Thierry Viard

MTG Flexible Combined Imager optical design and performances

Meteosat Third Generation is the next ESA Program of Earth Observation dedicated to Nowcasting and very short term Weather Forecasting (NWC), medium/short range Global and Regional Numerical Weather Prediction (NWP), and Climate and Air Composition Monitoring. The satellites will be operating from the Geostationary orbit using a 3 axes stabilized platform. The main instrument is called the Flexible Combined Imager (FCI), currently under development by Thales Alenia Space France (TAS-F). This instrument will provide full images of the Earth every 10 minutes in 16 spectral channels between 0.44 and 13.3 μm, with a ground resolution from 0.5 km to 2 km. The FCI is composed of a TMA telescope developed by Kayser-Threde (KT), which includes a Scan mirror, and a calibration mechanism with an embedded black body dedicated to accurate in-flight IR radiometric calibration and a Metallic Neutral density for dedicated VNIR Sun calibration. The image produced by the telescope is split into several spectral groups by a spectral separation assembly (SSA) with dichroïc beamsplitters. The output beams are collimated to ease the instrument integration, and reach the cold optics (CO-I) which focalize the optical beams onto the detectors. The cold optics and IR detectors are accurately positioned inside a common cryostat to improve registration between spectral channels. Spectral filters are integrated on top of the detectors in order to achieve the required spectral selection. This article will describe the optical design and the main optical performances of the FCI: image quality, very high line-of-sight stability, and an efficient stray-light rejection thanks to the implementation of dedicated baffles and a stringent control of contamination. The FCI currently under development is expected to exhibit a significant improvement of performances with respect to Meteosat Second Generation satellites.

Optical MEMS in space instruments for Earth observation and astronomy

Optical MEMS could be major candidates for designing future generation of space instruments. In addition to their compactness, scalability, and specific task customization, they could generate new functions not available with current technologies. We have listed new functions associated with several types of MEMS. Instrumental applications are derived and we propose two promising concepts using object selection and spectral tailoring techniques. In Earth Observation instruments, observation of scenes including bright sources leads to an important degradation of the recorded signal. We propose a new concept to remove dynamically the bright sources and obtain a field of view (FOV) with an optically enhanced SNR. Our concept consists in replacing the plain slit in classical designs by an active row of MOEMS. Experimental demonstration of this concept has been conducted on a dedicated bench: a scene with a contiguous bright area has been focused on a micromirror array and imaged on a CCD detector. After the programmable slit, the straylight issued from the bright zone is polluting the scene; the micromirrors located on the bright area are switched off, removing almost completely the straylight in the instrument. In Astronomy and Earth Observation, we propose an innovative reconfigurable instrument: a programmable wide-field spectrograph where both the FOV and the spectrum could be tailored thanks to a 2D micromirror array. The FOV is linear and each point spectrum could be modified dynamically along the second direction. A demonstrator has been designed and its realization is under way for testing the unique performances of this instrument.

Programmable spectrometer using MOEMS devices for space applications

A new class of spectrometer can be designed using programmable components such as MOEMS which enable to tune the beam in spectral width and central wavelength. It becomes possible to propose for space applications a spectrometer with programmable resolution and adjustable spectral bandwidth. The proposed way to tune the output beam is to use the diffraction effect with the so-called PMDG (Programmable Micro Diffraction Gratings) diffractive MEMS. In that case, small moving structures can form programmable gratings, diffracting or not the incoming light. In the proposed concept, the MOEMS is placed in the focal plane of a first diffracting stage (using a grating for instance). With such implementation, the MOEMS component can be used to select some wavelengths (for instance by reflecting them) and to switch-off the others (for instance by diffracting them). A second diffracting stage is used to recombine the beam composed by all the selected wavelengths. It becomes then possible to change and adjust the filter in λ and Δλ. This type of implementation is very interesting for space applications (astronomy, Earth observation, planetary observation). Firstly because it becomes possible to tune the filtering function quasi instantaneously. And secondly because the focal plane dimension can be reduced to a single detector (for application without field of view) or to a linear detector instead of a 2D matrix detector (for application with field of view) thanks to a sequential acquisition of the signal.

