Target Acquisition for MIRI Coronagraphs

International audienceCoronagraphs are powerful instruments to reduce diffraction from a bright source in order to detect planets. Four coronagraphs will be installed in MIRI, the Mid-InfraRed Instrument of the James Webb Space Telescope. To further reduce the diffraction in addition to the coronagraph, a calibration of the residual speckle pattern can be obtained, for instance, with a reference star (or alternatively on the target star at a different roll angle). For this calibration to be accurate, the diffraction pattern of the two coronagraphic images must be as similar as possible. We study the accuracy of the star image positioning onto the coronagraph to reach acceptable performance: we proved that pointing reproducibility must be better than 5 mas rms per axis while the absolute pointing can be relaxed to 10 mas rms. The choice of algorithm is driven by the level of accuracy to be reached in the presence of a nonlinear system like the coronagraph. We first study their bias, and then we estimate their sensitivity to different sources of noises in the context of MIRI. And finally, for practical matter, we derive the necessary exposure time to obtain the centroid on an actual star