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Sentinel-4 is an imaging UVN (UV-VIS-NIR) spectrometer, developed by Airbus Defence and Space as prime contractor under ESA contract in the frame of the joint EU/ESA COPERNICUS program. The mission objective is the operational monitoring of trace gas concentrations for atmospheric chemistry and climate applications. This paper gives an overview of the Sentinel-4 system architecture, its design & development status.

Sentinel-4 is an imaging UVN (UV-VIS-NIR) spectrometer, developed by Airbus Defence and Space as prime contractor under ESA contract in the frame of the joint EU/ESA COPERNICUS program. The mission objective is the operational monitoring of trace gas concentrations for atmospheric chemistry and climate applications. Stray light, which is unwanted light captured by the detectors, is one of the major contributors to such important instrument performance metrics as absolute and relative spectral and spatial radiometric accuracies. Amongst the different sources of stray light, Out-of-Band stray light can not be corrected. It is thus important to ensure its impact is limited. The main scope of this paper is to describe the stray light simulations performed in the frame of the Sentinel-4 project: the models, analysis approach and the results. Also, the technical challenges faced in building the models and performing the analysis and the solutions found to solve them are presented.

International audience; The Polarized Instrument for Long-wavelength Observation of the Tenuous interstellar medium (PILOT) is a balloon-borne experiment that aims to measure the polarized emission of thermal dust at a wavelength of 240 µm (1.2 THz). A first PILOT flight of the experiment took place from Timmins, Ontario, Canada, in September 2015 and a second flight took place from Alice Springs, Australia in April 2017. In this paper, we present the inflight performance of the instrument. Here we concentrate on the instrument performance as measured during the second flight, but refer to the performance observed during the first flight, if it was significantly different. We present a short description of the instrument and the flights. We measure the time constants of the detectors using the decay of the observed signal during flight following high energy particle impacts (glitches) and switching off the instrument's internal calibration source. We use these time constants to deconvolve the timelines and analyze the optical quality of the instrument as measured on planets. We then analyze the structure and polarization of the instrumental background. We measure the detector response flat field and its time variations using the signal from the residual atmosphere and from the internal calibration source. Finally, we analyze the spectral and temporal properties of the detector noise. The inflight performance is found to be satisfactory and globally in line with expectations from ground calibrations. We conclude by assessing the expected inflight sensitivity of the instrument in light of the measured inflight performance.

Earth observation measurements at wavelengths below 320nm are challenging due to the steep decrease of the earth irradiance towards shorter wavelengths. Stray light and ghosting of longer wave light can easily overwhelm the signals at short wavelengths. In the UV channel (270-320nm) of the TROPOMI instrument this challenge has been addressed using a number of coatings. Three black UV mirror coatings absorb light with a wavelength above 370nm. Together, these achieve more than four orders suppression of long wave out-of-band light. A lowpass transmission filter with a position dependent cut-off wavelength is deposited on the last lens surface, directly in front of the detector. At the position where short wavelength light passes the filter, longer wavelength in-band stray light and ghosts are blocked. A simulation predicts that this graded filter reduces ghosting by a factor 20 and scatter related stray light by factor 30.

PILOT (Polarized Instrument for Long wavelength Observations of the Tenuous interstellar medium) is a balloonborne astronomy experiment designed to study the polarization of dust emission in the diffuse interstellar medium in our Galaxy. The PILOT instrument allows observations at wavelengths 240 μm (1.2THz) with an angular resolution about two arc-minutes. The observations performed during the first flight in September 2015 at Timmins, Ontario Canada, have demonstrated the optical performances of the instrument.

PILOT is a balloon-borne astronomy experiment designed to study the polarization of dust emission in the diffuse interstellar medium in our Galaxy at wavelengths 240 and 550 µm with an angular resolution of about two arc-min. PILOT optics is composed of an off-axis Gregorian telescope and a refractive re-imager system. All these optical elements, except the primary mirror, are in a cryostat cooled to 3K. We used optical and 3D measurements combined with thermo-elastic modeling to perform the optical alignment. This paper describes the system analysis, the alignment procedure, and finally the performances obtained during the first flight in September 2015

International audience; The Polarized Instrument for Long‐wavelength Observation of the Tenuous interstellar medium (PILOT) is a balloon‐borne astronomy experiment designed to study the linear polarization of thermal dust emission in two photometric bands centred at wavelengths 240 mu m (1.2 THz) and 550 mu m (545 GHz), with an angular resolution of a few arcminutes. Several end‐to‐end tests of the instrument were performed on the ground between 2012 and 2014, in order to prepare for the first scientific flight of the experiment that took place in September 2015 from Timmins, Ontario, Canada. This paper presents the results of those tests, focussing on an evaluation of the instrument's optical performance. We quantify image quality across the extent of the focal plane, and describe the tests that we conducted to determine the focal plane geometry, the optimal focus position, and sources of internal straylight. We present estimates of the detector response, obtained using an internal calibration source, and estimates of the background intensity and background polarization.

