If one of the scan axes is the monochromator, you can choose start and stop energies and the energy step rather than the number of pixels and the pixel size. Note that on the fast axis (i.e., for point spectra), the microscope server program might switch start and stop energies if it doesn't like the scan direction (it has to do with the backlash of the monochromator motor). If you care, you can recall the actual start and stop energies later from the parameters start_ev and stop_ev in the sm_par structure (see Sec. 10.4). However, since the display in SM_GUI (and also the arrays fast_dist_um or ev which are returned from read_stxm5.pro, see Sec. 10.3) is always correct, there should rarely be a situation where you have to care.