Stray light and speckle modeling in spaceborne optical systems: LISA and ESA mission case studies

Stray light is defined as any unintended optical radiation propagating through an instrument and reaching its focal plane. Originating from scattering and unintended reflections, it can reduce sensitivity, degrade performance, and, in severe cases, jeopardize mission success. This thesis presents the analysis and characterization of stray light conducted primarily in support of the LISA mission, with further contributions to NEOMIR and NewAthena. For LISA (the first space-based gravitational-wave observatory, planned by ESA for launch in 2037) the study focuses on the Constellation Acquisition Sensor (CAS), designed to receive the incoming laser beam and establish the inter-spacecraft optical link. Coherent speckle arising from rough or contaminated surfaces represents a critical acquisition risk insufficiently captured by conventional tools. To address this limitation, an integrated workflow combining non-sequential ray tracing and numerical wave-optics modeling was developed. A comprehensive opto-mechanical model was implemented in FRED, with surface roughness and particulate contamination parameters accounted for. Both transmitted and received beams were simulated, and out-of-field illumination was analyzed via Point Source Transmittance (PST) scans. Dedicated MATLAB algorithms were developed to simulate static and dynamic speckle. Static speckle statistics were validated via a large-scale Monte Carlo campaign, generating over one million realizations. Dynamic speckle, induced by rigid-body motions of scattering elements, was synthesized by applying controlled phase shifts through the Fourier shift theorem. Experimental validation on an optical bench demonstrated excellent agreement with simulations, with typical discrepancies within 5-10%. For NEOMIR, an all-sky out-of-field analysis using an end-to-end opto-mechanical model and a tessellated sky mosaic identified the most critical observation geometries. Complementary MATLAB tools linked exposure strategy and apparent target motion for signal-to-noise optimization. For NewAthena, canonical stray light analyses mapped illuminated and critical surfaces, assessed absorber and baffling concepts for inter-module gaps, and simulated grazing-incidence reflections and through-pore stray light using dedicated numerical tools.