Blue = no AO, green = GLAO, red = LTAO.
ZD = 0 deg, pos. in FoV = centre, GLAO: FoV = 6 arcmin.

The dotted line shows the wavelength dependence of r50 (∝ λ-0.2) expected for an uncorrected PSF assuming an infinitely large outer scale of the atmospheric turbulence (L0). Note that the blue no AO points are significantly offset towards lower values compared to this line, and that this offset increases towards longer wavelengths. The offset is due to the finite outer scale of the atmospheric turbulence that was assumed in the simulations. In the case of a D = 42 m telescope the aperture is in fact larger than the outer scale, so that the telescope 'sees' significantly less turbulence than it would if the outer scale were infinite, resulting in a pronounced reduction of the telescope seeing. The upper dashed line shows r50 after approximately correcting for this effect (using equation 20 of Tokovinen (2002)). Both this curve and the dotted curve are normalised to the r0 value used in the simulations.

The lower dashed curve shows 0.5 rDLC, where rDLC = 1.22 λ/D is the radius of the diffraction limited core of a perfect PSF for a D = 42 m telscope. An ideal PSF will contain approximately half the flux within 0.5 rDLC so that this radius is an approximate lower limit of r50.