ELT-HIRES the high resolution spectrograph for the ELT: status of the polarization ray tracing tool for the polarimetric unit

ELT-HIRES (High Resolution Spectrograph for the Extremely Large Telescope) is a multifold fiber-fed spectrograph planned for the Nasmyth B focal station on ELT, covering a wavelength range between 0.4 and 1.8 μ. One of the most relevant science cases is represented by the detection of life signature in the extrasolar atmospheres for Earth-like planets. The polarimetric unit, feeding the UBVRI and the zYJH spectrograph modules, will consist of two subassemblies: the main one, aimed to be installed in the intermediate focus, will host the polarizer (a double Wollaston calcite prism) and the retarder plates splitting the optical beam into the four Stokes vector components; the other will be located in one of the four dedicated arms of the Front End on the Nasmyth platform, providing the atmospheric dispersion correction, field stabilization and selection of the operating modes before the fiber injection. At the conclusion of Phase A, we have presented this as the optimal design solution fulfilling the top level requirement of reaching a sensitivity of 10-5 S/I (S being equivalent to one of the Stokes vectors), a condition achievable only if the polarizers are installed in a rotationally symmetric focus. The current work, which can be considered a continuation of two preceding papers, illustrates new simulation results from the development of the polarization-ray-tracing tool. We have included also the crystal anisotropic properties of the polarizing components, analyzing the transmission loss that the light beam encounters propagating through the optical train before entering the fiber bundles in the different operating modes, bearing in mind the polarimetric aberrations induced by the active compensation of the primary mirror and the loads contributing as Zernike polynomials of different orders, namely the telescope cross-talk.