Using a machine learning-based hyperspectral image classification method for stray light pollution level assessment

Abstract Solar coronagraphs often include narrowband filters located at an image of the telescope pupil. These filters are commonly illuminated with collimated light coming from an intermediate telecentric image of the corona. In this configuration, the intermediate image is located at the front focal plane of a collimator, while the telescope pupil is formed at its back focal plane. Collimators employed to illuminate the filter have long focal lengths and usually need to be designed as telephoto lenses for the purpose of minimizing the distance from the intermediate image to the pupil. This is particularly important for spaceborne instruments, whose dimensions are strictly constrained. However, conventional telephoto lenses cannot be employed in this case as they unavoidably increase the front-to-back focal length of the system. Reducing the number of optical elements in coronagraphs is also essential to minimize stray light and ghost images. In this work, we derive the equations for two-component telephoto lenses needed to minimize the front-to-back focal distance. We apply these equations to design a collimator for CMAG coronagraph. We study the image quality as a function of the telephoto ratio, we athermalize our design and we carry out a tolerance analysis. We demonstrate that the front-to-back focal length can be reduced by $$\textbf{45}\,\mathbf {\%}$$ for a field of view as large as 1.28° while keeping the root mean square wavefront error below $$\varvec{\lambda /20}$$ .

Abstract The Nuclear Spectroscopic Telescope Array (NuSTAR) enables detailed high-energy X-ray observations from 3–79 keV, but its performance can be constrained by telemetry saturation when observing bright sources, leading to reduced effective exposure times. In this study, we investigate the use of serendipitous stray light (SL) observations to infer properties of an X-ray bright source in comparison to focused data. Our case study is performed on the neutron star low-mass X-ray binary GX 340+0, a prominent Z source, where we execute a spectral analysis comparing 25 SL and 7 focused NuSTAR observations. Our findings demonstrate that SL observations can significantly enhance long-term temporal coverage detecting variations in the thermal components of the system across the baseline of the mission, which could not be inferred from focused observations alone.

New modeling of the stray light noise in the main arms of the Einstein Telescope

This article demonstrates that 3D-printed parts can replace metal parts in optomechanics in the correct circumstances. Three examples are shown: a clamping fork for pedestal holders where stability is important, an adjustable mirror holder where the rigidity is the main criterion, and a stray light shield where the transmissivity is critical. By combining carbon-fiber-reinforced polymers (CFRPs) with 3D printing, it is possible to produce components that fill the gap between standard 3D-printed plastics and metal parts in terms of strength and stability. These parts are designed to be lighter, more compact, and easier to modify, while keeping good mechanical properties such as resistance to vibration, shape accuracy, and controlled thermal expansion. The article focuses on the application of composite 3D printing on optomechanical components. It compares different methods of composite 3D printing, including fused filament fabrication (FFF) with either chopped fibers or with continuous fiber reinforcement. Three examples from the HiLASE Centre demonstrate how these parts are used in practice, confirming that it is indeed possible to 3D print components that are lighter and cheaper yet still highly functional compared to their off-the-shelf counterparts—for example, lightweight and stiff mounts, shielding against stray laser light, or flexible elements allowing fine mechanical adjustments. Simulations of the deformations are included to compare the printed and metal versions. The article ends with a summary of the benefits and limitations of using 3D-printed composites in optomechanics.

Generation of metal particle contaminants excited from ablated aluminum alloys by stray light

We present a novel dual-mode vibrational and electronic sum-frequency generation (SFG) spectrometer designed to investigate the interface of organic functional materials. Our setup features a custom-designed detection module that effectively suppresses stray light components. The spectrometer is highly versatile, allowing for both vibrational SFG and electronic SFG spectra to be obtained from the same sample by simply switching infrared and near-infrared pulses. We applied this technique to two different types of organic light-emitting diodes. Our results reveal that the charge carrier behavior, reflected by the changes in VSFG and ESFG signal intensities under applied voltage, is distinct for each device.

