The Magnetic Distortion Calibration System of the LHCb RICH1 Detector
  The LHCb RICH1 detector uses hybrid photon detectors (HPDs) as its optical sensors. A calibration system has been constructed to provide corrections for distortions that are primarily due to external magnetic fields. We describe here the system design, construction, operation and performance. 
A. Borgia, W. Cameron, A. Contu, C. D'Ambrosio, C. Frei, N. Harnew, M.   John, G. Lefeuvre, R. Mountain, S. Stone, D. Websda...
physics.ins-det hep-ex