Study of the optical design of an elementary spacebased interferometer for the detection and characterisation of exoplanets

Abstract

This master thesis is dedicated to the optical design of an elementary spacebased interferometer for the detection and characterisation of exoplanets. More precisely, this work proposes a Bracewell-type interferometer design allowing light reduction and collimation with the use of two telescopes composed of two 45° off-axis afocal parabolic mirrors. This design aims at minimising the number of optical surfaces required to perform interferometric measurements as well as the complexity of the design in order to maximise the compactness and decrease the development costs. The sensitivity of this system to telescope pointing errors is investigated. The beam propagation deviations are corrected by a tip-tilt mirror. The position of this tip-tilt mirror is optimised to minimise the transverse shift of the beams induced after reflection. The use of a single mode fibre as modal filtering reduces the sensitivity of the system to wavefront distortions. However, the final design presents a variation in optical path length for the rays entering the entrance pupil at its extremities, namely the sagittal and meridional rays. Theses rays experience an optical path variation (with respect to the chief ray) proportional to ±1.74α, where α is the ray field angle in degrees with respect to the line of sight incident on the telescope. Thus, given the current capabilities of deformable mirrors which are able to perform a maximum stroke of ∼ 15 µm (compensating therefore about 30 µm of maximum wavefront distortion), this limits the telescope pointing accuracy to 0.02 degrees. Considering the capabilities of current attitude determination and control systems as well as pointing sensors precision this seems to be a totally achievable accuracy without significant costs increase. Finally, options for adapting the design to manage higher error field angles are proposed.