M 87 jet base collimation profile plotted vs. the core separation d. Top: jet width and power-law fit. The blue data points are the measurements, the red solid line is the best power-law fit to the data, and the gray broken line is a fit with a fixed slope of k = 0.58. Middle: same as above, but for the apparent opening angle. Bottom: fractional difference between the observed diameter and the power-law model with a corresponding slope, k ≈ 0.47. The arrow indicates a core distance where the fractional difference starts to grow significantly.