Jess Köhler, Rik Jansen, Juan Irizar, Alexander Sohmer, Markus Melf, Robert Greinacher, Matthias Erdmann, Volker Kirschner, Abelardo Pérez Albiñana, Didier Martin, Bryan de Goeij, Rob Vink, James Day, Daniël Ten Bloemendal, Wim Gielesen, Jan de Vreugd, Ludger van der Laan, Adriaan van’t Hof
Sentinel-5 is an Earth atmospheric monitoring mission developed within the European Union’s Copernicus program. The mission objective is to monitor the composition of the Earth’s atmosphere on a daily basis. Airbus DS GmbH acts as the prime contractor under a European Space Agency contract. The instrument design, development, and the instrument verification was and is in many aspects a fruitful co-operation between Airbus and TNO. The first part of this paper illustrates the optical design of the five optical channels of the Sentinel 5 instrument. The innovative compact optical design sets the basis for the acquisition of the variety of trace gas spectra. The design includes a two free-form mirror wide field telescope, a pointing insensitive Dual Babinet Pseudo Depolariser type polarisation scrambler, three one-dimensional waveguide type homogenizers, five reflective and refractive anamorphotic spectrometers including different disperser types as an a-spherical reflective diffraction grating for the ultra-violet, a grism for mainly the visible, a transmissive grating for the near-infrared, and immersed reflective gratings for the short wave infrared spectral ranges. The second part of this paper focusses on the design, qualification and verification of the UV1 spectrometer subsystem. Its optical design is based on an Offner-type spectrometer which has been adapted to employ freeform optics and an aspheric off-axis grating. A monolithic housing structure has been developed for optimum stability, accessibility and easy integration of the optical components. The qualification and performance verification of the first Proto Flight Model of the UV1 spectrometer was successfully completed in July 2020.
The first diffraction gratings on curved substrates were manufactured by employing ruling engines by H. A. Rowland. Due to the ruling principle, these gratings are characterized by equidistant parallel lines if the line pattern is projected along the optical axis onto a tangential plane in the vertex of the grating substrate. The Offner spectrometer is based on such a classical grating on a convex spherical substrate. This spectrometer type shows very good field correction properties. Therefore, it is among the most promising spectrometer types to meet the demands of hyper-spectral imaging. A further improvement of the optical performance is based on the modification of the surface figure of the substrate to an aspherical shape while keeping the mentioned constant grating line distribution. There are many reasons to employ interference lithography/holography - and in particular the direct blazing approach - even for the generation of the specific Offner grating line distribution on the convex substrate. A main benefit of this method is the attainable nearly perfect angular orientation of the blaze facets for the whole grating aperture. Here the achievable well defined blaze structure leads to best diffraction efficiencies close to the theoretical optimum - independent from the local curvature of the substrate. To manage the more complex recording setups of direct blazed Offner gratings, reliable methods for testing the wave front quality are a necessary pre-condition. A corresponding test method based on holographic principles will be introduced in the following text. The aspherical Offner grating was designed for the application in the UV-1 spectrometer within the Sentinel 5 mission, which is part of the European Earth Observation Program Copernicus". The spectrometer is a passive grating imaging spectrometer with a swath width of 2.670km and a spatial resolution of 50x50km2.
Sentinel-5 is an Earth atmospheric monitoring mission developed within the Copernicus program. The mission objective is to monitor the composition of the Earth’s atmosphere on a daily basis. TNO is developing the UV1 spectrometer subsystem as well as the two telescope subsystems. In this paper, the design of the UV1 spectrometer is described and its major design and verification challenges are discussed.
The UV1 optical design is based on a classical Offner-type spectrometer which has been adapted to employ freeform optics and an aspheric off-axis grating. It generates a magnification of 0.4. A structural, monolithic housing has been designed that is optimized for stability, accessibility and integration of optical components. To realize a thermo-mechanically stable construction, both housing and mirror bodies are built from aluminum. Four out of five mirrors have a dedicated black coating for absorbing out-of-band light. For further stray light suppression, a graded short-pass filter is employed on the last optical element just in front of the detector.
Performance verification of a standalone spectrometer without telescope, entrance slit, limiting aperture stop and flight detector is a major challenge. For this verification TNO developed dedicated ground support equipment that will be used under both ambient and vacuum conditions; a dedicated optical stimulus that mimics the illumination from the telescope in terms of pointing and NA, a Slit Assembly that mimics the homogenizer entrance slit and a Test Detector to determine the image plane and to measure the optical performance with high accuracy.
