Simulated correction factors for R Aqr Hα surface brightness sb_corr (upper) and aperture flux ap_corr (lower) as function of cloud diameter ⊘ _cl [pix] for different apertures and AO performance. The full lines are for round clouds and aperture diameters ⊘ _ap = ⊘ _cl + 20 and “average” conditions, and crosses indicate results for good and bad atmospheric conditions. The red dotted lines show the results for small ⊘ _cl + 10 or large apertures ⊘ _cl + 40. Results for elliptical model clouds with the same surface area but a ratio l:b = 4 are plotted with diamonds.