Earth observation from space is an important scientific and industrial activity that has applications in many sectors. The instruments employed are often large, complex, and expensive. In addition, they generate large amounts of data, which is challenging for storage and transfer purposes. Compressive spectral imaging would be a cheaper, more efficient, and well-adapted technique to perform Earth observation. An interesting architecture is compressive spectral imaging with diffractive lenses, which is extremely compact. This work investigates the possibility of replacing the diffractive lens in this system with a classical refractive lens. Taking advantage of the chromatic aberration of a lens makes the use of expensive diffractive lenses unnecessary. Simulations are performed to test the feasibility of the method. Signal recovery is a basis pursuit solved using the Douglas-Rashford algorithm.

This paper addresses the engineering aspects of implementing a cost-effective instrumental approach to measuring the Bidirectional Reflectance Distribution Function (BRDF) of optical diffusers. The measurement bench presented involves a PerkinElmer UV/Vis/NIR spectrophotometer equipped with a sample angular orientation system based on a compact motorized gimbal. This instrumental approach allows for measuring the BRDF with accuracy comparable to that achieved in much more expensive robot-based gonioreflectometer benches. Its practical implementation is straightforward and does not require significant costs for purchasing additional measuring equipment or adapting the laboratory clean room. The presented approach can be easily adapted to other available spectrophotometers. Additionally, we introduce a relatively simple yet effective data processing method and discuss various aspects of its practical use for analyzing measurement data obtained using the developed spectrometric bench.

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.

Image reconstruction in off-axis terahertz digital holography is complicated due to the harsh recording conditions and the non-convexity form of the problem. In this paper, we propose an inverse problem-based reconstruction technique that jointly reconstructs the object field and the amplitude of the reference field. Regularization in the wavelet domain promotes a sparse object solution. A single objective function combining the data-fidelity and regularization terms is optimized with a dedicated algorithm based on an alternating direction method of multipliers framework. Each iteration alternates between two consecutive optimizations using projections operating on each solution and one soft thresholding operator applying to the object solution. The method is preceded by a windowing process to alleviate artifacts due to the mismatch between camera frame truncation and periodic boundary conditions assumed to implement convolution operators. Experiments demonstrate the effectiveness of the proposed method, in particular, improvements of reconstruction quality, compared to two other methods.

We describe the state of the development of a coherence scanning interferometer to measure local changes in topology and local induced vibrations of a mirror at cryogenic temperatures. The metrology instrument incorporates an optical phase mask and a microlenses array, enabling the acquisition of complete white light interferograms within a single-camera frame. This stands in contrast to traditional temporal phase-shifting interferometers. We design the optical phase mask as a combination of steps of different thicknesses, so each step introduces a different optical path difference to the rays. The local interferograms for each camera frame provide us with information on the local topology of the mirror. The interferogram displacement between camera frames allows us to monitor the mirror's local induced vibrations. In this work, we report the metrology instrument's working principle through numerical simulations and present the latest results of a proof of concept developed at the laboratory. The metrology instrument shown is of extensive usability in diverse applications related to real-time measurements of various fast physical processes and real-time characterization of the optical components topology.

LiteBIRD is a future satellite mission designed to polarization-sensitive mapping of the Cosmic Microwave Background (CMB) anisotropies required to test the theory of cosmological inflation. The standard procedure of the on-ground characterization and calibration of the LiteBIRD Med- and High Frequency Telescopes (MHFT) requires the use a cryogenic reference unit, called also 'SKYLOAD', compatible with the vacuum environment. In this paper, we present one possible baseline design of such SKYLOAD, proposed and currently under development by our research group. The SKYLOAD is composed from anechoic panels, which consist of a planar impedance substrate and a structured layer involving an anechoic cone array with a honeycomb arrangement. Both structured layer and planar impedance substrates should be made of a microwave absorbing polymer, compatible with vacuum cryogenic environment. The SKYLOAD is cooled down by liquid helium to 2 K. Accordingly, in our work, we address also key issues related to no-thermal cracking conditions, as well as the deterioration of the performance of the SKYLOAD assembled from anechoic panels due to their thermal shrinkage when cooling to cryogenic temperatures. Finally, an original manufacturing method of the proposed anechoic panels, involving polymer casting, is currently under consideration.

