2016 September: Metrology of the mounted SPIRou Parabola
The SPIRou parabola in its mount was shown to have nominal optical performances
Integrating the SPIRou parabola in its mount implied bonding and mechanical clamping close to the mirror optical surface, potentially able to induce stresses and deformations, and thus to degrade optical performances. Although both mirror and mount were designed to minimize these effects, verification that the mirror optical performance is nominal was required.
To carry out this verification, the LAM (Laboratoire d’Astrophysique de Marseille) implements a dedicated optical bench based on the infinity focus method. The two clear apertures of the SPIRou mirror were successively measured using an interferometer and a large (400 mm wide) reference folding flat, both before and after mounting, in order to quantify the contribution of the mount to the wavefront error.
After a pre-alignment step, the fine tuning of the fringe pattern on the interferometer revealed a very low amount of form error (in particular a minimization of the main Zernike terms like coma, astigmatism and spherical aberration). The full tests indicated no significant mechanical stress caused by the mount and nominal optical quality of the mounted SPIRou parabola (wavefront error < 60 nm rms) on both clear apertures.
This validation step was performed with the expertise of Johan Floriot, Amandine Caillat and Kjetil Dohlen, from the LAM optical team.