CHIME (Copernicus Hyperspectral Imaging Mission for the Environment) is one of the six Sentinel Expansion missions currently developed by EU and ESA, to be launched by the end of the decade. It will provide hyperspectral imagery of the Earth's surface. The mission is expected to have a major impact on our understanding of the Earth's land surface and its processes and will help us to address some of the most pressing challenges facing our planet today by supporting a variety of applications including sustainable agriculture, water management, environmental monitoring, or disaster management. A pair of satellites will each carry a high-performance push broom hyperspectral imager able to measure the reflected light from the Earth's surface in 250 spectral bands over a swath of 130 km, at 30m x 30m ground sampling. The spectrometer system (SPS) consists of three identical spectrometer units (SU). The spectrometers are in a unique staggered and offset arrangement, offering a full swath of about 4500 pixels (including overlaps). The main technological challenges of the spectrometer unit, i.e. the broadband grating and the ultra-accurate long slit have been raised up to TRL6 during the Phase B2 and the final design is now frozen. The main optical and thermo-mechanical analyses are completed as well as the test approach. The manufacturing of the first fully functional Engineering Model is on-going.

The M6C mirror of the Extremely Large Telescope is a Zerodur elliptical flat with wavefront specification of 60 nm RMS at 45 degrees angle of incidence over an optical clear area of 694 x 504 mm. The mirror is being processed at Glyndwr Innovations Ltd via small tool CNC polishing, with a sub-aperture interferometric stitching test being used to measure the surface error. Here, the design and performance of the interferometric stitching test is discussed, complete with a full uncertainty budget to ensure conformance to the specification. Current work on the stitching test has found measurements of the M6C to be repeatable to 4 nm RMS surface error, and an overall uncertainty of measurement of 12.2 nm RMS. Using this test the optic will be polished to a measured surface error of less than 17.5 nm RMS.

A new step was taken in the development of the Chemical-Mechanical mirror polishing (CMP) manufacturing process for so-called “hyper-polished” optics. The main objective was to explore the limits of hyper polishing with the synthesis of Silica oxide and Cerium oxide nanoparticle slurries. Better control of the size and morphology of the synthesized SiO₂ and CeO₂ nanoparticles in the slurries will give us information on the surface chemistry and the interaction that happens while polishing but it will also allow us to take a step forward to meet the challenge of reducing the surface roughness below the Angstrom.

Quantum communication is considered to be a key feature for secure communication e.g. between government organisations or other institutions with high security requirements. Therefore, the QuNET initiative was founded. It focuses on developing a quantum-secure German governmental agency network based on quantum key distribution (QKD). Free-space optical (FSO) links are a valuable part of infrastructure because they can be deployed temporarily, such as at summits or to bridge the last miles where there is no fiber infrastructure. In particular, high-throughput telescopes are of great importance as optical antennas for terrestrial networks or links between mobile nodes. The paper describes the development and manufacturing of an unobscured, afocal four-mirror metal telescope which is already tested for ground-based quantum communication. The off-axis system, operating with a full telescope aperture of 200 mm, a magnification of 20x, and a FOV (field of view) of 3.5 mrad and is designed to yield diffraction-limited performance for an operational wavelength of 810 nm and 1550 nm. The addressed wavefront error-target of the whole system amounts to 66 nm RMS (root mean square). The use case of the telescope implied an operational temperature range of -40 °C up to +50 °C. Therefore, an athermal system is realized using an aluminum-silicon alloy substrate material combined with a nickel-phosphorus polishing layer that allows to reach the required surface quality of the mirrors. To simplify the alignment of the telescope, its mechanical concept relies on a snap-together approach using two substrates with two optical mirrors on a common substrate, each. The manufacturing chain of these two so called mirror substrates is described in detail. That includes the CNC pre-manufacturing, ultra precision diamond turning and subsequent polishing steps. The resulting quality of the mirror substrates as well as of the telescope system is demonstrated by optical measurements using interferometric setups.