At the time of writing this article, the first (proto-flight) model has been aligned and performance verification is about to start.
Tropomi, successfully launched in October 2017, and Sentinel-5, with launch due in 2021, are two pushbroom spectrometers measuring Earth’s radiance from a Low Earth Orbit (LEO) at a high spectral resolution. Both instruments have strongly overlapping spectral channels (UV-VIS, NIR, SWIR-3). While the Tropomi spectrometers are designed with standard slits, all Sentinel-5 channels make use of complex slit assemblies called “slit homogenizers” that aim at mitigating the slit heterogeneous illumination that results from the along track spatial non-uniformity of the observed scenes, and is known to distort the instrument spectral response function (ISRF). The similarity between the two missions will allow, in a few years from now, to evaluate the performance gain resulting from these devices. If their expected success is confirmed, slit homogenizers may become standard components of future space missions. This paper aims at providing a comprehensive, yet as simple and accessible as possible, overview of the slit homogenizer performance. The Sentinel-5 slit homogenizers, based on two parallel mirrors will be discussed, and a new and promising family of slit homogenizer designs will be presented for the first time. The new designs offer several advantages in terms of performance and manufacturability.
Within the context of the ESA TRP programme for DARWIN, a Nulling Interferometer Breadboard for the Near-Infrared was developed and tested. Its basic principle is recombining two light beams relying on a highly symmetric optical design (autobalanced Sagnac Core). Two different star simulators have been implemented, based on a) amplitude division and b) on wavefront division. The required achromatic Pi phase shift was implemented using a) dispersive phase shifter, and b) periscopes (geometrical pupil and field rotation). Due to the extremely symmetric optical design, very good star suppression up to 400 000 has been achieved. OPD control better than 1 nm RMS has been demonstrated over hours.
The EarthCARE satellite mission objective is the observation of clouds and aerosols from low Earth orbit. The payload will include active remote sensing instruments being the W-band Cloud Profiling Radar (CPR) and the ATLID LIDAR. These are supported by the passive instruments Broadband Radiometer (BBR) and the Multispectral Imager (MSI) providing the radiometric and spatial context of the ground scene being probed. The MSI will form Earth images over a swath width of 150 km; it will image the Earth atmosphere in 7 spectral bands. The MSI instrument consists of two parts: the Visible, Near infrared and Short wave infrared (VNS) unit, and the Thermal InfraRed (TIR) unit. Subject of this paper is the VNS unit.
In the VNS optical unit, the ground scene is imaged in four spectral bands onto four linear detectors via separate optical channels. Driving requirements for the VNS instrument performance are the spectral sensitivity including out-of-band rejection, the MTF, co-registration and the inter-channel radiometric accuracy. The radiometric accuracy performance of the VNS is supported by in-orbit calibration, in which direct solar radiation is fed into the instrument via a set of quasi volume diffusers.
The compact optical concept with challenging stability requirements together with the strict thermal constraints have led to a sophisticated opto-mechanical design.
This paper, being the second of a sequence of two on the Multispectral Imager describes the VNS instrument concept chosen to fulfil the performance requirements within the resource and accommodation constraints.
KEYWORDS: Sensors, Short wave infrared radiation, Spectroscopy, Space telescopes, Telescopes, Astronomical imaging, Calibration, Electronics, Near infrared, Space operations
The Tropospheric Monitoring Instrument TROPOMI is ready for system level verification. All sub-units have been integrated and tested and final integration at Dutch Space in Leiden has been completed. The instrument will be subjected to a testing and calibration program and is expected to be ready for delivery to the spacecraft early 2015. Using TROPOMI measurements, scientists will be able to improve and continue the study of the Earth’s atmosphere and to monitor air quality, on both global and local scale.
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.
