The MERTIS instrument is a thermal infrared imaging spectrometer onboard of ESA's cornerstone mission BepiColombo to Mercury. MERTIS will provide detailed information about the mineralogical composition of Mercury's surface layer by measuring the spectral emittance in the spectral range from 7-14 μm with a high spatial and spectral resolution. Furthermore MERTIS will obtain radiometric measurements in the spectral range from 7-40 μm to study the thermo-physical properties of the surface material. Under the lead of the German Aerospace Center DLR (Dep. Optical Information Systems, Berlin) a development model (DM) is in development which integrates all MERTIS sub-units of later flight models. With the DM the general design and performance goals of the system shall be investigated and verified. Besides a general overview about the instrument principles the following topics are addressed: Optics setup with a Three Mirror Anastigmatic (TMA) telescope and Offner Spectrometer, Manufacturing techniques for the robust and high precision optics and Radiometer Concept and Design
The MERTIS instrument is a thermal infrared imaging spectrometer onboard of ESA’s cornerstone mission BepiColombo to Mercury. MERTIS has four goals: the study of Mercury’s surface composition, identification of rock-forming minerals, mapping of the surface mineralogy, and the study of the surface temperature variations and thermal inertia. MERTIS will provide detailed information about the mineralogical composition of Mercury’s surface layer by measuring the spectral emittance in the spectral range from 7-14 μm at high spatial and spectral resolution. Furthermore MERTIS will obtain radiometric measurements in the spectral range from 7-40 μm to study the thermo-physical properties of the surface material. The MERTIS detector is based on an uncooled micro-bolometer array providing spectral separation and spatial resolution according to its 2-dimensional shape. The operation principle is characterized by intermediate scanning of the planet surface and three different calibration targets – free space view and two on-board black body sources. In the current project phase, the MERTIS Qualification Model (QM) is under a rigorous testing program. Besides a general overview of the instrument principles, the papers addresses major aspects of the instrument design, manufacturing and verification.
For the Euclid mission a pre-development phase is implemented to prove feasibility of individual components of the
system. The optical system of EUCLID Near-Infrared Spectrometer & Photometer (NISP) is composed of 4 lenses,
bandpass filters and grisms. The lenses are made of different materials: the corrector lens (fused silica) directly behind
the dichroic and the lenses L1 (CaF2), L2 (LF5G15), and L3 (LF5G15) that are mounted in a separate lens barrel design.
Each lens has its separate mechanical interface to the lens barrel, the so called adaption ring.
The adaption ring shall provide the necessary elasticity caused by different CTEs of the lens and ring materials, as well
as shall allow the high position accuracy of the lenses relative to the lens barrel and the optical axis.
The design drivers for the adaption ring are high precision, cryogenic operation temperature (150 K) and the large
dimension of the lenses (150 - 170 mm). The design concept of the adaption ring is based on solid state springs, which
shall both provide sufficient protection against vibration loads at ambient temperature as well as high precision (<
±10 μm) and stability at cryogenic temperatures.
Criteria for the solid state spring design shall be low radial forces at cryogenic conditions to avoid any refractive index
and polarization variations. The design shall be compliant to the large temperature differences between assembly and
operation, the high precision and non-deformation requirements of the lenses as well as to the deviating CTEs of the
selected lens materials. The paper describes the selected development approach including justification, thermal and
MERTIS is a miniaturized thermal infrared imaging spectrometer onboard of ESA's cornerstone mission BepiColombo
to Mercury. It shall provide measurements in the spectral range from 7-14 μm with a spatial resolution of maximal 300
m and 80 spectral channels in combination with radiometric measurements in the spectral range from 7-40 μm.
The instrument concept therefore integrates two detector systems sharing a common optical path consisting of mirror
entrance optics and reflective Offner spectrometer. Uncooled micro-bolometer and thermopile radiometer technology are
implemented for lowest power consumption. Subsequent viewing of different targets including on-board calibration
sources will provide the desired performance. Special attention is spent on the fully passive thermal design in the harsh
environment around Mercury.
The article will provide an overview of the 3 kg - instrument design and highlight the concept of the subsystems and
technologies used. The status of the development process will be reported.
Within an ESA funded project combustion chambers of Ariane V rockets are investigated for further development. Due
to temperature gradients of approximately 1300 K/mm in the combustion chamber during launch, material damages
occur because of the Doghouse effect. To avoid these damages, the combustion chambers have to be redesigned
wherefore the occurring temperatures have to be measured with an uncertainty of ±5 K. In order to measure the
temperature in the small layer between the hot exhaust emissions and the coolant, optical fiber sensing is deployed.
Embedding special optical sensor fibers that are high temperature resistant within the material allows measuring the wall
In order to demonstrate fiber optic sensing for high temperature and strain measurement, Thermo Mechanical Fatigue
(TMF) panels, constructed as sandwich structure have been developed that represent the combustion chamber walls.
Coated fibers which are installed in the the panel can be subjected to thermal loads up to 1000 K inside a high
temperature oven. Online measurements of FBG sensors inscribed in the embedded optical fibers can be carried out. The
measurement results of the FBG sensors exactly match the data of the electrical reference sensor.