DMD-based programmable wide field spectrograph for Earth observation

In Earth Observation, Universe Observation and Planet Exploration, scientific return could be optimized in future missions using MOEMS devices. In Earth Observation, we propose an innovative reconfigurable instrument, a programmable wide-field spectrograph where both the FOV and the spectrum could be tailored thanks to a 2D micromirror array (MMA). For a linear 1D field of view (FOV), the principle is to use a MMA to select the wavelengths by acting on intensity. This component is placed in the focal plane of a first grating. On the MMA surface, the spatial dimension is along one side of the device and for each spatial point, its spectrum is displayed along the perpendicular direction: each spatial and spectral feature of the 1D FOV is then fully adjustable dynamically and/or programmable. A second stage with an identical grating recomposes the beam after wavelengths selection, leading to an output tailored 1D image. A mock-up has been designed, fabricated and tested. The micromirror array is the largest DMD in 2048 x 1080 mirrors format, with a pitch of 13.68μm. A synthetic linear FOV is generated and typical images have been recorded o at the output focal plane of the instrument. By tailoring the DMD, we could modify successfully each pixel of the input image: for example, it is possible to remove bright objects or, for each spatial pixel, modify the spectral signature. The very promising results obtained on the mock-up of the programmable wide-field spectrograph reveal the efficiency of this new instrument concept for Earth Observation.

Optical MEMS for space spectro-imagers

In addition to their compactness, scalability and specific task customization, optical MEMS could generate new functions not available with current technologies and are thus candidates for the design of future space instruments. Most mature components for space applications are the Digital Mirror Device (DMD) from Texas Instruments (TI), the micro-deformable mirrors, the Programmable Micro Diffraction Grating and the tiltable micro-mirrors. Among 20-30 MEMS-based payloads concepts, two concepts are selected. The first concept is a programmable slit for straylight control for space spectro-imagers. This instrument is a push-broom spectro-imager for which some images cannot be exploited because of bright sources in the field-of-view. The proposed concept consists in replacing the current entrance spectrometer slit by an active row of micro-mirrors. The MEMS will permit to dynamically remove the bright sources and then to obtain a field-of-view with an optically enhanced signal-to-noise ratio. The second concept is a push-broom imager for which the acquired spectrum can be tuned by optical MEMS. This system is composed of two diffractive elements and a TI’s DMD component. The first diffractive element spreads the spectrum. A micro-mirror array is set at the location of the spectral focal plane. By putting the micro-mirrors ON or OFF, we can select parts of field-of-view or spectrum. The second diffractive element then recombines the light on a push-broom detector. Dichroics filters, strip filter, band-pass filter could be replaced by a unique instrument.

Convolution spectrometer demonstration using programmable diffraction grating

We demonstrate experimentally the concept of a convolution spectrometer exhibiting enhanced performance. It is based on a programmable diffraction grating for generating a shifting spectral window. The scene spectral density is obtained by deconvolution process.

MOEMS for prospective space applications

We are involved with ESA and CNES since several years, in the analysis of space applications using MOEMS components. A first concept using a Programmable Micro Diffracting Device (PMDG) has been proposed for an astronomical spectrometer with a small field of view. In this application the introduction of a MOEMS component has allowed to reduce the focal plane complexity (one mono detector) and to increase the mission adaptability to the target (programmable mission). An opto mechanical concept has been proposed and first performance assessed. A second concept has been studied and deals with the use of a MOEMS component to realize an innovative spectrometer, so-called convolution spectrometer. In the proposed solution, a MOEMS is used to realize a shifting spectral window (large spectral width) associated to a slight spectral increment. The signal given by the detector being the convolution between the target spectral density and the spectral window, it is then possible to recover the target spectral signal by a deconvolution. A breadboard has been developed, and the concept of the convolution spectrometer has been successfully demonstrated. Finally, some results of analysis will be also given concerning the use of a DMD for Earth observation associated to a push broom detection mode and a large field of view.

Micromirror arrays for spectroscopy in space

In Astronomy and Earth Observation, we propose two innovative reconfigurable instruments based on micromirror arrays: 1) a multi-object spectrograph with MOEMS programmable slit masks; 2) a programmable wide-field spectrograph where both the field of view (FOV) and the spectrum could be tailored thanks to a 2D micromirror array. The FOV is linear and each point spectrum could be modified dynamically along the second direction. Demonstrators of both concepts have been designed, fabricated and integrated: first results show the unique performances of these micromirrors-based instruments.

Batman and Robin: next generation spectro-imagers for space observation

In Earth Observation, Universe Observation and Planet Exploration, scientific return of the instruments must be optimized in future space missions.

Programmable wide field spectrograph for earth observation

In Earth Observation, Universe Observation and Planet Exploration, scientific return of the instruments must be optimized in future missions. Micro-Opto-Electro-Mechanical Systems (MOEMS) could be key components in future generation of space instruments. These devices are based on the mature micro-electronics technology and in addition to their compactness, scalability, and specific task customization, they could generate new functions not available with current technologies. French and European space agencies, the Centre National d’Etudes Spatiales (CNES) and the European Space Agency (ESA) have initiated several studies with LAM and TAS for listing the new functions associated with several types of MEMS, and developing new ideas of instruments.