Catoptrical systems fall into 2 broad categories: On-Axis and Off-Axis. The alignment of off-axis mirror-based optical systems are non-trivial compared to conventional on-axis mirror designs because of the lack of a central optical axis. To align an off-axis mirror-based laser beam expander (consisting of 2 off-axis parabolic mirrors), we have developed an alignment technique that is systematic, quantitative, and reproducible. This is a quick and efficient method of alignment that is based on wavefront sensing and its corresponding correction based on the Zernike polynomial coefficients. In this paper, we highlight the key elements of the beam expander’s design and detail the steps of the opto-mechanical alignment process as well as the performance results of the system after successful alignment.

Traditionally, the Fresnel-type lens design often assumes thin lenses, since lenses of finite thickness cannot be completely described in analytical terms, then a numerical solution has to be found. We implement an exact ray trace considering a plane wavefront incident by reducing as much as possible the area of light concentration, while increasing the concentration ratio of energy. In such a way that we have mathematically determined a function to compute groove angles, to refract a bundle of rays from each planar echelon, which are propagated into a predetermined area of energy collection. In addition, we can evaluate the performance of the Fresnel-type lens design for a solar collector. Thus, an analytical formula to calculate the optical efficiency is obtained, in terms of the geometrical losses widely studied for nonimaging systems, such as blocking losses. Finally, the proposed aspherical Fresnel lens design is fabricated using a commercial 3D printer and subsequently polishing the surface for better performance. In this fashion we could test the performance of the manufactured prototype.

In this work we propose a procedure to divide a regular off-axis conic surface, which defines the parent surface represented by a parametric equation, in order to describe the optical surface for an off-axis Fresnel-type reflector, in such a way that this mathematical representation allows us to implement an exact ray trace considering a plane wavefront incident on the surface. Additionally, we can simulate the performance of a preliminary prototype proposed for designing a solar collector.

Ion beam sputtering (IBS) is the state-of-the-art coating technique to produce highest quality optical filters and laser optics. In both fields the requirements regarding surface flatness have increased through the last decade. Since IBS thin films usually show a high compressive intrinsic stress, a suitable compensation approach as well as the precise determination of the material dependent stress values are essential to fulfill challenging specifications. This contribution compares intrinsic stress results for several coating materials realized with different IBS machines in side sputtering and sputter up configuration. Furthermore, a stress compensation approach is discussed on a concrete example including the influence of the substrate material. As a major topic the temporal evolution of the coating stress is investigated. Finally, we present a post coating process step to reduce the surface irregularity around the substrate rim induced by the fixturing.

Most corneal topographers are based on the Placido disk, which is consists of a set of concentric rings illuminated in black and white. Each luminous ring is projected onto the corneal surface and its image produced by reflection is captured by a camera. When the corneal surface is symmetrical and aligned with respect to the optical axis of the corneal topographer, the image consists of concentric rings; therefore, displacements occur only in the radial direction. However, if the cornea lacks revolution symmetry, the image suffers deformations in both radial and azimuthal directions; therefore, corneal topography depends not only on the radial direction, but also on the azimuthal direction. This is known as the skew ray error. In this work, we present a numerical analysis on the effects on the corneal topography caused by skew ray error and show the advantages of using illuminated elongated ellipses, which are designed using exact ray tracing, to recover the corneal topography considering a conical corneal topographer.

This work proposes the design of an off-axis null-screen that is displayed on an LCD screen and allows characterizing the surface quality of a sliding seismic isolator. For this purpose, we design a null-screen with a pattern of quasi-elliptical dots drawn on it, so that its image, which is formed by reflection on the concave surface, becomes a precise set of circular targets if the surface under test coincides with the designed surface. It is discussed how to integrate the off-axis measurement system to calibrate it by testing a reflecting concave surface, as a result optical parameters are obtained, at the surface vertex, such as radius of curvature, conic constant, and some deformation coefficients necessary for the characterization of the deformations associated to the test surface with respect to the design surface. Finally, based on the preliminary results, the advantages and disadvantages of the proposed test are discussed.

