Preliminary design of the full-Stokes UV and visible spectropolarimeter for UVMag/Arago

Abstract

The UVMag consortium proposed the space mission project Arago to ESA at its M4 call. Arago is dedicated to the study of the dynamic 3D environment of stars and planets. This space mission will be equipped with a high-resolution spectropolarimeter working from 119 to 888 nm. A preliminary optical design of the whole instrument has been prepared and is presented here. The design consists of the telescope, the instrument itself, and the focusing optics. Considering not only the scientific requirements, but also the cost and size constraints to fit an M-size mission, the telescope has a 1.3 m diameter primary mirror and is a classical Cassegrain-type telescope that allows a polarization-free focus. The polarimeter is placed at this Cassegrain focus. This is the key element of the mission and the most challenging one to be designed. The main challenge lies in the huge spectral range offered by the instrument; the polarimeter has to deliver the full Stokes vector with a high precision from the FUV (119 nm) to the NIR (888 nm). The polarimeter module is then followed by a high-resolution echelle-spectrometer achieving a resolution of 35000 in the visible range and 25000 in the UV. The two channels are separated after the echelle grating, allowing specific cross-dispersion and focusing optics for the UV and the visible ranges. Considering the large field of view and the high numerical aperture, the focusing optics for both the UV and the visible channels is a Three-Mirror-Anastigmatic (TMA) telescope, needed to focus the various wavelengths and many orders onto the detectors.