The invention provides for a method of operating an instrument (100). The instrument comprises a magnetic resonance system (102) for measuring dictionary magnetic resonance data (154) from a measurement zone (108). The magnetic resonance system comprises a magnet (104) for generating a main magnetic field within the measurement zone. The magnetic resonance system comprises a test fixture (124) for holding a test sample (132) within the measurement zone. The test fixture comprises a supplementary magnetic field coil (126) and a magnetic resonance antenna (128). The method comprises the steps of repeatedly: choosing (200) an electrical current; supplying (202) the electrical current to the supplementary magnetic field coil to adjust the main magnetic field within the measurement zone; acquiring (204) the dictionary magnetic resonance data from the test sample with the magnetic resonance antenna by controlling the magnetic resonance system according to a magnetic resonance fingerprinting technique; and appending (206) the dictionary magnetic resonance data to a magnetic resonance fingerprinting dictionary (156).