The GeoCoronal Imager (GCI) onboard the Carruthers Geocorona Observatory is the primary scientific instrument of the mission. It is designed to measure far ultraviolet light at 121.6 nm (Lyman-alpha) emitted by hydrogen (H) atoms in Earth's exosphere with the sensitivity, accuracy and precision to meet the mission's scientific objectives regarding the nature of terrestrial exospheric structure and dynamics on both global and regional scales. The GCI is comprised of two co-aligned UV imaging systems. The Narrow Field Imager (NFI) acquires nearly continuous images of exospheric Lyman-alpha radiance near and above the Earth's limb at relatively high spatial and temporal resolution, while the Wide Field Imager (WFI) uses relatively higher optical sensitivity and a wider field of view to detect faint Lyman-alpha emission from the exosphere's outermost extent. Both imaging channels feature identical active pixel sensor cameras, gain-intensifiers, and 6-position optical filter wheels. This paper outlines the instrument design requirements, informed by mission science goals, as well as its performance as measured in the vacuum ultraviolet laboratory test and calibration.

Straylight (SL) characterization using ultrafast time of flight imaging (ToF) has been demonstrated for the testing of refractive telescopes, using a streak tube with a femtosecond laser. It was shown that individual SL contributors such as different ghost reflections and scattering features can be measured individually and identified by temporal discrimination due to the specific optical path length of each of them. This allows to analyze them individually for a better understanding of straylight properties in instruments. Recently, we have used the ToF approach to characterize a testing facility that was then used in the frame of the calibration campaign for the Narrow Angle Camera (NAC) of the Earth Return Orbiter mission. The facility itself could generate its own SL that has to be retrieved from that coming from the instrument. Due to the large facility dimensions, optical path lengths can be discriminated by using a low temporal resolution that is enabled by picosecond lasers associated to a SPAD detector. At the end, the SL coming from the facility can be reverse engineered to find its origin and either removed by facility adaptation or by processing.

The Ultraviolet Imager (UVI) is a challenging instrument developed in the frame of the SMILE mission, a collaboration between ESA and CAS. The UVI instrument is a CMOS-based ultraviolet camera developed to image Earth's northern auroral regions. It is centered on the 160-180 nm UV waveband, with a 10 degrees x 10 degrees field of view. At the core of the instrument, four thin film-coated mirrors guide light into its detector and ensure most of the signal filtering, crucial to achieve an out-of-band rejection ratio as low as possible to reject light from solar diffusion, dayglow and unwanted atomic lines. We developed an interferometric coating based on an MgF2/LaF3 multilayer stack deposited by ion-assisted electron-beam deposition. We gradually improved our evaporation setup to reach a high degree of homogeneity, precision and repeatability on the material thicknesses, over the entire mirrors surface. The reflectivity maximum is above 85% and the wavelength at which it occurs is adjustable within 1 nm, while the out-of-band reflectivity between 120 and 155 nm and between 200 nm and 1100 nm is kept below 6% on average never exceeding 8 %. The coating has been space qualified and shows stable performances in conditions representative of the instrument operation environment (thermal cycling under vacuum, radiations, UV exposure...).

Stray light (SL) has emerged as a primary limiting factor for space telescopes. Pre-launch testing is essential for validating performance and identifying potential issues. However, traditional methods do not enable the decomposition and identification of individual SL contributors. Consequently, when problems arise, resolving them often involves a cumbersome and risky trial-and-error approach. The time-of-flight (ToF) method was recently introduced, employing a pulsed laser source and ultrafast sensor to characterize individual SL contributors. A proof of concept was achieved using a simple three-lens system. In this paper, we apply the ToF method to a real space optical system: the spare model of the CoRoT baffle. We successfully measured individual SL contributors over a dynamic range of 10−11, identifying direct scattering on vane edges and two-step scattering paths. Our results provide a performance breakdown, differentiating intrinsic baffle SL from contributions arising from experimental conditions. Notably, the ToF method allowed us to discriminate air scattering, eliminating the need for expensive vacuum testing. The ToF provides unparallel insights, including defects identification. For instance, we identified the presence of localized dust particles causing significant SL. These results confirm the utility of the ToF method even for the most challenging space systems.

