Gamma-ray extension, given as the 1σ radius of a 2D Gaussian fit, of the Crab nebula as derived from Holler et al. (2017) (blue) and using ctools (red) superimposed on a Chandra 0.3–10 keV X-ray image of the Crab nebula (credit: NASA/CXC/SAO). The crosses indicate the statistical uncertainty in the centroid position as quoted by Holler et al. (2017) (blue) and as derived from the Gaussian model fitting using ctlike (red). The systematic positioning accuracy of H.E.S.S. of 20″ (Holler et al. 2017) is indicated as a white circle. Coordinates are for the epoch J2000.