ATHENA [ESA] (The advanced telescope for high energy astrophysics) is an ESA X-ray observatory mission to address the hot and energetic universe theme. The instrument consists of a single x-ray telescope, supported by a large area mirror/ lens based on silicon pore optics technology, which will focus the photon flux alternatively onto two different instruments, 12m away: 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 large hexapod structure that is able to change the pointing of the main mirror and the focus plane position, according to strict operational requirements. In order to verify that the mirror optical axis is perfectly aligned with each of the instruments, the telescope shall be supported by an optical on-board metrology system (OBM) that is able to estimate the telescope pointing with accuracy better than 0.5 arcsec. The Athena OBM system currently under development is based on the concept of a projective metrology where a pattern of active fiducials point sources is imaged onto 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, thus allowing obtaining an estimate of the pointing absolute knowledge error