For FBG readout we use our newly developed Scanning Laser (SL) Interrogation System which uses a tuneable laser
source. The output wavelength is determined by a set of control currents. In order to archive the required accuracy a
Current Control Unit (CCU) stabilizes the control currents and thereby the output wavelengths. The CCU significantly
improves the accuracy and additionally enhances the measurement rate. The high temperature measurement results
demonstrate compatibility with the requirements.
Optical instruments for remote sensing applications frequently require measures for reducing the amount of external,
unwanted stray light in the optical instrument path. The reflective planet baffle design and manufacturing process for the
thermal infrared imaging spectrometer MERTIS onboard of ESA's cornerstone mission BepiColombo to Mercury is
presented. The baffle has to reflect the unwanted solar flux and scattered IR radiation, and minimize the heat load on the
Based on optical stray light simulations and analyses of different baffle concepts the Stavroudis principle showed the
best performance and the smallest number of internal reflections. The setup makes use of the optical properties of
specific conic sections of revolution. These are the oblate spheroid, generated by rotating an ellipse about its minor axis,
and the hyperboloid of one sheet, obtained by the rotation of a hyperbola around its conjugate axis.
Due to the demanding requirements regarding surface quality, low mass and high mechanical stability, electroforming
fabrication was selected for the baffle. During manufacturing, a layer of high strength nickel alloy is electrodeposited
onto a diamond turned aluminum mandrel. The mandrel is subsequently chemically dissolved. Not only the baffle, but
also the baffle support structure and other mating components are electroformed. Finally, the baffle and support structure
are assembled and joined by an inert gas soldering process. After the optimum baffle geometry and surface roughness
has been realized, the remaining total heat flux on the baffle is only dependent on the selection of the appropriate, high
The MERTIS reflective infrared optics can be beneficial implemented as diamond turned aluminium mirrors coated with
a thin gold layer. The cutting processes allow the manufacturing of both, the optical surface and mechanical interfaces, in
tight tolerances. This is one of the major advantages of metal optics and was consequently used for the MERTIS sensor
This paper describes the entire process chain of the MERTIS spectrometer optics including the manufacturing methods
for the mirrors and for the spherical grating, the coating with sputtered gold for infrared reflectivity as well as the
alignment and the verification of the spectrometer optics.
Fiber optic Bragg grating (FBG) sensors show promising capabilities in the measurement of strain and temperatures in structures at many locations. In this work, the potential of FBG sensors for high-precision deformation control in opto-mechanical applications is investigated. This requires a strain resolution of < 1 um/m. A test rig with a simply supported steel beam was developed which should represent the geometry of a lightweight optical mirror with a ribbed support structure. The deformation of this beam is controlled by a piezo actuator. The reference deformation measurement is done
using six capacitive displacement sensors with a resolution < 0.5 nm. It is being investigated to what level of accuracy FBG sensors can be used to reconstruct the displacement information. Different
methods to increase the accuracy are discussed: decreasing the sensor noise by oversampling and increasing the number of sensors. Tests were performed using different diffraction-based interrogation techniques for the wavelength detection: a CCD-based FBG sensor system and a PSD (Position Sensitive Detector)-based high-speed FBG sensor system which - to our knowledge - has not been used for an application of this kind yet. A comparison of both systems discussing the weaknesses and strengths is given for the recording of mechanical strain < 1 um/m. The results showed that a resolution of < 0.3 um/m for the strain measurement using FBG sensors can be achieved. This study shows an interesting application potential for FBG sensors in structural deformation control for various fields such as optics or high-precision machine tools.
In some specific applications the electronic speckle pattern
interferometry (ESPI) is superior to other optical surface
metrology methods. The two-wavelength ESPI for surface contouring
can achieve both high accuracy of height resolution in the micron
range and short measurement times far below a second. A further
advantage of this method is that compared to e.g. triangulation,
illumination axis and observation axis can be identical. A problem
of interferometric methods in general are phase ambiguities
originating from discontinuous measurement object surfaces. A
common idea to decrease the range of ambiguity is the fusion of
several interferograms recorded at different wavelengths. This
paper presents a concept for a loss free sequential superposition
of several spatially separated laser beams as well as algorithms
for the determination of measured surface discontinuities. Also a
solution of a stability control for fast wavelength tuning of
laser diodes is presented.
Over the course of the last few years, several readout techniques for fiber Bragg grating (FBG) sensors have been proposed. However, all of them suffer from specific restrictions concerning response speed, accuracy, sensor multiplexibility and cost. In the past, it was often assumed that diffraction grating spectrometers were suitable only for FBG applications with modest resolution. The achievable pixel resolution is nowadays in the range of several tens of pm. For FBG sensors with typical temperature coefficients of 5 pm/K and strain coefficients of 0.7 pm/με this resolution is not sufficient for the majority of applications. We present a discussion on different methods for the subpixel registration of FBG spectra and we introduce a novel detection algorithm: the linear phase operator technique (LPO). Even under extreme noisy conditions LPO ensures a significant resolution enhancement by a factor of three compared to conventional algorithms and is shown to be very efficient in its implementation. The efficiencies of several conventional algorithms and LPO is compared by simulations and by means of a test bench. With slight efforts LPO is adaptable to further applications like spectrometer based Fabry-Perot sensors and other sensors with CCD detectors.