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SPIRou : nIR rhombs and Wollaston prism

Par Jean-Francois Donati - 29/09/2008


SPIRou : nIR rhombs and Wollaston prism

To obtain an achromatic polarimetric analysis over the whole spectral domain of SPIRou (0.98-2.4µm), we need to find the adequate glass that can provide both the nominal retardation and the optimal transmission; we also need to check that ultra-low birefringence samples are available.

The first preliminary study indicates that several options are possible. The 2 best solutions apparently consist in using S-FTM16 with MgF2 coating or ZnSe with CVD diamond (better throughput but much thicker), both providing a nominal retardance within better than 0.5% over the whole wavelength range and field of view (see image on the right in the case of ZnSe). In the next stage of the design study, we will order several chunks of both glasses and ask for accurate measurements of the residual stress birefringence.



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