To convert from airy disc diameter to FWHM, multiply the angular size of the airy disc by ~0.45. So, for say a 250mm aperture and an 85mm aperture:
A250 = 2.44 * 0.000555 / 250 * 206264.8 * 0.45 = 0.5" FWHM
A85 = 2.44 * 0.000555 / 85 * 206264.8 * 0.45 = 1.5" FWHM
Now that you have diffraction in terms of FWHM, you could convolve that with your assumed seeing. If you have 1.5" seeing, your total spot sizes would be:
S250 = SQRT(0.5^2 + 1.5^2) = 1.6" FWHM
S85 = SQRT(1.5^2 + 1.5^2) = 2.1" FWHM
This is actually a meaningful difference. Having imaged with subs at 1.6" and 2.1" myself on several occasions, the difference is quite noticable in both star sizes, star peakness, and detail sharpness. Now if you actually have 3" seeing (rare, but it does occur in heavily populated areas with large heat plume, and under areas with high jetstream velocities and turbulence, such as right under the edge of the polar vortex):
S250 = SQRT(0.5^2 + 3^2) = 3.1" FWHM
S85 = SQRT(1.5^2 + 3^2) = 3.4" FWHM
A smaller difference, and also a less meaningful difference. In both cases, your stars are going to be more blurry, and that blur is dominated by seeing. If you have seeing this bad, a big telescope isn't going to do you much good.
So, there is the basic theory to determine your ballpark FWHMs for different scopes under known seeing conditions. Again, most people OVER-estimate their seeing, especially if they are measuring FWHM with small aperture scopes. Keep in mind, there are other sources of error and blur, so these formulas just get you in the ballpark. Blur terms convolve with each other much like noise terms...the larger terms will dominate the smaller, rendering smaller terms largely moot. However, if your airy FWHM is around 0.5", then it may not be a larger term...tracking error may actually be larger, depending on your mount and environment.
Anyway... I, too, would like to see someone put the theory to the test. I would do it myself, and I guess in some sense I probably can. I have data with an IMX183 from a 150mm aperture scope, and I am now using a 106mm aperture scope. I could try to get useful comparison data from the new scope on an object I've imaged before. Of course, to actually get useful comparisons, it might take me until next winter, which wouldn't be all that useful to you now. And, a 150mm aperture is a good one for a refractor, but there are MUCH larger apertures to be had, and for a lot less money as well. You could easily have a 250, 300, 350mm aperture with an SCT, RC or Newt at much less cost. With small-pixel cameras, an 8-10" aperture newt should be plenty to reduce diffraction enough that even ~1.5" seeing would dominate.