2016 July: SPIRou off-axis parabola delivered
The silver-coated off-axis parabola was delivered by Thales-SESO
The collimator of the SPIRou spectrograph is an off-axis parabola cut from an 880 mm parent and coated with protected sliver. With a curvature radius of 2381 mm and a focal length of 1220 mm, the SPIRou collimator was manufactured and coated by Thales-SESO in Aix-en-Provence (France), and will work in a cryogenic environment at 80 K.
More specifically, the collimator features two clear apertures on both sides of its optical centre. The first clear aperture (~440 mm, double-pass) collimates the beam coming from the pupil slicer towards the prism cross disperser and the echelle grating, then redirects the diffracted beam at an intermediate focus. The second clear aperture (~355 mm, single-pass) sends the diffracted focused beam back to the fully-dioptric camera and the detector. The optical quality is nominal, with a wavefront error smaller than 60 nm rms on both clear apertures.
The initial blank was circular (see Fig) and featured hollow sections on its backside along two symmetrical axes so that we could choose the best orientation (based on the optical quality) between four cutting options, once polishing was completed.