66926_ICSOpaperEuclid.doc-ORBi-UniversitédeLiège.docVIP

Thermo-elastic deformation measurement of the EUCLIDnear infrared focal plane array by long wave infrared digital holographY J.-F.?Vandenrijt1, C.?Thizy1, L.?Salvador1, F.?Beaumont2, J.?Garcia2, L.?Martin2, C.?Fabron2, E.?Prieto2, T.?Maciaszek2, M.?Georges1, E.?Renotte1. 1Centre Spatial de Liège – Université de Liège, Avenue du Pré Aily, 4031 Angleur, Belgium. 2Laboratoire d’Astrophysique de Marseille, Rue Frédéric Joliot Curie, 13013 Marseille, France. I. INTRODUCTION The Near Infrared Spectrometer and Photometer (NISP) is an instrument onboard the Euclid mission from the European Space Agency. Its role is to measure the redshift of galaxies by observing adjacent non-overlapping areas of sky. The instrument focal plane array (FPA) is composed of 16?detectors mounted on a structure cooled down to 95?K. The flatness of the FPA has to be ±?10?μm to achieve the required imaging performance of the instrument. In this paper we present the thermal vacuum test performed on the demonstration model of the FPA to measure its thermo-elastic deformation when cooled down to its operational temperature. The measurement system selected is a long-wave infrared (LWIR) digital holographic interferometer. It is composed of a CO2 laser emitting at a wavelength of 10.6?μm combined with a commercial thermographic camera. The surface of the FPA being specular, the speckle wavefront is produced by illuminating a diffuser with the laser beam, and is then observed after reflection on the FPA surface. This reflected speckle wavefront behaves exactly as if the FPA was a diffusive surface, producing its own speckle, allowing the measurement of its deformation. This technique has been previously demonstrated to measure the surface displacements of specular sample under thermal vacuum conditions?[1]. The choice of this system was driven by the fact that the required measurement accuracy is ±?1?μm, which is compatible with this kind of instrument. Also, this type of interferometer is less sens