When performing X-ray observations with a Wolter-I telescope, the presence of bright off-axis sources can introduce unfocused rays, known as stray light, which contaminate the detector and compromise the scientific analysis. Among the different components of stray light, single reflections off the hyperboloid section of the mirror shells often produce arc-like patterns on the detector. These arcs depend on not only the off-axis angle of the source but also the geometrical alignment of the individual shells. In this paper, we introduce the SHell misAlignment Detection for stray light Estimation (SHADE) algorithm, a novel and flexible tool designed to infer the misalignment parameters of individual shells, reproduce the geometry of stray light arcs, and predict its pattern on the detector. SHADE allows us to model each shell displacement with two parameters, $(γ,ξ)$, which represent the tilt amplitude and direction. While the algorithm is general and applicable to any Wolter-like telescope, we demonstrate its effectiveness using a set of XMM-Newton observations of the low-mass X-ray binary GX5-1. As a proof of concept, we recover the best-fit misalignment parameters for a selected shell, obtaining γ = 21.9''^ and ξ = 5.88^ rad. SHADE represents a new approach to diagnosing mirror misalignments from stray light patterns and can support both pre- and postlaunch calibration efforts and future telescope designs.

Abstract In widefield fluorescence imaging of neurons, out‐of‐focus and scattered light from the bright cell body often obscures nearby dim fibers and degrades their contrast. Scanning techniques can solve this problem but are limited by reduced imaging speed and increased cost. In this study, stray light in widefield imaging is greatly reduced by modulating the illumination intensity to different structures. An iterative approach is used to identify fibers by real‐time image processing and target illumination to fibers by a digital micromirror device add‐on to a common widefield microscope. Bright cell bodies are illuminated with minimal light intensity, and in‐focus fibers with high light intensity. This procedure minimizes the background and enhances the visibility of fibers while maintaining a fast imaging speed, low photobleaching rate, and low cost. By updating the targeting pattern, illumination is maintained on the structures of interest, even in moving samples. Using this targeted illumination approach, high contrast, optically sectioned imaging of complex neurons is demonstrated in anesthetized C. elegans, ex vivo mouse brain slice, and restrained zebrafish larva, as well as high‐speed imaging of dynamic changes in C. elegans.

Analysis of the Influence of micrometeoroids on Stray Light of the TianQin Telescope

We report the development and optimization of a dual-path Thomson scattering (DPTS) system in GAMMA 10/PDX, designed for simultaneous measurements of electron temperature and density in both the central cell and divertor-simulated plasma regions. The system comprises a multi-pass central-cell TS and an upgraded end-cell TS (EC-TS) double-pass configuration, both tailored to enhance signal intensity. Short-pass filters were added to the polychromators to improve spectral selectivity, and newly installed apertures in the EC-TS system were used to suppress stray light. Experimental applications involving supersonic molecular beam injection, central electron cyclotron heating, and ion cyclotron heating confirmed successful concurrent observations of plasma parameters in the central cell.

Research on the influence of metal ion deposition on the optical properties and the stray light damage behavior of Al-Based functional gradient films

The development of ultrablack coatings with exceptional absorption (>98%) has historically faced significant scientific and engineering challenges, primarily due to limitations in material selection, structural design, and practical durability. Considering the difficulties in practical applications of ultrablack materials with micro/nano structures and the limitations of planar ultrablack coatings in optical performance, we introduce an innovative integration of conventional planar ultrablack coatings with a specifically engineered trilayer antireflection architecture. This hybrid system incorporates a refractive index distribution (1.6-1.8-1.3 from substrate to air interface) achieved through a "dry-wet hybrid" deposition methodology. This coating exhibits outstanding optical performance, achieving 99.1% average visible-light absorption and peak absorption reaching 99.9%, which is in excellent agreement with theoretical simulations. The hybrid fabrication strategy offers dual advantages: it maintains antireflection performance comparable to vacuum-deposited coatings while significantly enhancing mechanical robustness. After 20,000 bending cycles, the coating retains optical stability with less than 1% performance degradation, showing no delamination or microcracking. A subsequent SiO2 chemical vapor deposition (5 nm) further improves surface durability, limiting optical loss to less than 0.5% while achieving tribological properties on par with those of commercial antireflective films. In addition, it also exhibits a remarkable ability to convert light into heat and is resistant to high temperatures and acid erosion. This breakthrough combines unparalleled optical performance with excellent mechanical properties, demonstrating substantial potential for advanced optical applications such as stray light suppression and photothermal conversion. The dry-wet hybrid approach establishes a paradigm for developing multifunctional optical coatings that surpass traditional single performance.