The Carruthers Geocorona Observatory is a NASA Heliophysics mission designed to study the variability of Earth's hydrogen exosphere. Launching in 2025, the Carruthers GeoCoronal Imager (GCI) will observe the exosphere at Far Ultraviolet wavelengths from an Earth-Sun L1 vantage point. The GCI consists of two co-aligned imagers that simultaneously provide both wide field observations of the entire exosphere, in addition to high spatial resolution observations near the Earth's limb. The optical prescription for both the narrow field imager (NFI) and wide field imager (WFI) is discussed, including critical analyses that were performed during the design phase of the project. A deterministic alignment approach was adopted to verify performances of the imagers at visible wavelengths prior to verification in the vacuum ultraviolet. The details of this alignment plan, along with opto-mechanical considerations and requirements are discussed in detail. Finally, we discuss the imaging performance of the system in the ultraviolet utilizing a ground calibration facility previously developed for another NASA spaceflight mission.

Stray light characterization using ultrafast time of flight imaging was demonstrated recently for the testing of refractive telescopes, using a streak tube with a femtosecond laser.It was shown that individual contributors such as ghost reflections and scattering features can be measured individually and identified, allowing unprecedented understanding of stray light properties in telescopes.This opens the door to the development of higher performing instruments, with stray light properties significantly reduced compared to the state of the art.In this paper, we will present the latest advances in the domain of stray light characterization by ultrafast time-of-flight imaging.This includes the characterization of imaging instruments, and the use of the time-of-flight measurements for reverse engineering instruments properties.In addition of using the time-of-flight approach for characterizing instruments, we will show that this method can be used to validate and improve conventional stray light measurement devices and facilities.In the case of large facilities, the typical optical path lengths is of the order of several centimeters to tens of meters.Therefore, in that case streak cameras can be replaced by a less expensive alternative, namely SPAD detectors.We will present the dedicated SPAD detector that we developed and the results obtain in the validation and improvement of the stray light facility for the FLEX Earth observation instrument.This system will be also used in the near future also for the NAC instrument in the ERO mission to Mars.

ALTIUS is the next ESA limb-sounding mission for monitoring of stratospheric ozone at high vertical resolution and of NOx molecules and aerosols.With a platform based on the PROBA-NEXT concept flying in a Sun-Synchronous orbit, the data provided by the ALTIUS Mission will support the scientific community addressing key questions related to atmospheric chemistry composition and climate changes.The ALTIUS Instrument features wavelength-tuning capabilities in the UV (250-355nm), VIS (440-675 nm) and NIR (600-1020) bands using a Fabry-Perot interferometer (FPI) stack in the UV band and an Acousto-Optic Tunable Filter technologies (AOTF) in the VIS and NIR bands.This Instrument topology allows ALTIUS to perform 2D imaging with high resolution in the vertical profile of the Earth limb.The optical layout of 2D imagers, characterized by a more extensive field of view (FOV), makes them more susceptible to stray light issues in comparison to more conventional optical designs such as grating systems.These particular design aspects in combination with the use of novel technologies (FPI's and AOTF's) and the irradiance distribution of the observed bright limb scenes makes the stray light prediction very interesting and complex.An accurate modelling of scatter contributors involving optical and mechanical surfaces is, therefore, required.Due to the cost-effective model philosophy applied for the ALTIUS Instrument, no hardware model is available for stray light correlation purposes prior to the Instrument Proto-flight.Hence, a study was performed to benchmark the stray light analyses results obtained with Optic Studio with the ones obtained with FRED.This paper provides a description of the optical modelling features of the ALTIUS Instrument with specific attention to the novel optical devices, AOTF and FPI stack.It also addresses the particularities and differences observed when modelling the Instrument using two different commercial optical design suites.A comparison of scattered stray light computations for the ALTIUS Instrument ran in OpticStudio and FRED is also presented highlighting reflections on modelling approach and used mathematical models, with an outlook on consistency at L1. Finally, lessons learned from this exercise are presented along with the conclusions and plans for future work.

In order to characterize and calibrate the Metimage spectro-radiometer instrument response in Solar Spectral Bands, an Optical Ground Support Equipment (OGSE), a sun simulator has been designed and developed at CSL. This Sun Simulator (SUSI) is designed for wavelength ranging from 400 nm to 2300 nm. In order to reach radiance and both spatial and angular homogeneity requirements a specific design allowing mixing up to 4 plasma lamp sources has been designed and implemented. This mixing and homogenizing device is placed at the focal plane of an Off Axis parabolic mirror which will produce an 0.5° full divergence beam over a pupil of 350 mm. First the design and its challenges will be given. Simulation of SUSI performances are then proposed. Finally, experimentally measured performances of the OGSE are presented.