The fact that every spectrometer can sort light by wavelength at the speed of light is intriguing. The field of spectrometry is a long-existing and ever-changing one. The application areas extend from optical communication to possible extraterrestrial life detection, health monitoring, environmental monitoring and quite a long list of other topics. TNO has played a role in several of these areas, always using state of the art designs and components. Some of the recent developments are described, as well as a possible path for (near) future developments. Any spectrometer consists of a telescope, slit, collimator, disperser and an imager. Each of these functions is discussed using and even pushing progress in the manufacturing and design capabilities of the industry. The progress from a two-mirror spherical telescope for a pushbroom space-based daily global coverage spectroscopy instrument OMI to a two-mirror freeform telescope for TROPOMI is described, the design and manufacturing of supergratings showing very little straylight, freeform mirrors and the use of deliberately decentered lenses is shown. A near-future small-satellite system is shown that is being built and tested as this paper was written.
Designing a novel optical system is a nested iterative process. The optimization loop, from a starting point to final system is already mostly automated. However this loop is part of a wider loop which is not. This wider loop starts with an optical specification and ends with a manufacturability assessment. When designing a new spectrometer with emphasis on weight and cost, numerous iterations between the optical- and mechanical designer are inevitable. The optical designer must then be able to reliably produce optical designs based on new input gained from multidisciplinary studies. This paper presents a procedure that can automatically generate new starting points based on any kind of input or new constraint that might arise. These starting points can then be handed over to a generic optimization routine to make the design tasks extremely efficient. The optical designer job is then not to design optical systems, but to meta-design a procedure that produces optical systems paving the way for system level optimization. We present here this procedure and its application to the design of TROPOLITE a lightweight push broom imaging spectrometer.
The main challenges of fabricating diffraction gratings for use in earth monitoring spectrometers are given by the requirements for low stray light, high diffraction efficiency and a low polarization sensitivity. Furthermore the use in space also requires a high environmental stability of these gratings. We found that holography in combination with ion beam plasma etching provides a way to obtain monolithic, robust fused silica gratings which are able to meet the above mentioned requirements for space applications. Holography accompanied by plasma etching allows the fabrication of a wide range of different grating profiles to optimize the efficiency including the polarization behavior according to a wealth of applications. Typical profile shapes feasible are blazed gratings, sinusoidal profiles and binary profiles and this allows to tailor the efficiency and polarization requirements exactly to the spectral range of the special application. Holographic gratings can be fabricated on plane and also on curved substrates as core components of imaging spectrometers. In this paper we present our grating fabrication flow for the example of plane blazed gratings and we relate the efficiency and stray light measurement results to certain steps of the process. The holographic setup was optimized to minimize stray light and ghosting recorded by the photoresist during the exposure. Low wave front deviations require the use of highly accurate grating substrates and high precision optics in the holographic exposure.
We have developed a novel diffraction grating based on lithographical techniques and anisotropic etching in silicon. The
grating is designed for the short-wave-infrared channel of the TROPOMI imaging spectrometer that will be launched on
ESA's Sentinel 5 Precursor mission to monitor trace gases in the earth atmosphere. Stringent requirements on both the
imaging properties and the quality of the spectra translate to a high-tech grating. In our design the dispersion and
resolution is increased with a factor 3.4 with respect to conventional gratings by using the grating in immersion, such
that diffraction takes place inside the silicon grating material. By lithographic patterning and anisotropic etching of the
mono-crystalline silicon we precisely control line spacing and blaze angle. The grating has a line spacing of 2.5 μm and
is operated in sixth order. We show that an efficiency of 60% is reached on a 50 x 60 mm2 grating surface. We compare
our test results with numerical calculations for grating efficiency for both polarizations and find good agreement.
To measure the relative motions of GAIA's telescopes, the angle between the telescopes is monitored by an all Silicon
Carbide Basic Angle Monitoring subsystem (BAM OMA). TNO is developing this metrology system. The stability
requirements for this metrology system go into the pico meter and pico radian range. Such accuracies require extreme
measures and extreme stability.
Specific topics addressed are mountings of opto-mechanical components, gravity deformation, materials and tests that
were necessary to prove that the requirements are feasible. Especially mounting glass components on Silicon Carbide
and mastering the Silicon Carbide material proved to be a challenge.
