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SPIRou / Infrared SpectroPolarimeter

2016 April: Inspection of SPIRou Optics at NRC-H

Most optics of the SPIRou spectrograph were optically controlled at NRC-H

ZnSe Prism

Visual inspection of one of the SPIRou ZnSe prism at NRC-H

Most of the optics of the SPIRou cryogenic spectrograph, i.e. the R2 grating, the Infrasil prism, the twin ZnSe prisms and the folding flat mirror, were inspected and optically controlled at NRC-H, during the visit of Driss Kouach (SPIRou PM at IRAP / OMP), Zalpha Challita (SPIRou spectrograph AIT at IRAP / OMP) and Greg Barrick (CFHT) to Victoria, Canada. 

The three prisms will form the double-pass cross-disperser of the SPIRou spectrograph. Their specific materials ensure both high throughput and adequate dispersion in the full SPIRou spectral range (0.98-2.35 µm), with large thicknesses of up to 150 mm and 47 mm for the Infrasil and ZnSe prisms respectively, and a clear square aperture of 180 mm for the whole prism train.  The R2 échelle grating is replicated on a Zerodur substrate, features 22 grooves per millimeter and diffracts the beam in a quasi-Littrow configuration. The flat mirror, also in Zerodur, allows the beam to be folded back to the parabolic mirror and to obtain as compact and efficient an optical system as possible.

Zerodur folding flat mirror

Visual inspection of the Zerodur folding flat mirror, at NRC-H

R2 échelle grating

The SPIRou R2 échelle grating in Zerodur on its testing mount

Measurement on the R2 échelle grating

Wavefront error measurement of the SPIRou R2 échelle grating at NRC-H

For all optical elements, matching specifications is essential for achieving the performances that our science goals require.  Inspection and measurements were carried out by our optical experts at NRC-H and CFHT (John Pazder and Greg Barrick), using dedicated facilities of NRC-H. Wave-front precision, throughput, coating quality and the possible presence of cosmetic defects are among the points that were checked. All optics are now ready to be integrated in their mounts.