The photon tagging system for Hall D is essential for defining the incident photon energy of the GlueX experiment, and also for the alignment of the diamond crystal radiator in order to place the desired coherent peak at the correct photon energy. The photon energy is determined by precise measurement of the energy of the bremsstrahlung electrons by their deflection in the field of the tagger dipole magnet. The energy calibration of the system thus requires precise
knowledge of this magnetic field.