Radiochromic film is widely recognized for its high spatial resolution and ease of use in radiation dosimetry, but accurate and artifact-free readout remains a challenge. In this work, we evaluate and compare three different radiochromic film imaging systems: a conventional flatbed scanner, a diffuse light field and camera setup, and a custom-built scanning point source and detector system. We investigate each system's optical characteristics, imaging performance, and clinical utility, with a particular focus on small field dosimetry, where conventional systems often fall short. Our results highlight significant limitations in the diffuse light field system due to stray light contamination which leads to dose underestimation in small irradiated fields. The point source system demonstrated excellent accuracy and robustness across all field sizes with no measurable stray light or polarization effects. This work demonstrates that a purpose-built point source scanning system offers a reliable alternative to commercial flatbed scanners.

Optical-enabled identification and interaction provide an integral link between the digital and physical realms. However, nowadays optic-encodings, predominantly reliant on light’s intensity and wavelength, are hindered by environmental light interference and limited information capacity. The introduction of unusual polarization states, such as circular polarization—which is absent from ordinary surroundings—holds promise for higher-dimensional interaction. Here, we propose a circularly polarized optical mapper capable of generating high-entropy, noise-resistant keys, serving as a physical interface for unique interaction process between parties. To materialize this mapper, we developed an automated, in situ synthesis platform that facilitates the self-acting fabrication of robust, solid-state, chiral optical spin constrained assemblies. Our mappers, formed by randomized arrays of discrete assemblies, demonstrate near-theoretical performance in uniformity (0.4917), uniqueness (0.4968), and reliability (0.9355). By emitting high-dimensional spin-polarized light, our mappers enable both far-field readout and near-field authentication, with resistance to stray light interference, offering promising applications in the internet of things, augmented reality, and beyond.

HighlightsWhat are the main findings?This paper describes a compact electro-optical system designed to generate synthetic star fields in apparent motion for realistic ground-based testing of star trackers.Working principles of the system and capability of the first prototype are described: simulation of stars and other celestial bodies, user-defined objects, and disturbance sources.What is the implication of the main finding?Early advancements are reported, enabling the transition from the initial MINISTAR prototype to the next-generation STARLITE system.Initial steps for instrument validation, characterization, and transition to a commercial model, and early results are presented.Star trackers are critical electro-optical devices used for satellite attitude determination, typically tested using Optical Ground Support Equipment (OGSE). Within the POR FESR 2014–2020 program (funded by Regione Toscana), we developed MINISTAR, a compact electro-optical prototype designed to generate synthetic star fields in apparent motion for realistic ground-based testing of star trackers. MINISTAR supports simultaneous testing of up to three units, assessing optical, electronic, and on-board software performance. Its reduced size and weight allow for direct integration on the satellite platform, enabling testing in assembled configurations. The system can simulate bright celestial bodies (Sun, Earth, Moon), user-defined objects, and disturbances such as cosmic rays and stray light. Radiometric and geometric calibrations were successfully validated in laboratory conditions. Under the PR FESR TOSCANA 2021–2027 initiative (also funded by Regione Toscana), the concept was further developed into STARLITE (STAR tracker LIght Test Equipment), a next-generation OGSE with a higher Technology Readiness Level (TRL). Based largely on commercial off-the-shelf (COTS) components, STARLITE targets commercial maturity and enhanced functionality, meeting the increasing demand for compact, high-fidelity OGSE systems for pre-launch verification of attitude sensors. This paper describes the working principles of a generic system, as well as its main characteristics and the early advancements enabling the transition from the initial MINISTAR prototype to the next-generation STARLITE system.

Abstract Stray light is one of the critical noise components in space-based gravitational wave observatories, such as TianQin and LISA. The noise contribution of stray light effect depends on its phase difference and power ratio compared with the nominal light, which are determined by the overall optical system design rather than being solely dictated by the characteristics of the stray light source. To accurately characterize the propagation of interfering lights, this study establishes a comprehensive model including optical path, efficiency, drag-free performance and the mounting stability between the telescope and the optical bench. Furthermore, we examine noise transfer functions based on the time-delay interferometry Michelson combination and conduct simulations to quantify the stray light noise. Our findings identify the constraints imposed by mounting stability on the stray light from telescope and the impact of the distance between the test mass and the optical bench on the stray light from backlink. Finally, we propose two stray light noise budgets, encompassing requirements for stray light source characteristics, related noise sources, link efficiency, and balanced detection performance.

