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.
Stray-light components are an unavoidable part of the signal detected by most optical instrument; they can originate from different sources and optical design solutions can help reducing some of them. Diffuse stray-light results from the same scene observed by the instrument, and can deteriorate the radiometric performances especially in the presence of highlycontrasted scenes, as it often occurs in earth observation applications. When strict radiometric requirements are applied, as in the case of more recent mission studies, an assessment of stray-light levels can be relevant already in the early phases of an instrument development, to compare the performances of different concepts. For this task commercial stray-light tools using Monte-Carlo simulations can be extremely time consuming.
Here we present the software StrayLux, a tool to calculate the diffuse stray-light component of optical instruments. This software uses a semi-analytical approach to approximate stray-light contribution of the optical components of an instrument, resulting in shorter calculation times than Monte-Carlo simulations. The tool is completely written in Python, is provided with a graphical interface, and can interact with Zemax to extract the relevant parameters of an optical design.
The latest version of the software is currently made available to ESA industrial partners as a possible benchmark tool for stray-light estimation, within the instrument pre-development activities for future missions.
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 and 550 μm with an angular resolution of about two arcminutes. The observations performed during the two first flights performed from Timmins, Ontario Canada, and from Alice-springs, Australia, respectively in September 2015 and in April 2017 have demonstrated the good performances of the instrument. Pilot optics is composed of an off axis Gregorian type telescope combined with a refractive re-imager system. All optical elements, except the primary mirror, which is at ambient temperature, are inside a cryostat and cooled down to 3K. The whole optical system is aligned on ground at room temperature using dedicated means and procedures in order to keep the tight requirements on the focus position and ensure the instrument optical performances during the various phases of a flight. We’ll present the optical performances and the firsts results obtained during the two first flight campaigns. The talk describes the system analysis, the alignment methods, and finally the inflight performances.
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.
Three mirror anastigmat (TMA) telescope designs  had been implemented in different projects ranging from the narrow Field-Of-View large instruments as Quickbird (2° FOV)  to smaller telescopes as JSS 12° FOV developed for RapidEye mission .
This telescope configuration had been also selected for the PROBA-V payload, the successor of Vegetation, a multispectral imager flown on Spot-4 and subsequently on Spot-5 French satellites for Earth Observation and defence. PROBA-V, small PROBA-type satellite, will continue acquisition of vegetation data after the lifetime of Spot-5 expires in 2012.
The PROBA-V TMA optical design achieves a 34° FOV across track and makes use of highly aspherical mirrors. Such a telescope had become feasible due to the recently developed Single Point Diamond Turning fabrication technology. The telescope mirrors and structure are fabricated in aluminium and form an athermal optical system.
This paper presents the development of the compact wide FOV TMA, its implementation in PROBA-V multispectral imager and reviews optics fabrication technology that made this development possible. Furthermore, this TMA is being used in combination with a linear variable filter in a breadboard of a compact hyperspectral imager. Moreover, current technology allows miniaturization of TMA, so it is possible to use a TMA-based hyperspectral imager on a cubesat platform.
ESA is currently running two parallel, competitive phase A/B1 studies for MetOp Second Generation (MetOp-SG). MetOp-SG is the space segment of EUMETSAT Polar System (EPS-SG) consisting of the satellites and instruments. The Phase A/B1 studies will be completed in the first quarter of 2013. The final implementation phases (B2/C/D) are planned to start 2013. ESA is responsible for instrument design of five missions, namely Microwave Sounding Mission (MWS), Scatterometer mission (SCA), Radio Occultation mission (RO), Microwave Imaging mission (MWI), Ice Cloud Imaging (ICI) mission, and Multiviewing, Multi-channel, Multi-polarization imaging mission (3MI). This paper will present the instrument main design elements of the 3MI mission, primarily aimed at providing aerosol characterization for climate monitoring, Numerical Weather Prediction (NWP), atmospheric chemistry and air quality. The 3MI instrument is a passive radiometer measuring the polarized radiances reflected by the Earth under different viewing geometries and across several spectral bands spanning the visible and short-wave infrared spectrum. The paper will present the main performances of the instrument and will concentrate mainly on the performance improvements with respect to its heritage derived by the POLDER instrument. The engineering of some key performance requirements (multiviewing, polarization sensitivity, etc.) will also be discussed.
