Dpierce54
For a circular aperture, the angular FWHM (in radians) of the point spread function from diffraction limited optics is about 1.025 * wavelength/diameter of the aperture. Notice that Dawes Limit in radians (at a wavelenth of 550 nm) is 115.8 / diameter (in mm).
From this, you can derive the FWHM at any wavelength, in arc seconds from the Dawes Limit in arc seconds. You can get the latter number from many web sites like this one https://astronomy.tools/calculators/field_of_view/ .
If the atmospheric turbulence has a point spread function with an equivalent diameter of 3 arc seconds (colloquially, a "seeing of 3 arc seconds") , then a 4 arc second star as seen by your sensor would look like it is bloated to sqrt( 4 * 4 + 3 * 3) -- i.e., assumes that the atmospheric turbulence is ergodic and independent of the star itself, and the argument of the square root is simply the addition of variances when two independent point spread functions are convolved with one another.
So, the amount a star increases in size will depend on both the ideal FWHM and the atmospheric turbulence. If you have a small aperture telescope, its FWHM would be already quite large -- "seeing" still affects it, but won't affect it as much percentage-wise than if you start with a smaller star from a larger telescope.
Chen