The benefits Astronomy could gain by performing multi-slit spectroscopy in a space mission is renown. Digital
Micromirror Devices (DMD), developed for consumer applications, represent a potentially powerful solution. They are
currently studied in the context of the EUCLID project. EUCLID is a mission dedicated to the study of Dark Energy
developed under the ESA Cosmic Vision programme. EUCLID is designed with 3 instruments on-board: a Visual
Imager, an Infrared Imager and an Infrared Multi-Object Spectrograph (ENIS). ENIS is focused on the study of Baryonic
Acoustic Oscillations as the main probe, based on low-resolution spectroscopic observations of a very large number of
high-z galaxies, covering a large fraction of the whole sky. To cope with these challenging requirements, a highmultiplexing
spectrograph, coupled with a relatively small telescope (1.2m diameter) has been designed. Although the
current baseline is to perform slit-less spectroscopy, an important option to increase multiplexing rates is to use DMDs as
electronic reconfigurable slit masks. A Texas Instrument 2048x1080 Cinema DMD has been selected, and space
validation studies started, as a joint ESA-ENIS Consortium effort. Around DMD, a number of suited optical systems has
been developed to project sky sources onto the DMD surface and then, to disperse light onto IR arrays. A detailed study
started, both at system and subsystem level, to validate the initial proposal. Here, main results are shown, making clear
that the use of DMD devices has great potential in Astronomical Instrumentation.
In 2007 TNO started to fly some sensors on an unmanned helicopter platform. These sensors included RGB, B/W and
thermal infrared cameras. In 2008 a spectrometer was added. The goal for 2010 is to be able to offer a low altitude flying
platform including several sensors. Development of these sensors will take place the next years. Since the total weight of
the payload should be < 7kg, the weight requirements for the individual sensors will be quite strict. Applications include
gas concentrations, water quality, pipelines, etc. Collaboration still is possible.
Combining the information of several sensor systems is a difficult task. The first steps have been performed in 2007
where RGB and thermal infrared images have been combined together with the coordinates of the platform itself. The
offline data processing includes stitching video images and classification, and correcting for instability of the helicopter
itself. As environmental regulation will become even more strict than today, it is expected that high spatial resolution
sensors that can measure pollution near highways and urban areas, water quality of rivers and lakes, find and track
pollution sources etcetera are key systems in the near future.
In September 2007 and April 2008 flight campaigns have been carried out, demonstrating two applications of the system.
These include the detection of inland salty water, and the detection of benthic diatoms on an estuarine tidal flat. The
results of the two cases are discussed.
To measure the relative motions of GAIA's telescopes, the angle between the telescopes is monitored by an all Silicon
Carbide Basic Angle Monitoring subsystem (BAM OMA). TNO is developing this metrology system. The stability
requirements for this metrology system go into the pico meter and pico radian range. Such accuracies require extreme
measures and extreme stability.
Specific topics addressed are mountings of opto-mechanical components, gravity deformation, materials and tests that
were necessary to prove that the requirements are feasible. Especially mounting glass components on Silicon Carbide
and mastering the Silicon Carbide material proved to be a challenge.
The GAIA satellite, scheduled for launch in 2010, will make a highly accurate map of our Galaxy. It will measure the position of stars with an accuracy of 50 prad using two telescopes, which are positioned under a 'basic' angle between the the lines-of-sight of the telescopes of 106°. With a Basic Angle Monitoring system, variations of this angle will be measured with 5 prad accuracy, to correct for these variations on the measured position of stars. A conceptual design of the Basic Angle Monitoring system is presented. Two pairs of parallel laser bundles are sent to the telescopes, which create two interference patterns. If the basic angle varies, the interference patterns will shift. The optical design is such that the rotation of one pair of beams with respect to the other pair, does not affect the measured basic angle. The position stability requirement of the mirrors is a maximum shift of 1 pm in 6 hours. For material stability and good thermal and mechanical properties, Silicon Carbide has been chosen. The structural design is such that the design is as much monolithic as possible. The alignment is performed along the horizontal plane with external and removable alignment mechanisms. The components are locked by adhesives.
Nulling interferometry is a direct method to detect earth-like planets. To determine whether a planet is earth-like spectrometry is performed on a broadband infra-red (l = 4-20 mm) input signal from the planet. The star signal in this region is roughly 106 times stronger than the planet signal. Nulling interferometry should decrease the broadband star signal by about this factor of 106. This can be performed using an achromatic phaseshifter based on dispersive elements. The design of a complete breadboard under an ESA contract including a prism based (eight prisms in total) dispersive achromatic phaseshifter is presented including error budget and implied tolerances on the mechanical components. Measurements with this breadboard resulted in nulling depths of 3.5.105 for polarized laser light and just below 103 for polarized visible broadband light in the wavelength range of 530-750nm.
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