Image in I, Q/I and U/I of the Crab nebula in the [2–8] keV band, assuming fully unpolarized radiation and computed using a full ixpeobssim plus GEANT4 simulation (upper panel). Data were reduced using a standard ixpeobssim data analysis tool. Gaussian smoothing was applied to reduce Poisson noise. The original source input file was the Chandra ObsId 16364. Lower rows show the same above, but computed by convolution of the Mueller matrix element with the intensity map of Chandra ObsId. In order, from the second to the fourth row, the functions 𝒢(r) and 𝒦(r) were derived as: nonlinear formalism (second row), fit of the results from a GEANT4 point source simulation (third row), and fit of the Cygnus X-1 data (forth row). The reference frame is chosen such that Q is positive for polarization aligned along the north-south direction.