Distant Quasar Light Doesn't Show Discrete Space-Time Distortions

One observable that might be impacted by a discrete structure of space-time is the way that light from distant quasars appears when observed (i.e. "the degradation of the diffraction images of distant sources"). Experimentally, these effects are not seen. If a=1 is the naiively expected amount of distortion from a discrete space-time structure at a Planck scale, the data supports distortion of not more than something on the order of 0.05 to 0.15 relative to that value.