We study different approaches to describe the evolution of wavefronts refracted through two simple lenses forming an achromat separated by finite distance (dialyte for short), assuming a plane wavefront incident propagated along the optical axis impinging on the optical system. This allows us to quantitatively evaluate the best optical design containing the minimum amount of spherical aberration produced by the optical system under test. Based on this study, we have implemented an interferometric array for testing a dialyte placing a reference mirror at predefined position along the optical axis, which permits to compensate the optical phase and properly produce a null interferogram to evaluate the performance for this kind of optical systems.

Measurement and fabrication process for a reference concave mirror with NA 0.9 is explained. The mirror's surface figure error is measured by using a Fizeau interferometer and a computer-generated hologram (CGH). The result is compared with a stitched figure error using a small NA transmission sphere. Also, the system error is calibrated by a Random Ball Test (RBT). Ion beam figuring (IBF) is employed to enhance the surface figure of the reference mirror. Achieving a high NA of 0.9 opens new opportunities in advanced metrology and optics applications.

Safran Reosc was awarded the manufacturing of the Secondary Mirror of the Extremely Large Telescope by ESO in 2016. The secondary mirror is a 4-meter convex mirror, the world’s largest convex precision mirror ever made. As of spring 2024, this mirror is in its final phase of polishing. It is regularly controlled on a dedicated interferometric test bench (ITB) specifically designed to achieve the best accuracy on such a large mirror. The test bench was thoroughly validated in September 2023 and all its requirements were demonstrated. This bench includes a convex-concave test plate together with a unique custom interferometer system, both specifically designed and manufactured for this test bench. In fact, the specific requirements of this very large mirror forbid the use of off-the-shelf component or interferometer and it was necessary to design a specific interferometer to cope with all the requirements. Furthermore, it was necessary to manufacture an almost perfect 2-m reference plate to control the mirror. It took about 3 years to manufacture this reference plate with the requested accuracy. The test bench makes a measurement on 24 sub-pupils which are stitched together to reconstruct the final surface error. The full aperture measurement and the map stitching lasts around 2 hours. Specific sensors are used to monitor the stitching process and the stitching algorithm had to be adapted in order to fulfil the stringent performance requirements. In this paper, we present the main features of the bench, the alignment strategy, the test results obtained on the bench and the stitching algorithm developed to reconstruct the surface error.

In 2016, Safran Reosc was awarded the manufacturing of the Secondary Mirror of the Extremely Large Telescope by ESO. The secondary mirror is a 4-meter convex mirror, the world’s largest convex precision mirror ever made. This mirror is currently in its final phase of polishing. It is regularly measured on a dedicated interferometric test bench specifically designed to achieve the best accuracy on such a large mirror. The mirror is quite thin compared to its diameter and one challenge was to design a supporting tool able to hold the mirror during the interferometric measurements without stress and deformation. The thickness is about 100 mm for a 4m diameter, leading to a ratio of 40 between thickness and diameter. Such a thin mirror requires a very good control on the force field applied during the measurement, which shall be exactly the same as the one seen in the telescope. An accuracy on each force applied in the range of 0.1 to 0.01% is requested. In order to guarantee this performance, the full system had to be thoroughly designed and tested, to avoid any bias, which could degrade the optical performances at the customer's facility. In this paper, we present the specification and the design of the supporting tool, along with the validation results obtained on sub-systems and finally obtained with the full mirror. The ELT M2 is now close to its final shape and any defects coming from the supporting tool can be easily detected. Thanks to that, it was possible to demonstrate the performance achieved in use.

