Contour Plot of Impact Flash
A contour map of the flash, so bright that it is saturated, shows that the North (top) and South (bottom) of the flash are not identical. There is a clear elongation on the South side in the direction of the motion.
The elongations at 45 degrees are due to the diffraction pattern from the secondary mirror supports (the spider).
The units on the axes of the graph are WIRCam pixels: 0.3" or ~0.5km.