A control system for controlling a magnet of a magnetic sector mass spectrometer comprises a magnetic field sensor for sensing the magnetic field of the magnet and generating an output representative thereof; a set point generator configured to generate an output representative of, or related to, a desired magnetic field of the magnet; and a digital controller configured to receive a variable digital input signal from the output of the magnetic field sensor and a set point digital input signal from the output of the set point generator, and to generate a digital output from which is derived a control signal for controlling a current to the magnet so as to control the magnetic field thereof. The control system is arranged to apply to the digital controller a selected one of a plurality of different controller settings, in accordance with the desired magnetic field of the magnet.