Calibration and test of the Athena On-board Metrology system

Athena is ESA's X-ray observatory mission designed to address the Hot and Energetic Universe theme. The instrument consists of a single x-ray telescope, supported by a large area mirror based on Silicon Pore Optics technology, focusing the photon flux alternatively onto two different instruments: the Wide Field Imager (WFI) and the X-ray Integral Field Unit (X-IFU). The process of switching the focus between the two instruments is produced by a hexapod structure changing the pointing of the main X-ray mirror. To verify that the mirror optical axis is aligned with each of the instruments, an optical On-board Metrology System (OBM) will be used to estimate the telescope pointing with accuracy better than 0.5 arcsec. The OBM system currently in development at the Institute of Astrophysics and Space Sciences is based on the concept of a projective metrology where a pattern of active fiducials is imaged by an array detector. The position and orientation of the imaged pattern provides the necessary information to reconstruct the fiducial displacements with respect to the calibrated reference points, allowing obtaining an estimate of the pointing absolute knowledge error. The design of the metrology system is presently at Engineering Model level, although it will be tested for performance in normal laboratory conditions. In this work we will present the experimental setup and processing architecture designed for calibration and testing the OBM demonstrator close to operational conditions, namely in what respects to the distance from the OBM Optical Head to the fiducial plane (12 meter). This experimental setup is calibrated to reproduce verifiable fiducial displacements over the OBM telescope field of view at 7.5 micrometre level (1 sigma). This setup allows evaluating the performance of OBM and verify its compliance with requirements of the on-board metrology system for the Athena X-ray Observatory mission.