Stray light characterization using ultrafast time of flight imaging was demonstrated recently for the testing of refractive telescopes, using a streak tube with a femtosecond laser. It was shown that individual contributors such as ghost reflections and scattering could be measured individually and identified through their optical path length with a time-resolved measurement. This allows unprecedented understanding of stray light properties in optical instruments, especially for high-end space telescopes. In this paper, we demonstrate the extension of this method for the validation and improvement of stray light rejection in an optical calibration facility for large space instruments. Here, the stray light paths to be characterized have long range, up to 20 meters. Therefore, the ultrafast sensor considered is a single photon avalanche diode (SPAD) and the illumination is achieved by a picosecond laser. We demonstrate that we are able to measure the individual stray light contributors, for example the scattering on the optical surfaces or the multiple scattering events occurring between baffles and vanes. As each contributor can be identified, the time-of-flight results are used to determine how to improve the optical calibration facility. Moreover, while the measurement is affected also by the detection system intrinsic contribution, this effect can be removed so that the final result only shows the performance of the optical facility. This new approach is extremely useful as it allows pushing further the achievable performances of space telescope characterization, where conventional methods were currently reaching a plateau.

Space telescopes require always better stray light control. However conventional characterization methods rely on the same principles since several decades and we are reaching the limits of their capabilities. We have developed a disruptive approach which solves these limitations: stray light characterization by ultrafast time of flight imaging. By using a pulsed laser source and an ultrafast sensor, we are able to discriminate and identify individual stray light contributors, thus providing the ultimate understanding of the stray light origins in an instrument. In this paper, we present our approach and its potential for the future of space telescopes. We will present the experimental characterization of a refractive telescope with this method, using a streak camera and a fs laser. We will present how we use this approach was used to experimentally characterize, validate and improve a stray light measurement facility, using a SPAD sensor and a ps laser.

Stray light represents a major performance limitation for optical instruments. Analyses are done with ray-tracing software to evaluate the stray light performances of a design and, if necessary, improve it before manufacturing. Accurate simulations, however, require sending a sufficient number of rays. Hence, the process can be very time-consuming. We introduce the concept of stray light entrance pupil (SLEP) and demonstrate how it can be an efficient tool for simulating stray light for point sources. The SLEP defines a pupil over which light entering the optical system generates stray light reaching the detector. When that pupil is smaller than the first lens of the system, rays can be sent only through that pupil instead of the full lens aperture. Therefore, the time required to perform the simulation is reduced. Moreover, the efficiency can be further improved by defining a source with nonuniform ray density. The SLEP method is demonstrated on a wide-angle Earth observation camera and a time reduction up to about 20 is obtained. The SLEP concept can also be used to facilitate experimental characterization. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE).

Shearography nondestructive inspection (NDI) based on the use of thermal stress in studied in view of detecting defects in composite materials. In particular, we would like to extract the content of information that is present in the deformation due to the evolving deformation during the variation of the stress. For that a good approach is to try to apply processing techniques which are largely applied in thermography NDI for the same purpose. We will discuss the necessity of pre-processing the shearography data to make them compatible with thermography inspired processing techniques. After that we will present the statistical analysis based on principal components, and we will discuss the possibility of deterministic analysis based on the temporal behavior during and after heating.

We introduce the concept of stray-light entrance pupil (SL-EP), which can be seen as the analogous regarding stray-light of the entrance pupil in optical design. It takes the shape of the rays footprint which by entering the system undergo stray-light effects before ultimately reaching the detector, modulated by the relative area importance. This paper discusses the properties of the SL-EP and the way it can be computed. The SL-EP can be used to improve stray-light simulations, as it allows to improve significantly the simulation accuracy while limiting the ray-tracing time. For that, sources can be defined based on the SL-EP and non-uniform ray densities can even be used to reach even higher performances. Time reduction factor up to more than 20 times can be demonstrated on the design of the 3MI earth observation instrument. Furthermore, the SL-EP concept can be used to facilitate experimental characterization of spatial point source transmittance maps.