THz digital holography with camera sensor in off-axis configuration suffers from difficult recording conditions.In order to obtain a high resolution on the object, the latter must be close to the sensor, yielding high recording angles. It was already shown that iterative reconstruction methods can be used to limit the impact of spurious fringes obtained in such schemes. In this paper, we propose an inverse problem-based reconstruction technique that jointly reconstructs the object field and the amplitude of the reference field. Regularization in the wavelet domain promotes a sparse object solution. A single objective function combining the data-fidelity and regularization terms is optimized with a dedicated algorithm based on an ADMM framework. Each iteration alternates between two consecutive optimizations using projections operating on each solution and one soft thresholding operator applying to the object solution. The method is preceded by a windowing process to alleviate artifacts due to the mismatch between camera frame truncation and periodic boundary conditions assumed to implement convolution operators. Experiments demonstrate the effectiveness of the method.

We study the possibility of using the $LiteBIRD$ satellite $B$-mode survey to constrain models of inflation producing specific features in CMB angular power spectra. We explore a particular model example, i.e. spectator axion-SU(2) gauge field inflation. This model can source parity-violating gravitational waves from the amplification of gauge field fluctuations driven by a pseudoscalar "axionlike" field, rolling for a few e-folds during inflation. The sourced gravitational waves can exceed the vacuum contribution at reionization bump scales by about an order of magnitude and can be comparable to the vacuum contribution at recombination bump scales. We argue that a satellite mission with full sky coverage and access to the reionization bump scales is necessary to understand the origin of the primordial gravitational wave signal and distinguish among two production mechanisms: quantum vacuum fluctuations of spacetime and matter sources during inflation. We present the expected constraints on model parameters from $LiteBIRD$ satellite simulations, which complement and expand previous studies in the literature. We find that $LiteBIRD$ will be able to exclude with high significance standard single-field slow-roll models, such as the Starobinsky model, if the true model is the axion-SU(2) model with a feature at CMB scales. We further investigate the possibility of using the parity-violating signature of the model, such as the $TB$ and $EB$ angular power spectra, to disentangle it from the standard single-field slow-roll scenario. We find that most of the discriminating power of $LiteBIRD$ will reside in $BB$ angular power spectra rather than in $TB$ and $EB$ correlations.

The adaptive optics (AO) technology is crucial to achieve the full potential of ground-based telescopes. Over the last three decades, the world has witnessed the successful advent and operation of AO systems on large ground-based telescopes. The complexity and cost of AO systems have largely gone down in the last decade thanks to advances in deformable mirror, wavefront sensor, and real-time computing technologies. Here, we present a robust Rayleigh scattered laser-guided single conjugated adaptive optics system called SALTO, which was designed, built, and tested in the Belgian countryside on a 1-meter class telescope. This project aims to demonstrate the possibility of rejuvenating the scientific goals of medium-class telescopes (1-3 m) with AO technology, as well as to enable optical telecommunication from relatively poor observing sites. This paper discusses the overview of the design, integration and calibration of SALTO. It concludes with the presentation of successful on-sky results at 1.55 μm under 2-3" seeing.

