In-flight performance of the IBIS calibration unit

We describe the in-flight performance of the on-board calibration unit for the IBIS telescope on INTEGRAL. Both intrinsic performance and the quality of the calibration signals provided to the IBIS detector planes are discussed. The calibration unit intrinsic performance is assessed based on the diagnostic information in IBIS housekeeping. The flux of tagged photons, i.e. those arriving at the detector planes in coincidence with a calibration strobe, is assessed from in-flight data, and analysed in conjunction with a detailed simulation. Proposed usage of the tagged photon flux is discussed, and the expected calibration accuracy derived. The effect on science data from untagged calibration unit photons is assessed based on the predicted rate and distribution of such photons.