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Depends on what the software means by "radius". Some possibilities (approx.):
radius of the inner disk = 0.61*lambda/a
1/2 standard deviation of PSF in the radial direction = 0.22*lambda/a
1/2 full width at half max (FWHM) = 0.26*lambda/a
where
lambda = wavelength (approx: red 650nm, green 510nm, blue 470nm)
a = aperture
This is in arc second (and use same units for a and lambda, e.g., metres). Multiply by the number of pixels / arc second appropriate for your scope + camera to get the radius in pixels.
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