Posterior probability distribution of the power-law ellipsoid fit described in Sect. 4. Blue-filled contours represent the posterior probability obtained assuming a fixed uniform prior on the source position, and solid red contours the posterior probability of the model with the prior of Eq. (19). The quantities shown are: the Einstein radius (in arcsec), density slope, position angle of the major axis (in degrees), axis ratio, source position in polar coordinates (distance from the optical axis in arcsec and angle in degrees), and quad caustic area (in square arcsec). The dashed lines indicate the true values of the parameters. The median and 68% credible region of the marginal posterior in each quantity is listed at the top of each column.