Characterisation and compensation of magnetic distortions for the pixel Hybrid Photon Detectors of the LHCb RICH

The photon detectors need to operate in the fringe field of the LHCb dipole magnet which will produce distortions of the image detected on the pixel chip which is encapsulated inside the HPD. This paper reports on the experimental characterisation of the image distortions caused by an external magnetic flux density. These measurements allow for the development of a parameterisation of the effects and a compensation algorithm, which are also presented.