Optical and mechanical architecture for the E-ELT HIRES polarimeter

We introduce the opto-mechanical architecture of a high precision, full Stokes vector, dual-channel polarimeter for the European Extremely Large Telescope’s High Resolution spectrograph (E-ELT HIRES). It is foreseen to feed two spectrograph modules simultaneously through the standard Front End subunit located on the Nasmyth platform via two fiber bundles; one optimized for the optical (BVRI), the other optimized for the infrared (zYJH) bands. The polarimeter is located below M4 in the f/4.4 intermediate focus, representing the only rotationally symmetric focus available, and is retractable. We illustrate the strategy of repositioning and aligning the instrument, provided that it has to withstand wind and earthquake loads and that the PSF is varying in width and position due to the active compensation by the co-phasing corrections. Preliminary results of its expected polarimetric sensitivity and accuracy are also analyzed for several configurations of M1 segments and suggest a stunning performance in the intermediate focus with cross talks of the order of 10-7 but 10-2 if it were located in the Nasmyth focus.