Spectropolarimeter’s optical design for the Arago space mission project

International audienceArago is a concept of space mission submitted to the European Space Agency's M7 science program. It will target a number of science cases in stellar physics including the characterisation of star-planet interactions. The concept is based on a 1-m class Ritchey-Chretien F/13 telescope mounted on an Ariel-type platform. The scientific payload includes a common polarimetric unit and 2 spectrographs connected via a dichroic splitter. The polarimetric unit consists of 6 MgF2 plates in pairs connected by optical contact and a Wollaston analyzer. Each of the spectral channels represents an echelle spectrograph. The first one will operate in the ultraviolet range 119-320 nm with the spectral resolving power of R >= 25000. It consists of an off-axis parabolic collimator, an echelle grating in a quasi-Littrow mounting, a cross-disperser concave grating, and a CMOS camera. The cross-disperser grating works also as a camera mirror and represents a holographic grating imposed on a spherical substrate. It is recorded holographically with an auxiliary deformable mirror to correct the aberrations and has a blazed grooves profile. The spectral image is projected onto a δ-doped CMOS detector. The second spectral channel operates in the visible range 350-888 nm with R >= 35000. Its design uses an immersed grating, i.e. a blazed reflective grating ruled on the backside of a fused silica prism. This solution should allow us to operate in a spectral range wider than one octave with sufficient spectral orders separation. The dispersed beams are focused with a 4-lens objective onto a CMOS detector. In addition to the main scientific payload, the optical design includes two stages of fine-guiding system. The first stage represents a simple projecting system tracking the image around the entrance pinhole and communicating with the platform actuators. The second stage is fed by the 0th diffraction order of the visible channel echelle grating. Its information is communicated to a tip-tilt mirror in front of the dichroic. The first stage should improve the pointing accuracy from 8" to 200 mas precision to compensate the platform jitter and drift and guarantee that the star images passes through the instrument's entrance pinhole. The second stage should correct the pointing accuracy further as well as some thermo-elastic deformations. Additional presentation content can be accessed on the supplemental content page