This paper reports on an IC-Compatible process for the fabrication of Linear Variable Optical Filter (LVOF). The LVOF is integrated with a detector array to result in a micro-spectrometer. The technological challenge in fabrication of an LVOF is fabrication of a well-controlled tapered cavity layer. Very small taper angles, ranging from 0.001 degrees to 0.1 degrees. are fabricated in a resist layer by just one lithography step and a subsequent reflow process. The 3D pattern of resist structures is subsequently transferred into SiO2 by an appropriate etching. Complete LVOF fabrication involves CMOS-compatible deposition of a lower dielectric mirror using a stack of dielectrics on the wafer, tapered layer formation and the deposition of the top dielectric mirror. The design principle, IC-Compatible processing and the characterization results on fabricated LVOFs are presented. (C) 2010 Elsevier B.V. All rights reserved.

This paper presents spectral measurements using a linear variable optical filter. A LVOF has been developed for operation in the 530 nm–720 nm spectral band and has been fabricated in an IC-compatible process. The LVOF has been mounted on a CMOS camera. A Least Mean Square algorithm has been implemented to calculate the spectrum of light from the recorded image on the CMOS camera. A spectral resolution of 0.5 nm has been achieved using the algorithm. The spectral resolution is limited by the monochromator accuracy used for calibration.

The spectral resolution of a MEMS-based Infrared (IR) microspectrometer with a plainer grating for projection of dispersed light on a detector array critically depends on the thermal cross-talk between adjacent Thermo-Electric (TE) elements in the array. Bridge-shaped TE detector elements are cut out of a membrane using MEMS process. The air gap between pixels is the cause of the cross-talk. The static and dynamic characterizations of thermal cross-talk have been evaluated, with an emphasis on the effect of the thermal conductivity of air as the function of the package pressure. The developed models accurately predict the behaviour of the thermal cross-talk under different package pressure.

This paper reports on a IC-Compatible fabrication process for Linear Variable Optical Filters (LVOF). Very small taper angles, ranging from 0.001° to 0.1°, can be fabricated flexibly in a resist layer by just one lithography step and a subsequent reflow process. The 3D pattern of resist structures is subsequently transferred into SiO2 by appropriate etching. Complete LVOF fabrication involves CMOS-compatible deposition of a lower dielectric mirror using a stack of dielectrics on the wafer, tapered layer formation and deposition of the top dielectric mirror. Design principle and processing plus experimental validation of the technique on the profile in resist and after transfer of the taper into SiO2 are presented.

PILOT is a stratospheric experiment designed to measure the polarization of dust FIR emission, towards the diffuse interstellar medium. The first PILOT flight was carried out from Timmins in Ontario-Canada on September 20th 2015. The flight has been part of a launch campaign operated by the CNES, which has allowed to launch 4 experiments, including PILOT. The purpose of this paper is to describe the performance of the instrument in flight and to perform a first comparison with those achieved during ground tests. The analysis of the flight data is on-going, in particular the identification of instrumental systematic effects, the minimization of their impact and the quantification of their remaining effect on the polarization data. At the end of this paper, we shortly illustrate the quality of the scientific observations obtained during this first flight, at the current stage of systematic effect removal.

Future cosmology space missions will concentrate on measuring the polarization of the Cosmic Microwave Background, which potentially carries invaluable information about the earliest phases of the evolution of our universe. Such ambitious projects will ultimately be limited by the sensitivity of the instrument and by the accuracy at which polarized foreground emission from our own Galaxy can be subtracted out. We present the PILOT balloon project, which aims at characterizing one of these foreground sources, the polarized continuum emission by dust in the diffuse interstellar medium. The PILOT experiment also constitutes a test-bed for using multiplexed bolometer arrays for polarization measurements. This paper presents the instrument and its expected performances. Performance measured during ground calibrations of the instrument and in flight will be described in a forthcoming paper.

