This is a constant factor used in calculating the spatial step size. For each radial zone, the size of the next zone is chosen to be
where emult is defined below, and is the maximum opacity from the previous step calculation. is only calculated from energies where the optical depth to the illuminated face of the cloud is less than taumax, where taumax is defined below. The default value for nsteps is 2. The default value of emult is 0.5, and values outside the range 0.1 - 1 are unlikely to be of any practical value.