Labelled by the AMU and the CNRS, the POLARIS shared technology platform offers polishing and optical metrology services. It has polishing equipment covering the full range of dimensions from 10 mm to 1.2 m in diameter. For optical metrology, it is equipped with an interferential microscope, several optical interferometers with spherical calibres with apertures up to f/1.2 and planes up to 250 mm, as well as a UV-Vis-NIR spectrophotometer.
Polishing by active optics method
The platform implements the active optics method for polishing aspheric and free-form surfaces. This method, developed by LAM researchers over many years, consists of elastically deforming the substrate during the surfacing stages. Relaxed after obtaining the spherical shape, the surface takes on the desired aspherical or free-form shape.
Originally developed for Schmidt blades, the method has been used to manufacture variable-curvature mirrors for the VLTI delay lines in Chile. More recently, taking advantage of the method’s ability to obtain aspherical surfaces of extreme polish quality, the method has demonstrated its effectiveness for instruments dedicated to the search for extrasolar planets, enabling them to produce very high contrast images. The platform has delivered the three toric mirrors for the VLT-SPHERE instrument (Chile) and several toric mirrors for HiCAT, a test bench installed at the Space Telescope Science Institute (STScI) in Baltimore, as well as off-axis parabolas for NASA’s Nancy Grey Roman space telescope.
Active optics method
The use of the shared platform makes it possible to carry out steps a) and b) using the LAM’s active optics method. In this case, after active deformation of the mirror, the surface to be roughened and polished (steps a and b) is a simple sphere. The difficulty of realization which was linked to the aspherical surface is thus replaced by the simplicity (in relative terms) of realizing a spherical surface.
It is this transposition which constitutes the significant time saving in the realization of the surface. In addition, the “spherical” polish converges much faster, so this time saving also includes the reduction in the number of intermediate measurements. Step c) is necessary to finish the part.
Over the years, a large collection of optical measurement instruments has been built up to ensure the acceptance of the optics installed in the various astronomical instruments designed by the LAM. The Fizeau interferometers form the basis of this collection, allowing the measurement of the shape of lenses and mirrors or the precise alignment of these components within their mounts. The interference microscope allows the characterisation of these surfaces at smaller scales, enabling roughness control (thus ensuring their scattering behaviour) or pattern metrology for micro-structured components such as diffraction gratings or phase masks. The spectrophotometer is essential for the measurement of transmission or spectral reflectivity of optical coatings such as filters or dichroic plates.