The performance of astronomical space telescopes can be greatly impacted by straylight. That is why characterizing the straylight in such telescopes before they are deployed is paramount. Nowadays such characterization can be done by simulation or by test. Simulation can provide very useful information on the origin of straylight, helping devise solutions to reduce it and improve the performance of the telescope. However, simulation suffers from limitations due to processing power needed and assumptions made in the model which can lead to simulation results quite far from the actual performances. Standard straylight tests on the other hand provide accurate measurement of the straylight but without any insight about its origin, making it difficult to mitigate. Emerging technologies now offer new possibilities for straylight measurement using time-of-flight technics to help identify the origin of the straylight. Such technologies were reviewed and analysed in a first activity called TRIPP (Time-Resolved Imaging of Photon Paths). The results and outcome of this study are presented in the first chapter of this paper. A second chapter then presents the ongoing status of a second activity, SLOTT (Straylight Lidar Ogse verificaTion Tool) which aims to develop a demonstrator for such a time-resolved straylight verification system. With the development and test of such a tool, CSEM and its partners (TAS-CH, Difrotec, CSL, LusoSpace), supported by ESA, hopes to establish new methods to characterize and reduce the straylight propagation in future space-based telescopes.

We present the experimental results of the proof of concept of a metrology instrument developed to characterize the cryogenic mirror of the Einstein Telescope (ET) prototype. ET is a proposed gravitational-wave observatory. The metrology instrument uses the principle of low-coherence interferometry to measure the local change in topology and local induced vibrations of the mirror resulting from the cooling down process. We implement an innovative optical phase mask and a microlens array to obtain a depth map of the mirror on a single camera frame. With our instrument prototype, we can obtain 25 interference patterns of the same mirror spot for each camera frame. Each interference pattern corresponds to a difference Optical Path Difference (OPD). Then by reconstructing the interference patterns, we can measure the mirror’s local topology change and local induced vibration. Moreover, in this proceeding, we describe the analysis of the white-light interference patterns through numerical simulations and depict the metrology instrument’s optical design. Finally, we discuss how we can use the metrology instrument for real-time characterization of other optical components with all the advantages of white light interferometry.

With increasing attention paid to the protection of cultural relics, non-destructive testing (NDT) technologies are thought to be profoundly rewarding, resulting in a widespread uptake of feature-extraction algorithms and defect-detection techniques. Among various alternatives, infrared thermography (IRT) and Terahertz time-domain spectroscopy (THz-TDS) are non-invasive in nature and thus are appropriate for applications involving ancient buildings and artworks. The present study is motivated by the fact that online/offline background segmentation algorithms based on the Gaussian mixture model and widely used in video processing to distinguish between foreground and background, can be successfully integrated as a feature extraction tool with NDT. Since IRT and THz-TDS image sequences resemble a video, the image length and width can be taken as the first two dimensions, and time can serve as the third one. Such sequences can be processed effectively by the background segmentation algorithms, thus can help detect defects of different types at varying depths. The experimental section of the paper considers a tempera painting (a replica of Botticelli’s “The Birth of Venus”) with artificiallyintroduced defects. For benchmarking purposes, the background segmentation algorithms (based on a mixture of Gaussian models) are compared with the fast Fourier transform and principal component analysis to demonstrate the superior performance of the proposed novel algorithms.

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.

Since its invention, holography has been mostly applied at visible wavelengths in a variety of applications.Specifically, non-destructive testing of manufactured objects was a driver for developing holographic methods and all related ones based on the speckle pattern recording.One substantial limitation of holographic non-destructive testing is the setup stability requirements directly related to the laser wavelength.This observation has driven some works for 15 years: developing holography at wavelengths much longer than visible ones.In this paper, we will first review researches carried out in the infrared, mostly digital holography at thermal infrared wavelengths around 10 micrometers.We will discuss the advantages of using such wavelengths and show different examples of applications.In nondestructive testing, large wavelengths allow using digital holography in perturbed environments on large objects and measure large deformations, typical of the aerospace domain.Other astonishing applications such as reconstructing scenes through smoke and flames were proposed.When moving further in the spectrum, digital holography with so-called Terahertz waves (up to 3 millimeters wavelength) has also been studied.The main advantage here is that these waves easily penetrate some materials.Therefore, one can envisage Terahertz digital holography to reconstruct the amplitude and phase of visually opaque objects.We review some cases in which Terahertz digital holography has shown potential in biomedical and industrial applications.We will also address some fundamental bottlenecks that prevent fully benefiting from the advantages of digital holography when increasing the wavelength.

