Measurement and modelling of the chromatic dependence of a reflected wavefront on the Euclid space telescope dichroic mirror

Euclid is the second M-class mission of ESA's Cosmic Vision Program. It implements a space telescope to be launched at L2. The objective is to characterize the dynamics of the early Universe by using two instruments: the high definition camera VIS (visible instrument) and the spectrophotometer NISP (Near Infrared Spectrometer and Photometer). Light entering Euclid is either reflected toward VIS in the visible band, or transmitted to NISP in the infrared band by a dichroic mirror. In order to guarantee the quality of scientific data delivered by the mission, the knowledge of any chromatic dependence of the optical payload's Point-Spread function (PSF) is critical. However, previous works showed that complex coatings, such as high-performance dichroic coating, are likely to induce high chromatic variations in reflection, either as a chromatic "Wave-Front-error" (WFE) and/or as inhomogeneous reflectance profile (R), both affecting PSF morphology. In-depth knowledge of the reflected wavefront by the Euclid Dichroic is then necessary in order to calibrate the in-flight Euclid Observations. This work focuses on two aspects. On the one hand, we present an experimental campaign to measure the dichroic WFE and R at any wavelength, incidence, and polarization state, with an extreme precision. This metrology work implements a bench funded by ESA, designed by Imagine Optic Company, and commissioned at LMA. On the other hand we build a numerical model of the dichroic based on these on-ground measurements. By reproducing the experimental optical properties of the dichroic mirror, we ensure the subjacent thinfilms physics at play is well understood, ultimately providing adequate inputs for the in-flight calibration of Euclid with a suitable level of accuracy.