PSF and Strehl Ratio

What you are seeing: the Airy PSF for a circular aperture; the first null at $1.22 \lambda / D$ . With rms wavefront error $\sigma$ , the Strehl ratio drops as $S = e^{-(2\pi\sigma)^2}$ (Maréchal). The standard "diffraction-limited" criterion is $S \gt 0.8$ .

Figure 1. 2D Airy PSF (left), radial cut + Strehl (right).

λ (nm)650

D (m)8.0

σ_RMS (λ)0.05

WHAT TO TRY

Increase the aperture D: the Airy core narrows as 1.22 lambda/D, so resolution improves. Bigger apertures resolve finer detail at fixed wavelength.
Dial up the RMS wavefront error: the Strehl ratio collapses as exp(-(2 pi sigma)^2), light bleeds from the core into the halo, and you cross below the S = 0.8 diffraction-limited line.
Change the wavelength: longer lambda widens the PSF but tolerates the same physical aberration better, since the error is smaller measured in wavelengths.