The magnetic calibration procedure is performed in dedicated LED scans with dipole field ON and OFF. Scans with field OFF are used to reconstruct the calibration grid pattern with data. Figure 8 illustrates the grid pattern used in RICH 1(8a), RICH 2 (8b) on a single HPD. The positions of the patterns recorded in field-ON scans are then compared with the grid points constructed in field-OFF scans, to evaluate the correction parameters. The parameterisations are stored in the LHCb Conditions Database to be applied to the LHC collision data reconstruction. The LHCb experiment will search for new physics in CP violation and rare decays from heavy flavour production at the LHC. Particle identification is provided by two RICH detectors which cover a momentum range between 1 and 100 GeV/c. The RICH detectors incorporate a total of 484 Hybrid Photon Detectors (HPDs) to measure Cherenkov photons with a spatial granularity of 2.5mm per pixel. The RICH detectors are located in the fringe field of the LHCb dipole magnet, and fields as large as 2.5mT have been measured in some regions occupied by the RICH 1 photodetectors. Due to the cross-focusing feature (Figure 1) of the HPDs they are susceptible to magnetic field. Hence it is necessary to make corrections for the distortion of the ring-images measured by the HPDs. Iron shielding boxes are constructed for the RICH1 detector and the field inside the shielding where the photodetectors locate is illustrated in Figure 2. Additional Mu-metal shields are appended to every HPD, reducing the field strength to under 5 Gauss within the HPDs. Magnetic Distortion Calibration for LHCb RICH Detectors Fúnaì Xing on behalf of the LHCb RICH Group 7th International Workshop on Ring Imaging Cherenkov detectors, 2-5 May 2010, Cassis, France RICH 1 incorporates a magnetic distortion calibration system (MDCS) comprising a series of collimated LED boards, each with 5 arrays of 28 LEDs (Figure 3). 19 of these boards are aligned in series and mounted on x-y stages (Figure 4). The systems are mounted inside the Photon Funnel (Figure 5) directly in front of the HPDs. In a calibration run, the LED light bar scan over the front-faces of the HPD arrays with a dedicated grid calibration patterns from the LEDs. The system allows mapping of the magnetic distortions to a precision better than 1mm at each HPD photocathode. In RICH 2, a simpler system based on 2 commercial projectors are used for the calibration pattern projection (Figure 6). An example of the RICH 1 calibration grid pattern, and the results of the calibration in one HPD is shown in Figure 9. Hardware Control Calibration Patterns and Analysis RESULTS Before Corrections:After Corrections: Introductions Figure 1Figure 2 Figure 3Figure 4 Figure 5Figure 6 Figure 12 Figure 10 Figure 9 Figure 11 The MDCS for RICH 1 is operated via PVSS software (under TCP/IP protocol), the standard operation software for LHCb. The control interface is shown in Figure 7. For RICH 2 the control is a simple manual turn ON/OFF. Figure 7 Figure 8(a) The results in terms of spatial resolution after MDCS corrections is 0.51mm for RICH 1 (Figure 10) and 0.47 mm for RICH 2 (Figure 11). In comparison, the intrinsic pixel resolution of the HPDs is 0.72mm. The MDCS corrections were applied in the reconstruction of the 2009 LHC collisions data. The result on RICH 1is shown in Figure 12. On the left is the Cherenkov angle resolution before corrections. On the right, after alignment and MDCS corrections, the resolution improves to 2.4 mrad. The improvement in RICH 2 is less visible, as expected due to a less hostile field strength. Figure 8(b) Before Corrections:After Corrections: