The VERITAS (NASA, 2031) and EnVision (ESA, 2028) space missions will carry an advanced spectroscopy instrument, the Venus Emissivity Mapper (VEM), which is designed to map the surface of Venus. Developed by a consortium that includes the DLR (German Aerospace Center), LESIA (Laboratoire d’Etudes Spatiales et d’Instrumentation), and CNES (Centre National d’Etudes Spatiales), the VEM will operate in extreme environments while detecting very low-intensity infrared signals. Stray light is a major source of disturbance that can compromise the quality of measurements by saturating an instrument’s sensors and altering the data. To prevent these risks on the VEM and ensure reliable performance during its crucial missions, LESIA entrusted the analysis of stray light issues to PISÉO.
In this context, the main objective of the mission entrusted to PISÉO was to use our exprertise and advanced simulation tools to identify the different sources of stray light, quantify their influence, and propose actions for reduction.
Source : NASA (https://science.nasa.gov/mission/veritas/overview/)
The VEM instrument (called VenSpec-M for the EnVision mission) is a multispectral imager designed to map the surface of Venus and its lower atmosphere. Mapping is accomplished by observation through narrow atmospheric windows in the near-infrared spectral range. This allows the VEM to detect thermal emissions such as volcanic activity, surface rock composition, and water and cloud formation.
To image the surface of Venus, the instrument features an optical design consisting of a single lens on a filter assembly composed of 14 individual spectral bands, and a two-lens relay optical system to reform the spectrally-filtered image on an InGaAs detector.
Given the low intensity of the scientific signal that the instrument must detect, any external contribution to the effective signal-to-noise ratio must be studied, and if possible, mitigated. As with most optical instruments, one of the main sources of interference is stray light.
Stray light can create undesirable effects such as :
• veiling effect : a reduction of image contrast,
• light halo : parasitic dispersion around intense sources,
• ghost images : uncontrolled internal reflections,
• star effects : light artifacts distorting the interpretation of data.
Stray light control is crucial for ensuring the reliability of the images produced by the sensor and avoiding any scientific interpretation errors. By combining mechanical engineering, advanced optics, and simulation strategies, the VEM system represents a cutting-edge approach that meets the challenges associated with space environments and the mapping of planetary surfaces.
In collaboration with LESIA and DLR, PISÉO identified multiple sources of stray light that could potentially affect the quality of detection provided by the VEM. We then quantified the impact of these sources to assess the performance achievable by the VEM under its intended operating conditions.
These are the different sources of stray light identified by PISÉO :
• Reflections on the lenses : even treated lenses can generate stray radiation that is detected by the sensor.
• Reflections on mechanical elements : despite the use of a baffle, internal mechanical elements can generate unwanted radiation, depending on their geometries.
• Reflections on the sensor : InGaAs sensors like the one used in the VEM can reflect up to 30% of infrared radiation.
• Interference (“crosstalk”) between filters : the location of the 14 IR spectral bands is crucial for minimizing crosstalk.
• Materials and treatments : the optical properties and treatments (in particular, their diffusion) of the materials used significantly influence the propagation of light in optical systems.
• Dust contamination : particles generated during takeoff increase the diffusion of radiation..
• Solar influence : sun glare can saturate the sensor, depending on the orientation and launch date.
In order to precisely quantify these sources contribution to stray light, the VEM optical system was modeled in a powerful ray-tracing tool, and simulations were performed.
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.
PISÉO’s in-depth stray light analysis for LESIA demonstrates the importance of optical design expertise and advanced simulation and modeling tools to ensure the performance of optical instruments in extreme space environments. By identifying critical sources of stray light and proposing innovative solutions, e.g., optimizing coatings, absorbing materials, and mechanical elements such as the baffle and deflector, PISÉO has ensured the reliability of scientific measurements for the VERITAS and EnVision missions.
If you are developing advanced optical systems or are facing stray light challenges, contact PISÉO today. We will ensure you benefit from customized solutions that ensure the performance of your optical systems even in the most demanding conditions.