A method for minimizing the intensity of ghost stray light during the optical design process is presented in this paper. Dimensions and, consequently, intensities of ghost images formed in an optical system can be estimated using an analytical algorithm. This algorithm enables construction of a merit function in optical design software, which returns a number proportional to the total intensity of ghost stray light. In this way, an optical system can be simultaneously optimized for the best image quality and for the minimum ghost stray light intensity. The proposed method has been applied to reoptimize a diffraction-limited system of four lenses, and a reduction of ghost stray light by a factor of 6 with almost no degradation of image quality has been achieved.

This paper reports on the functional and spectral characterization of a microspectrometer based on a CMOS detector array covered by an IC-Compatible Linear Variable Optical Filter (LVOF). The Fabry-Perot LVOF is composed of 15 dielectric layers with a tapered middle cavity layer, which has been fabricated in an IC-Compatible process using resist reflow. A pattern of trenches is made in a resist layer by lithography and followed by a reflow step result in a smooth tapered resist layer. The lithography mask with the required pattern is designed by a simple geometrical model and FEM simulation of reflow process. The topography of the tapered resist layer is transferred into silicon dioxide layer by an optimized RIE process. The IC-compatible fabrication technique of such a LVOF, makes fabrication directly on a CMOS or CCD detector possible and would allow for high volume production of chip-size micro-spectrometers. The LVOF is designed to cover the 580 nm to 720 spectral range. The dimensions of the fabricated LVOF are 5×5 mm2. The LVOF is placed in front of detector chip of a commercial camera to enable characterization. An initial calibration is performed by projecting monochromatic light in the wavelength range of 580 nm to 720 nm on the LVOF and the camera. The wavelength of the monochromatic light is swept in 1 nm steps. The Illuminated stripe region on the camera detector moves as the wavelength is swept. Afterwards, a Neon lamp is used to validate the possibility of spectral measurement. The light from a Neon lamp is collimated and projected on the LVOF on the camera chip. After data acquisition a special algorithm is used to extract the spectrum of the Neon lamp.

This paper reports on the IC-compatible fabrication of vertically tapered optical layers for use in linear variable optical filters (LVOF). The taper angle is fully defined by a mask design. Only one masked lithography step is required for defining strips in a photoresist with trenches etched therein of a density varying along the length of the strip. In a subsequent reflow, this patterned photoresist is planarized, resulting in a strip with a local thickness defined by the initial layer thickness and the trench density at that position before reflow. Hence a taper can be flexibly programmed by the mask design to be from 0.001o to 0.1o, which enables the simultaneous fabrication of tapered layers of different taper angles. The 3D pattern of resist structures is subsequently transferred into Si or SiO2 by appropriate etching. Complete LVOF fabrication involves CMOS-compatible deposition of a lower dielectric mirror using a stack of dielectrics on the wafer, tapered layer formation and deposition of the top dielectric mirror. Design principle, processing and simulation results plus experimental validation of the technique on the profile in the resist and after transfer of the taper into Si and SiO2 are presented.

We report on a miniature spectrometer with a volume of 0.135 cm(3) and dimensions of 3x3x11 mm, mounted directly on the surface of a CCD sensor. The spectrometer is formed by two flat diffraction gratings that are designed to perform both the dispersion and imaging functions, eliminating the need for any spherical optics. Two separate parts of the device were fabricated with the single-mask 1 mum lithography on a single glass wafer. The wafer was diced and the device was assembled and directly mounted onto a CCD sensor. The resolution of 3 nm, spectral range of 450 to 750 nm and the optical throughput of ~9% were measured to be in a complete agreement with the model used for the development of the device.

A concept for a highly miniaturized spectrometer featuring a two-component design is presented. The first component is a planar chip that integrates an input slit and aberration-correcting diffraction grating with an image sensor and is fabricated using microelectromechanical systems (MEMS) technologies. Due to the fabrication in a simple MEMS batch process the essential elements of the spectrometer are automatically aligned, and a low fabrication cost per device can be achieved. The second component is a spherical mirror, which is the only external part. The optimized grating structure compensates for aberrations within the spectrometer operating range, resulting in a diffraction-limited performance of the spectrometer optics. The prototype of the device has been fabricated and characterized. It takes a volume of 0.5 cm(3) and provides a FWHM spectral resolution of 0.7 nm over a 350 nm bandwidth from 420 nm to 770 nm combined with an etendue of 7.4x10(-5) mm(2) sr.

