Thurairethinam, Vinooja; (2024) Optical and physical modelling of dichroic filters for the Ariel exoplanet spectroscopy mission. Doctoral thesis (Ph.D), UCL (University College London).

Text PhD_Thesis_Submission_Format.pdf - Accepted Version Access restricted to UCL open access staff until 1 April 2025. Download (29MB)

Abstract

The Atmospheric Remote-sensing Infrared Exoplanet Large-survey mission aims to perform spectroscopic observations of a large population of known exoplanets. For this, Ariel uses a system of dichroic filters and mirrors to spectrally and spatially separate the incoming beam into various channels. These dichroics rely on optical interference within thin-film layers to ensure the transmission and reflection of selective wavelengths of light at a given incident angle. As the mission demands simultaneous observation across extreme wavelength ranges, the necessity for an exceedingly complex broadband dichroic has emerged. Consequently, the uncertainties surrounding its optical performance have also become more intricate. This thesis focuses on the effects of uncertainties on the performance of the Ariel dichroic, D1. However, due to the generality of the effects presented here, the findings of this study hold relevance to any instrument that relies on the use of dichroic filters. As the most complex Ariel dichroic, D1 comprises over 50 coating layers and will operate at cryogenic temperatures over wavelengths of 0.5-7.8 microns. Using transmission line modelling, the spectral performance of multilayer coatings deposited on a substrate material can be evaluated for given coating layer thicknesses, materials, angle of incidence and light polarization. A dichroic recipe in line with the D1 requirements was designed using Monte Carlo techniques, and the tolerance of its performance to uncertainties from the manufacturing process and operational changes in space was studied. With accurate manufacturing recipes and uncertainty amplitudes, this tool can predict variations in optical performance from the propagation of each of these uncertainties for various hypothetical scenarios. The consequent effects on the optical throughput to the Ariel instruments and the point spread functions can hereby be modelled.

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