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 μm with an angular resolution about two arcminutes. Pilot optics
is composed an off-axis Gregorian type telescope and a refractive re-imager system. All optical elements, except the
primary mirror, are in a cryostat cooled to 3K. We combined the optical, 3D dimensional measurement methods and
thermo-elastic modeling to perform the optical alignment. The talk describes the system analysis, the alignment
procedure, and finally the performances obtained during the first flight in September 2015.
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.
The Meteosat Third Generation (MTG) Programme is being realised through the well-established and successful cooperation between EUMETSAT and ESA. It will ensure the continuity with, and enhancement of, operational meteorological and climate data from Geostationary Orbit as currently provided by the Meteosat Second Generation (MSG) system. The industrial Prime Contractor for the Space segment is Thales Alenia Space (France) with a core team consortium including OHB-Bremen (Germany) and OHB-Munich (Germany. This contract includes the provision of six satellites, four Imaging satellites (MTG-I) and two Sounding satellites (MTG-S), which will ensure a total operational life of the MTG system in excess of 20 years. A clear technical baseline has been established for both MTG-I and MTG-S satellites, and confirmed through a rigorous Preliminary Design Review (PDR) process that was formally concluded during 2013. Dedicated reviews have been held for all the main elements including the core instruments (Flexible Combined Imager (FCI) and Infrared Sounder (IRS)), the Platform (which is largely common for the two satellites), the Lightning Imager (LI) and the MTG-I and MTG-S satellites as a whole. The satellites and instruments are at the moment in preparation for the Structural and Thermal Models (STM). The FCI is designed to provide images of the Earth every 10 to 2.5 minutes in 16 spectral channels between 0.44 and 13.3 μm, with a ground resolution ranging from 0.5 km to 2 km. The on-board calibration is based on the use of a Metallic Neutral Density (MND) filter for VIS/NIR channels and a blackbody for the IR channels. This paper introduces the overall FCI design and its calibration concept covering VIS/NIR and IR domains and it describes how the use of the MND makes it possible to accurately correct the medium and long term radiometric drifts of the IR3.8 μm channel.
The MetOp-SG programme is a joint Programme of EUMETSAT and ESA. ESA develops the prototype MetOp-SG
satellites (including associated instruments) and procures, on behalf of EUMETSAT, the recurrent satellites (and
associated instruments). Two parallel, competitive phase A/B1 studies for MetOp Second Generation (MetOp-SG) have
been concluded in May 2013. The implementation phases (B2/C/D/E) are planned to start the first quarter of 2014.
ESA is responsible for instrument design of six missions, namely Microwave Sounding Mission (MWS), Scatterometer
mission (SCA), Radio Occultation mission (RO), Microwave Imaging mission (MWI), Ice Cloud Imager (ICI) and
Multi-viewing, Multi-channel, Multi-polarisation imaging mission (3MI).
The paper will present the main performances of the 3MI instrument and will highlight the performance improvements
with respect to its heritage derived by the POLDER instrument, such as number of spectral channels and spectral range
coverage, swath and ground spatial resolution. The engineering of some key performance requirements (multi-viewing,
polarisation sensitivity, straylight etc.) will also be discussed. The results of the feasibility studies will be presented
together with the programmatics for the instrument development.
Several pre-development activities have been initiated to retire highest risks and to demonstrate the ultimate
performances of the 3MI optics. The scope, objectives and current status of those activities will be presented. Key
technologies involved in the 3MI instrument design and implementation are considered to be: the optical design featuring
aspheric optics, the implementation of broadband Anti Reflection coatings featuring low polarisation and low de-phasing
properties, the development and qualification of polarisers with acceptable performances as well as spectral filters with
good uniformities over a large clear aperture.
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.
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.
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.