LESIA used Zemax to design an imaging optical system that optimizes image formation on the sensor while minimizing optical aberrations. However, although PISÉO also has Zemax, our analysis of stray rays outside the main image was done with LightTools – a more suitable non-sequential simulation software. LightTools allows the modeling of sources and surfaces to study irradiance maps and intensity indicators, thus identifying the contributions of stray rays, including “ghost images”.
PISÉO exploited LightTools to its limits. In doing so, we determined the origin of the stray rays, evaluated their energy in relation to the main image, and proposed corrective actions. Moreover, the LightTools “imaging module”, used in beta version, made it possible to isolate the critical paths of stray light and optimize the results by focusing on the most relevant trajectories, thus guaranteeing a good signal-to-noise ratio.
 
The optical simulation used a far-field source to represent the instrument’s field of view (FoV). An approach combining an ideal lens and a point source, positioned at three strategic locations (optical axis, horizontal edge, and vertical edge), allowed for accurate analysis of stray rays relative to the main image. These analyses highlighted distortions and aberrations due to an excessively wide FoV, causing artifacts and asymmetries on the sensor. An optimization of the optical design then corrected these defects, aligning the simulated performance with the system’s real-world requirements.
 
The lenses were accurately modeled in LightTools by integrating material characteristics and optical properties, including Fresnel losses and internal reflections. The filters were simulated from a mechanical model, with specific surface treatments based on the spectral transmittance data provided by CILAS. The surfaces of the filters and spectral bands were modeled with distinct treatments to ensure high accuracy. The diaphragm, treated with a black absorbent paint validated by CNES, was designed to absorb non-reflected rays.
 
Ultimately, the analysis of stray light, with a precise threshold, revealed that the main sources of disturbance were internal reflections in the sapphire window and lenses, and interference between filters. Anti-reflective coatings were subsequently optimized to reduce these reflections, and studies were conducted to evaluate the impact of dust and surface roughness. Moreover, irradiance maps were used to identify critical areas and refine treatments to minimize stray light.
 
Analysis of the impact of the mechanical system on stray light revealed that internal reflections can cause ghost images. To counteract this, PISÉO used and then adjusted a mathematical diffusion model, supplemented by BRDF measurements at 850 nm and 1550 nm. These data made it possible to precisely evaluate the absorbing materials and select the most suitable one to reduce stray light.
 
Finally, the analysis evaluated the “off-axis” stray light caused by the contribution of the Sun. The asymmetric deflector used for the VERITAS mission was modeled to simulate the interactions between visible and infrared rays. A uniform planar source was adopted to improve the efficiency of the simulations, reducing the detection threshold and the computation times. These optimizations made it possible to choose a suitable material and design for effectively reducing stray light.