Delamination in carbon fiber reinforced polymer (CFRP) laminated composites is an important problem for industry. Nondestructive inspection (NDI) methods aim at locating such defects. Shearography is a full-field NDI method that could be considered to detect them. Reference plates for assessing performances of NDI methods for detecting delamination use artificial defects of several types introduced in reference CFRP matrices. Although they are standardized for usual ultrasound testing, these artifacts are not necessarily adequate for shearography. We have studied this problem by comparing shearography experiment and simulations by finite element analysis. We show the convergence with experiments on the case of flat bottom hole artifacts. Then we discuss the adequateness of other artifacts through simulations.

The Ultraviolet Imager (UVI) instrument is a very challenging imager developed in the frame of the SMILE-ESA mission. The UV camera will consist of a single imaging system targeted at a portion of the Lyman-Birge-Hopfield (LBH) N2 wavelength band. The baseline design of the imager meets the requirements to record snapshots of auroral dynamics with sufficient spatial resolution to measure cusp processes (100 km) under fully sunlit conditions from the specified apogee of the spacecraft. To achieve this goal, the UVI instrument utilizes a combination of four on-axis mirrors with an intensified FUV CMOS based camera. The mirrors will be coated with spectral selective interferometric layers to provide most of the signal filtering. The objective of these filters is to select the scientific waveband between 160 and 180 nm. The combined four mirrors have to give an out-of-band rejection ratio as high as possible to reject light from solar diffusion, dayglow and unwanted atomic lines in a range of 10-8 - 10-9. Different multilayer coatings are considered and optimized according to the pi-multilayer equation for different H/L ratio and for different angles of incidence. Our theoretical evaluation shows a least a modification of the reflectance spectrum as a function of the angle of incidence, so that the optical beams hitting the different mirrors can have different optical properties depending on the optical fields and the distribution of the rays on the pupil. In this paper the effect of fields and coating homogeneity on the spectral throughput of the UVI instrument will be assessed and described.

PLATO (PLAnetary Transits and Oscillation of stars) is a medium-class space mission part of the ESA Cosmic vision program. Its goal is to find and study extrasolar planetary systems, emphasizing on planets located in habitable zone around solar-like stars. PLATO is equipped with 26 cameras, operating between 500 and 1000nm. The alignment of the focal plane assembly (FPA) with the optical assembly is a time consuming process, to be performed for each of the 26 cameras. An automatized method has been developed to fasten this process. The principle of the alignment is to illuminate the camera with a collimated beam and to vary the position of the FPA to search for the position which minimizes the RMS spot diameter. To reduce the total number of measurements which is performed, the alignment method is done by iteratively searching for the best focus, decreasing at each step the error on the estimated best focus by a factor 2. Because the spot size at focus is similar to the pixel, it would not be possible with this process alone to reach an alignment accuracy of less than several tens of microns. Dithering, achieved by in-plane translation of the focal plane and image recombination, is thus used to increase the sampling of the spot and decrease the error on the merit function.

The Ultraviolet Imager (UVI) instrument is a very challenging imager developed in the frame of the SMILE-ESA mission. The UV camera will consist of a single imaging system targeted at a portion of the Lyman-Birge-Hopfield (LBH) N2 wavelength band. The baseline design of the imager meets the requirements to record snapshots of auroral dynamics with sufficient spatial resolution to measure cusp processes (100 km) under fully sunlit conditions from the specified apogee of the spacecraft. To achieve this goal, the UVI instrument utilizes a combination of four on-axis mirrors with an intensified FUV CMOS based camera. The mirrors will be coated with spectral selective interferometric layers to provide most of the signal filtering. The objective of these filters is to select the scientific waveband between 160 and 180 nm. The combined four mirrors have to give an out-of-band rejection ratio as high as possible to reject light from solar diffusion, dayglow and unwanted atomic lines in a range of 10-8 – 10-9. Different multilayer coatings are considered and optimized according to the π-multilayer equation for different H/L ratio and for different angles of incidence. Our theoretical evaluation shows a modification of the reflectance spectrum as a function of the angle of incidence, so that the optical beams hitting the different mirrors can have different optical properties depending on the optical fields and the distribution of the rays on the pupil. We will evaluate the effect of fields on the spectral throughput of the UVI instrument based on its optical design. This analysis will be done using the Code V ray-trace software and proprietary scripts.

The Sentinel-4 mission (S4) is part of the Global Monitoring for Environment and Security (GMES) initiative and covers the needs for continuous monitoring of Earth atmospheric composition and air pollution [1].