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SPIRou / Infrared SpectroPolarimeter

2016 December: Optical alignment of the SPIRou spectrograph

Most of the optical components are being aligned on the SPIRou optical bench at IRAP/OMP

Spectrograph alignment

Aligning the spectrograph optics on the
SPIRou optical bench
(©S.Chastanet – CNRS/OMP)

December marked the first steps of the spectrograph optical alignment, carried out at ambient temperature. In practice, this meant locating all optical components and their mounts on the bench at their nominal position, then fine tuning their position with respect to a reference. We successively implemented all components on the bench, starting with the off-axis parabola, the folding mirror, the triple-prism cross-disperser and the R2 échelle diffraction grating.

Additional test optics (mirrors, collimating lens) are also used in conjunction with lasers, reference masks and pinholes to make sure the nominal alignment precision is reached, at selected visible and infrared wavelengths (635, 980 and 1550 nm). The last optical component to be added is the spectrograph camera, whose delivery at IRAP is scheduled for mid January.

The alignment masks were designed at IRAP/OMP. Two of them, featuring 1 mm removable pinholes, are mounted on the bench and define the reference path that the optical beam must follow. Two additional masks can be mounted on the off-axis parabola to ease its positioning. Fine tuning of each optical assembly ensures that the laser beam successfully passes through all pinholes. Each optical mount is indexed on the bench through a dedicated set of stepped brass pins and calibrated shims.

Alignment steps were performed at IRAP/OMP in the dedicated P2IS clean room, by Zalpha Challita, Sébastien Baratchart, Jean-François Donati and Marielle Lacombe, from the IRAP/OMP team and with the precious help of Leslie Saddlemyer from NRC-H (Victoria, Canada).

Spectrograph alignment 2

Overview of the SPIRou spectrograph in the alignment phase, within the P2IS clean room at IRAP/OMP (©S.Chastanet – CNRS/OMP)

Spectrograph alignment 3

R2 échelle grating (right) and triple-prism cross-disperser (middle, the orange and yellow prisms are made of ZnSe and Infrasil respectively) in their dedicated mounts (©S.Chastanet – CNRS/OMP)

Spectrograph alignment 4

Reference masks (left), prism train and folding mirror in their mounts (middle), whose position on the optical bench is indexed with stepped brass pins (© S. Chastanet – CNRS/OMP)

Spectrograph alignment 5

Reference mask with the pinholes on which the beam must be centred (©Z.Challita – IRAP/OMP)