This paper reports on the development and validation of a new technology for the fabrication of variable line-spacing non-planar diffraction gratings to be used in compact spectrometers. The technique is based on the standard lithographic process commonly used for pattern transfer onto a flat substrate. The essence of the technology presented here is the lithographic fabrication of a planar grating structure on top of a flexible membrane on a glass or silicon wafer and the subsequent deformation of the membrane using a master shape. For the validation of the proposed technology we fabricated several reflection concave diffraction gratings with the f-numbers varying from 2 to 3.8 and a diameter in the 4 - 7 mm range. A glass wafer with circular holes was laminated by dry-film resist to form the membranes. Subsequently, standard planar lithography was applied to the top part of the membranes for realizing grating structures. Finally the membranes were deformed using plano-convex lenses in such a way that precise lens alignment is not required. A permanent non-planar structure remains after curing. The imaging properties of the fabricated gratings were tested in a three-component spectrograph setup in which the cleaved tip of an optical fiber served as an input slit and a CCD camera was used as a detector. This simple spectrograph demonstrated subnanometer spectral resolution in the 580 - 720 nm range.

The design and performance of a highly miniaturized spectrometer fabricated using MEMS technologies are reported in this paper. Operation is based on an imaging diffraction grating. Minimizing fabrication complexity and assembly of the micromachined optical and electronic parts of the microspectrometer implies a planar design. It consists of two parallel glass plates, which contain all spectrograph components, including slit and diffraction grating, and can be fabricated on a single glass wafer with standard lithography. A simple analytical model for determining spectral resolution from device dimensions was developed and used for finding the optimal parameters of a miniaturized spectrometer as a compromise between size and spectral resolution. The fabricated spectrometer is very compact (11 × 1.5 × 3 mm3), which allowed mounting directly on top of an image sensor. The realized spectrometer features a 6 nm spectral resolution over a 100 nm operating range from 600 nm to 700 nm, which was tested using a Ne light source.

An analytical model has been developed and applied to explore the limits in the design of a highly miniaturized planar optical microspectrometer based on an imaging diffraction grating. This design tool has been validated as providing the smallest possible dimensions while maintaining acceptable spectral resolution. The resulting planar spectrometer is composed of two parallel glass plates, which contain all components of the device, including a reflective slit and an imaging diffraction grating. Fabrication is based on microelectromechanical system technology and starts with a single glass wafer; IC-compatible deposition and lithography are applied to realize the parts in aluminum, which makes the microspectrometer highly tolerant for component mismatch. The fabricated spectrometer was mounted directly on top of an image sensor and takes up a volume of only 50 mm(3). The measured spectral resolution of 6 nm (FWHM) in the 100 nm operating wavelength range (600-700 nm) is in agreement with a model calculation.

In this paper, design and fabrication procedure of a thermopile detector array for use in a fully integrated Infrared optical spectrometer is presented. IC-compatible MEMS technologies are used for fabrication of the spectrometer components, such as slit, flat imaging diffraction grating and detector array. The IR micro-spectrometer was designed for operation in the 1.5-3 mum wavelength range. The largest dimension of it is ~8 mm. The imaging properties of the diffraction grating result in non-uniform dispersion, which imposes special requirements on the dimensions of each single detector in the array. The result is an array of unequal elements. The final design considers technological constraints, sensitivity and cross-talk between elements. Simulations results and the final design are presented.

The spectral resolution of a MEMS-based IR microspectrometer critically depends on the thermal cross-talk between adjacent TE elements in the detector array. Thermal isolation between elements is realized by using bulk micromachining directly following CMOS processing. This paper reports on the characterization results of bridge-shaped TE detector elements that are cut out of a membrane. Elements with dimensions of 650 × 36 ?m2 are separated by 10 ?m wide gaps in order to minimize the thermal cross-talk by heat conduction through the support structure. The static and dynamic aspects of thermal cross-talk have been evaluated with an emphasis on the effect of the thermal conductivity of air as a function of the package pressure.

The statistical analysis of 529 compounds belonging to the group of cephalosporins revealed a significant influence of oxygen-containing descriptors of the radical at position 7 in the cefem nucleus on the biological activity of the compounds. It was concluded that prediction of the antimicrobial properties of the antibiotics by the pattern recognition was possible with using factors reflecting the presence and space arrangement of the following functional groups in the cephalosporin molecules:-OH (phenolic group), C-O-C (ether), C = O (ester, amide) and -O- (in the heterocycle). The results of the study were compared with the data on the impact of nitrogen-containing descriptors on the biological activity of cephalosporins and practical recommendations for synthesis of new active compounds were proposed.

