In a grating monochromator, both the entrance slit and the exit slit are fixed, and the only moving part is the grating. The angle formed between the entrance and the slit arms is called the deviation angle or angular deviation (AD). In constant-deviation monochromators, where the angular deviation is held constant, the gratings are rotated, allowing the spectrum of the incident light to be scanned. While plane or concave gratings can be used in constant deviation mounts, only concave gratings allow imaging to be acceptable over an entire spectrum without auxiliary focusing optics.