Cross-fringe technique for sensing piston errors of segmented mirror telescope

For the disaster prevention and mitigation, large segmented Earth observation telescope in the geostationary orbit is studied in JAXA. In order to obtain diffraction limited performance with the segmented telescope, it is necessary to co-phase the segmented primary mirrors with sub-wavelength accuracy. And since the displacement of the mirrors induced by environmental stress occurs during launching and in orbit, the phasing sensor also need to cover the measurement range exceed tens of micrometers. In this paper, we propose a technique for the purpose and enable us to obtain piston errors of multiple segments simultaneously independent of the edge orientation. The optical scheme for the telescope model is presented and some fundamental experiments are described.