For aspherical and freeform surfaces not only the surface form error but also the position of the surface relative to external references is important (e.g. fiducials like markers, the outside edge, plane orthogonal surfaces, dowel pin holes). The NMF non-contact measurement machine range for freeform optics by Dutch United Instruments (DUI) can be employed for measurements during the entire production process, measuring ground, polished and coated optical surfaces. This machine has now been extended with a second sensor, either a marker camera or a tactile probe, to measure the position of the optical surface relative to the references features. Tactile probe results are discussed in this paper.

Glyndwr Innovations Limited (GIL) has a requirement to use a Ritchey-Common optical test during the processing and final verification of a large elliptical flat mirror. The geometry of the test requires a reference sphere that is larger in diameter than the minor axis of the optical flat under test. In this implementation, the reference sphere measures approximately 60 inches in diameter, has an aspect ratio of 9:1, is a meniscus shape and has a mass of 430kg. The reference sphere support uses a tunable Schwesinger design, supporting the mirror radially, in a horizontal line of sight. FEA predicts the full aperture optical surface deformation resulting from the effects of gravity and the support of the mirror to be 95.9nm RMS, once optimized. Once integrated and tuned the interferometric testing delivers an actual optical surface deformation of 134mn RMS, with the key Zernike terms being spherical and trefoil aberrations. The final optical form of the reference sphere, measured over its illuminated diameter within the optical test, meets our test wavefront requirements.

For future development of components for the DUV/VUV spectral range, the precise knowledge of the optical transfer functions such as spectral transmission or reflection are essential. Due to the strong atmospheric absorption of radiation in this range, spectral measurements must be carried out in the absence of oxygen. Tests are usually performed in vacuum or by purging the spectrometer with an inert gas. In vacuum environment, often a radiation induced formation of hydrocarbon contaminants on the surface of the optic is observed, that deteriorates the optical functioning and leads to erroneous measurement results. We report in this article on spectrometry tests under vacuum and purge conditions and compare the results for both environmental conditions. To avoid the contamination of the optics tests were carried out in the DUV spectral range under nitrogen purging. N2 itself shows vibrational and rotational molecular absorption bands in the wavelength range from 110 nm to 150 nm, which are present as peaks with arbitrary amplitudes in the spectrometry results. Several time domain parameters such as integration time, integration range, power monitoring aspects, and frequency bandwidth of the data acquisition modules of the spectrometer are studied to investigate the origin of the disturbing peaks. Also, different polarization states of the radiation, angle resolved scattering and pressure variation of nitrogen gas are considered to adapt the test procedures for managing the presence of these absorption lines in the measured data and to obtain reliable results.

Maskless microlens arrays (MLA) for multi-aperture projection offer high transmission due to absence of buried absorbing mask structures for shaping the pattern. Arbitrary shaped lenslets arranged in a high fill-factor array form the entrance MLA, the apertures of each entrance lenslet are projected towards the far-field by regularly shaped lenslets of the exit array. Resulting tandem MLA is arranged in a fly’s eye homogenizer (FEH) configuration. Such MLAs are mastered by grayscale photolithography and replicated as UV-molded polymer-on-glass (POG) elements. Furthermore, such maskless projectors can be realized using alternative replication technologies, primarily - injection molding (IM), which offers low cost for large area replication. MLA designed for POG replication can be adapted for IM replication by adjustment of certain parameters, e.g., thickness due to material substitution. Since the shape of the lenslets remain same for both processes, the same MLA masters can be used to generate the tooling molds necessary for IM replication. However, unlike POG replication, IM technology lacks active alignment of the entrance and exit arrays which makes IM replicated MLAs more vulnerable to crosstalk and stray light. Hence, a trade-off between ease of manufacturing (low costs) and projected image quality (sharpness, contrast, stray light) should be considered when dealing with IM replication. In this work we describe an exemplary IM replicated maskless MLA, based on masters of a previously realized POG replicated MLA for an automotive projected blinker. We discuss the design adaptation to IM and present profilometric and photometric characterization of the IM replicated MLA. We also compare the characterization results of the IM MLA samples with that of ‘gold-standard’ POG replicated MLA and discuss performance, quality of projection and limitations of IM technology.