Deep surveys have helped to unveil the history of past and present galaxy mergers, and, in particular, uncovering their tidal debris and co-located globular clusters (GCs). Euclid’s unique combination of capabilities (spatial resolution, depth, and wide sky coverage) will make it a groundbreaking tool for galactic archaeology in the Local Universe, bringing low-surface-brightness (LSB) science into the era of large-scale astronomical surveys. Euclid’s Early Release Observations (ERO) demonstrate this potential with a field of view that includes several galaxies in the Dorado group. In this paper, we aim to derive from this image a mass assembly scenario for its main galaxies: NGC 1549, NGC 1553, and NGC 1546. We detected their internal and external diffuse structures, and identified candidate GCs. By analysing the colours and distributions of the diffuse structures and candidate GCs, we can place constraints on the galaxies’ mass assembly and merger histories. The results demonstrate that feature morphology, surface brightness, colours, and GC density profiles are consistent with galaxies that have undergone different merger scenarios. We classify NGC 1549 as a pure elliptical galaxy that has undergone a major merger. NGC 1553 appears to have recently transitioned from a late-type galaxy to early type, after a series of radial minor to intermediate mergers. NGC 1546 is a rare specimen of galaxy with an undisturbed disk and a prominent diffuse stellar halo, which we infer has been fed by minor mergers and then disturbed by the tidal effect from NGC 1553. Finally, we identify limitations specific to the observing conditions of this ERO, in particular, stray light in the visible and persistence in the near-infrared bands. Once these issues are addressed and the extended emission from LSB objects is preserved by the data-processing pipeline, the Euclid Wide Survey will allow for studies of the Local Universe to be extended to statistical ensembles over a large part of the extragalactic sky.

Trilobites had a unique visual system in which the focusing component of their compound eyes were calcite crystals. Biogenic calcite emits photoluminescence (PL) in the blue-green part of the visual spectrum in all directions when excited by long-wavelength ultraviolet (UV-A) radiation. This PL had no information about the image being formed by the incident light from objects (such as other animals and rocks) near the trilobite and would have acted as stray light, thereby obscuring its vision. The PL spectrum from biogenic calcite from extant red algae is used to model the PL emitted by the calcite in the eyes of trilobites. The visible range and the wavelength sensitivity of the extant marine crustacean Squilla empusa (a distant living relative of the trilobites) is used to model the vision of trilobites. In the first theoretical calculation, the magnitude of this PL is estimated using data from extant red algae (including the concentration of its PL centers) and is compared to the expected photon noise in the visible sunlight directly from the illuminated objects. Due to the differences in the transmission of visible and UV-A light by seawater, PL stray light obscuration is expected to lessen with depth. A second more general calculation determines the dependence of this obscuration effect on ocean depth, independent of the concentration of PL centers in the calcite, showing that the obscuring effect of PL diminishes with depth. This PL is insignificant at 10 meters deep and, possibly, at all depths.

The Tokamak à Configuration Variable has a unique Fast Ion Loss Detector (FILD) that allows the simultaneous measurement of co- and counter-current fast-ion losses in forward or reverse magnetic field. Recently, multiple novel upgrades have been implemented that further extended its capabilities. The optical system throughput was improved by adding anti-reflective coatings to the internal components, in direct view of the camera, and installing a low wavelength filter to reduce unwanted stray light. The mechanical rigidity was increased by adding additional contact points closer to the heat shield, which significantly reduced the system vibration amplitude during operation. The entrance slit pinholes were reduced from 1 to 0.8mm, obtaining an energy resolution improvement of over 40% for only an ∼20% signal level decrease. Finally, a rotary system now permits the FILD head angle to be scanned from ±30° on a shot-to-shot basis. These upgrades, described herein, greatly increase the functionality of the FILD diagnostic.