We describe the state of development of a white light interferometer to characterize the cryogenic mirrors for GW detector on operation. We include the first experimental results from the proof of concept of the metrology instrument. The instrument will characterize the topology as well as the vibration of the mirrors. This development takes place in the frame of the E-TEST project. E-TEST is one of the technology demonstrators for the future Einstein Telescope (ET). ET is dedicated to the measure and characterization of gravitational waves. The prototype built by E-TEST includes a large silicon mirror of 40 cm diameter suspended by innovative vibration isolation hanging modules. To reach the detection specification, the mirror is cooled down at cryogenic temperatures around 20 K. Nevertheless, even after the isolation, the mirror may not reach perfect stability once at cryogenic temperatures. Furthermore, the mirror may experience surface topology changes and wavefront deformation due to the extreme variations in temperature and gradient. With our metrology instrument, we can obtain on a single camera frame a set of interferogram maps of the area observed on the mirror at different optical path differences. To do this, we design an innovative phase mask for a white light low-coherence interferometer. In addition, we implement new algorithms for the white light interferogram analysis, avoiding the limitations of the conventional Phase Shifting Interferometry algorithms.

Feedhorn- and orthomode transducer- (OMT) coupled transition edge sensor (TES) bolometers have been designed and micro-fabricated to meet the optical specifications of the LiteBIRD high frequency telescope (HFT) focal plane. We discuss the design and optical characterization of two LiteBIRD HFT detector types: dual-polarization, dual-frequency-band pixels with 195/280 GHz and 235/337 GHz band centers. Results show well-matched passbands between orthogonal polarization channels and frequency centers within 3% of the design values. The optical efficiency of each frequency channel is conservatively reported to be within the range 0.64 $$-$$ 0.72, determined from the response to a cryogenic, temperature-controlled thermal source. These values are in good agreement with expectations and either exceed or are within 10% of the values used in the LiteBIRD sensitivity forecast. Lastly, we report a measurement of loss in Nb/SiN $$_x$$ /Nb microstrip at 100 mK and over the frequency range 200–350 GHz, which is comparable to values previously reported in the literature.

Ptychography is a lensless coherent diffraction imaging method that retrieves both the amplitude and the phase from a set of diffraction patterns. The phase-contrast imaging capacity combined with the unique penetration ability of THz radiation offers potential for applications such as biomedical imaging and nondestructive testing. We present two optimization strategies that allow THz ptychography to achieve efficiently a large field of view (FOV) with high resolution. We show that using a larger probe beam paired with a proportionally enlarged scanning step increases the imaging FOV without causing the imaging quality degradation as long as the overlap ratio is respected. Thus, a centimeter-scale object can be imaged with reasonable numbers of scan positions. We create a structural illumination by simply inserting a porous polymer foam that acts as a diffuser. The presence of the diffuser offers a higher spatial resolution and a better reconstruction for pure-phase objects. We experimentally demonstrate the reconstruction improvement with both an amplitude-contrast USAF target sample and a phase-contrast sample. The proposed THz ptychographic setup successfully images the phase variation distribution of a sample consisting of paraffin-embedded human breast cancer tissue.

Haze is an undesirable effect in images caused when atmospheric particles, such as water droplets, ice crystals, dust, or smoke, are lit directly or indirectly by the sun. This effect can be counteracted by image processing, to bring back the details of a hazy image. Unfortunately, the execution time is often long, which prevents deployment in some video or real-time applications. In this paper, we propose to tune several parameters of the Dark Channel Prior method (DCP) algorithm combined with the fast guided filter. We evaluate the optimization in terms of execution time, and quantify the output image quality using different image quality metrics.

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.

We propose a new inverse-problem-based reconstruction technique to off-axis THz digital holography. The proposed method takes into account the non-uniformity of the reference wavefront and reconstructs the object field as well as the amplitude of the reference field.