A titania grating with a high diffraction efficiency was fabricated using sol-gel method and imprinting technique with silicone mold. For this process, titania sol was dropped onto a silicone mold transferred from the photoresist pattern by the imprinting process, and then retransferred onto a glass substrate to form a titania diffraction grating. The first -order diffraction transmittance of the fabricated element with a 1.0-μm pitch and a depth of 0.48 μm was 31.7% at a wavelength of 632.8 nm. This grating can be fabricated without using expensive apparatuses such as reactive ion etching and molding equipment.

In this project we propose a new corneal topographer that improves the accuracy of measurement from previous versions, by gathering all those details that made them to have great accuracy. The corneal topographer uses the null-screen method which is based on the idea that a reflected image in a surface contains information about the form of the reflective surface. The topographer uses a long cone, and it is aimed to evaluate corneas removing the symmetry of revolution of the algorithms. At first, the corneal topographer is utilized in a spheric reference surface, where the results obtained show that the radio of the reconstructed surface varies 0.11 % from the radio of the tested surface. This project is still in process since the evaluation algorithms must be adapted to evaluate corneas without symmetry of revolution. Keywords: null screen, corneal topography, topographic maps.

The technique proposed in this paper provides inspection of functional surfaces quality of micro-components by nondestructive testing, with high resolution and increased sensitivity. The control is done in real time and instantaneously on all inspected surface. Component geometry accuracy is one of parameters which influences, function accuracy. Moiré topography is full-field optical technique in which the shape of object surfaces is measured by means of moiré interferometry. The technique has found various applications in diverse fields, from biomedical to industrial and scientific applications. In many industrial optical metrology applications, contactless and nondestructive shape measurement is a desirable tool for, quality control and contour mapping. on surfaces that are of the order of few mm² and more. Optical device used allows a significant dimensional surface magnification of up to 1000 times the area inspected, which allows easy processing and reaches an exceptional nanometric imprecision of measurements.

To assess the curvature of a fast free-form convex surface, such as the cornea of the human eye, a smartphone corneal topographer based on the null-screen method has been developed. The null screen consists of a semiradial distribution of ellipse-shape dots arranged on a conical surface so that, when reflected from a test surface, it forms an ordered set of circular dots if the surface is perfect. Any deviation from this geometry is indicative of surface defects. To demonstrate that the corneal topographer provides reliable results on the topography of the human cornea, in this work, we set out to perform a calibration of the corneal topographer using a reference sphere with geometric parameters 7.8 mm radius and 12 mm diameter. We use a normal iterative method to reconstruct the surface shape and obtain the corneal topography.

Achromats represent a widely employed optical component in optical design and engineering. Traditionally, an achromat consists of a pairing of two lenses: a positively refracting crown glass element and a negatively refracting flint glass element, which are cascaded together. The Cooke triplet comprises three distinct lenses arranged at a finite separation distance to control the aberrations. However, the glass materials employed in their construction are costly and necessitate access to a specialized fabrication facility for shaping their structure for custom design. In this study, we demonstrate the feasibility of removing aberrations using cheap, single material like transparent epoxy resin, exhibiting a refractive index of 1.55. We fabricated 3D molds for the lenses and subsequently filled them with resin, producing lenses. These lenses' straightforward and cost-effective production demonstrates that custom designs can be created without requiring specialized fabrication facilities. The lens design started with the lens maker equation and was further optimized in the ray tracing software Opticstudio Zemax. The Seidel diagrams and focal shift graphs provided empirical evidence of our successful aberration reduction, highlighting that we effectively mitigated up to 74% of aberrations. At the same time, complete elimination of aberrations through using a single material is unattainable. The production of these lenses will contribute to the realization of cost-effective imaging systems, lens assemblies, and 3D-printed camera setups.