The relationship between the in vitro antimicrobial activity of cephalosporin antibiotics and the presence and space arrangement of sulfur-containing descriptors of the radical at position 7 in the cefem nucleus was studied. It was noted that the sulfur-containing fragments were markedly less frequent in the structures of the known cephalosporins than the nitrogen- and oxygen-containing descriptors. It was concluded that at present information related only to two of the fragments under investigation might be used as characteristics in predicting antimicrobial activity of cephalosporins by the pattern recognition. On the basis of the studies there were defined two principle paths for synthesis of new antibiotics: introduction into the molecule structure of the functional groups providing with high probability improvement of the antimicrobial properties and preparation of compounds containing descriptors whose impact on the biological activity is not known.

The impact of the nitrogen containing functional groups of the radical at position 7 in the cefem nucleus on antimicrobial activity of cephalosporins against gram-positive and gram-negative organisms was studied. 235 compounds were tested. It was shown that the presence or absence of the above mentioned functional groups could not be used as the only factor for estimating the pharmacological properties of the antibiotics under investigation. There were different and, occasionally, diametrically opposite effects of the spatial arrangement of the nitrogen-containing descriptors on gram positive and gram negative organisms. It was concluded that prediction of the biological activity of cephalosporins by the pattern recognition according to the presence and spatial arrangement of the functional groups was possible. A way for designing novel compounds with the required biological activity is proposed.

The design and fabrication of a thermopile detector array for use in a fully integrated infrared optical spectrometer are described. IC-compatible MEMS technologies are used for fabrication of the spectrometer components, such as the slit, planar imaging diffraction grating and detector array. The IR micro-spectrometer was designed for operation in the 1.5‐3 µm wavelength range with the size of the largest dimension about 8 mm. The imaging properties of the diffraction grating result in non-uniform dispersion, which imposes special requirements on the dimensions of each single detector in the array. The result is an array of unequally sized elements. The design considers technological constraints, sensitivity and cross-talk between elements. Simulation results, final design, fabrication technique and fabricated devices are presented. (Some figures in this article are in colour only in the electronic version)

The design, fabrication and performance of a highly miniaturized optical spectrometer are described. The volume of the opto-electronic system is only 50 mm3. The main components are a planar imaging diffraction grating and a commercially available CCD camera. System integration is based on MEMS technologies on a glass wafer. The imaging grating circumvents the need for collimating optics, while the planar grating design is essential for limiting fabrication complexity and enabling assembly of the optical and electronic parts. The system consists of two glass plates placed in parallel with well-defined spacing and comprises all spectrometer components including slit and diffraction grating. The glass plates were fabricated using a single glass wafer with standard lithography applied. The spectrometer demonstrated a 6 nm FWHM spectral resolution in an operating range from 600 nm to 700 nm.

We have investigated the possibility of using transparent stretchable diffraction gratings for spectrometric applications. The gratings were fabricated by replication of a triangular-groove master into a transparent viscoelastic. The sample length, and hence the spatial period, can be reversibly changed by mechanical stretching. When used in a monochromator with two slits, the stretchable grating permits scanning the spectral components over the output slit, converting the monochromator into a scanning spectrometer. The spectral resolution of such a spectrometer was found to be limited mainly by the wave-front aberrations due to the grating deformation. A model relating the deformation-induced aberrations in different diffraction orders is presented. In the experiments, a 12-mm long viscoelastic grating with a spatial frequency of 600 line pairs/mm provided a full-width at half-maximum resolution of up to ~1.2 nm in the 580-680 nm spectral range when slowly stretched by a micrometer screw and ~3 nm when repeatedly stretched by a voice coil at 15 Hz. Comparison of aberrations in transmitted and diffracted beams measured by a Shack- Hartmann wave-front sensor showed that astigmatisms caused by stretch-dependent wedge deformation are the main factors limiting the resolution of the viscoelastic-grating-based spectrometer.

This paper reports on a microspectrometer using a concave grating fabricated in a MEMS compatible process. The fabrication technique is based on a standard lithography commonly used for pattern transfer onto a flat substrate. A planar grating structure is fabricated lithographically on top of a flexible membrane on a glass or silicon wafer and the membrane is subsequently deformed into a pre-defined shape using a master. The proposed technology had been used for the fabrication of the concave gratings for a microspectrometer composed of entrance slit, grating and image sensor. The fabricated microspectrometer demonstrates a subnanometer spectral resolution.