Past exam of the physics A course of the University of Cambridge / 2014 / Paper 2 / Section A / 1 / Solution

For a diffraction-limited circular aperture, the Rayleigh criterion gives $\theta =1.22\lambda /D$. The photon wavelength is $\lambda =hc/E=1.24\mu \mathrm{m}$, hence $\theta \simeq 1.5\times 10^{-6}\mathrm{rad}$. This assumes adequate optical quality and sampling; being in space removes atmospheric seeing, not diffraction.