The ESA DARWIN project uses a set of telescopes based on free flying satellites and operated as an aperture synthesis system. The relative position of all these satellites has to be maintained with a very high accuracy. This is only possible with the help of a very high precision metrology system. We present here in a first part the requirements for the optical metrology of DARWIN and the proposed definition of the metrology systems needed to answer these requirements. In the second part we present the design of the various systems. Eventually we give the results of the performance tests of breadboards that have been manufactured in the framework of the ESA study "High Precision Optical Metrology" to validate these designs.
One of ESA’s future missions is the Darwin Space Interferometer, which aims to detect planets around nearby stars using optical aperture synthesis with free-flying telescopes. Since this involves interfering white (infra-red) light over large distances, the mission is not possible without a complex metrology system that monitors various speeds, distances and angles between the satellites. One of its sub-systems should measure absolute distances with an accuracy of around 70 micrometer over distances up to 250 meter. To enable such measurements, we are investigating a technique called frequency sweeping interferometry, in which a single laser is swept over a large known frequency range. Central to our approach is the use of a very stable, high finesse Fabry-P´erot cavity, to which the laser is stabilized at the endpoints of the frequency sweep. We will discuss the optical set-up, the control system that controls the fast sweeping, the calibration and the data analysis. We tested the system using long fibers and achieved a repeatability of 50 micrometers at a distance of 55 meters. We conclude with some recommendations for further improvements and the adaption for use in space.
The primary goal of the LISA mission is the detection of gravitational waves from astronomical sources in a frequency range of 10-4 to 1 Hz. This requires operational stabilities in the picometer range as well as highly predictable mechanical distortions upon cooling down, outgassing in space, and gravity release.
In March 2011 ESA announced a new way forward for the Lclass candidate missions, including LISA. ESA and the scientific community are now studying options for European-only missions that offer a significant reduction of the costs, while maintaining their core science objectives. In this context LISA has become the New Gravitational wave Observatory (NGO).
Despite this reformulation, the need for dimensional stability in the picometer range remains valid, and ESA have continued the corresponding LISA Technology Development Activities (TDA’s) also in view of NGO. In such frame Astrium GmbH and xperion (Friedrichshafen, Germany) have designed and manufactured an ultra-stable CFRP breadboard of the LISA telescope in order to experimentally demonstrate that the structure and the M1 & M2 mirror mounts are fulfilling the LISA requirements in the mission operational thermal environment. Suitable techniques to mount the telescope mirrors and to support the M1 & M2 mirrors have been developed, with the aim of measuring a system CTE of less than 10-7 K-1 during cooling down to -80°C. Additionally to the stringent mass and stiffness specifications, the required offset design makes the control of relative tilts and lateral displacements between the M1 and M2 mirrors particularly demanding.
The thermo-elastic performance of the telescope assembly is going to be experimentally verified by TNO (Delft, The Netherlands) starting from the second half of 2012.
This paper addresses challenges faced in the design phase, shows the resulting hardware and present first outcomes of the test campaign performed at TNO.
Aerosols affect Earth’s energy level by scattering and absorbing radiation and by changing the properties of clouds. Such effects influence the precipitation patterns and lead to modifications of the global circulation systems that constitute Earth’s climate. The aerosol effects on our climate cannot be at full scale estimated due to the insufficient knowledge of their properties at a global scale. Achieving global measurement coverage requires an instrument with a large instantaneous field of view that can perform polarization measurements with high accuracy, typically better than 0.1%. Developing such an instrument can be considered as the most important challenge in polarimetric aerosol remote sensing.
Using a novel technique to measure polarization, we have designed an instrument for a low-Earth orbit, e.g. International Space Station, that can simultaneously characterize the intensity and state of linear polarization of scattered sunlight, from 400 to 800 nm and 1200 to 1600 nm, for 30 viewing directions, each with a 30° viewing angle. In this article we present the instrument’s optical design concept.
We present SPEX, the Spectropolarimeter for Planetary Exploration, which is a compact, robust and low-mass spectropolarimeter designed to operate from an orbiting or in situ platform. Its purpose is to simultaneously measure the radiance and the state (degree and angle) of linear polarization of sunlight that has been scattered in a planetary atmosphere and/or reflected by a planetary surface with high accuracy. The degree of linear polarization is extremely sensitive to the microphysical properties of atmospheric or surface particles (such as size, shape, and composition), and to the vertical distribution of atmospheric particles, such as cloud top altitudes. Measurements as those performed by SPEX are therefore crucial and often the only tool for disentangling the many parameters that describe planetary atmospheres and surfaces. SPEX uses a novel, passive method for its radiance and polarization observations that is based on a carefully selected combination of polarization optics. This method, called spectral modulation, is the modulation of the radiance spectrum in both amplitude and phase by the degree and angle of linear polarization, respectively. The polarization optics consists of an achromatic quarter-wave retarder, an athermal multiple-order retarder, and a polarizing beam splitter. We will show first results obtained with the recently developed prototype of the SPEX instrument, and present a performance analysis based on a dedicated vector radiative transport model together with a recently developed SPEX instrument simulator.
The LISA Optical Stability Characterization project is part of the LISA CTP activities to achieve the required Technology Readiness Level (TRL) for all of the LISA technologies used. This activity aims demonstration of the Telescope Assembly (TA), with a structure based on CFRP technology, that a CTE of 10-7 1/K can be achieved with measures to tune the CTE to this level. In addition the demonstration is required to prove that the structure exhibits highly predictable mechanical distortion characteristics when cooling down to - 90°C, during outgassing in space and when going from 1g environment to 0g.
This paper describes the test facilities as well as the first test results. A dedicated test setup is designed and realized to allow monitoring dimensional variations of the TA using three interferometers, while varying the temperature in a thermal vacuum chamber. Critical parameters of the verification setup are the length metrology accuracy in thermal vacuum and the thermal vacuum flexibility and stability. The test programme includes Telescope Assembly CTE measurements and thermal gradient characterization.
Climate change and environmental conditions are high on the political agenda of international governments. Laws and regulations are being setup all around the world to improve the air quality and to reduce the impact. The growth of a number of trace gasses, including CO2, Methane and NOx are especially interesting due to their environmental impact. The regulations made are being based on both models and measurements of the trend of those trace gases over the years. Now the regulations are in place also enforcement and therewith measurements become more and more important. Instruments enabling high spectral and spatial resolution as well as high accurate measurements of trace gases are required to deliver the necessary inputs. Nowadays those measurements are usually performed by space based spectrometers. The requirement for high spectral resolution and measurement accuracy significantly increases the size of the instruments. As a result the instrument and satellite becomes very expensive to develop and to launch. Specialized instruments with a small volume and the required performance will offer significant advantages in both cost and performance. Huib’s Innovative Gas Sensor (HIGS, named after its inventor Huib Visser), currently being developed at TNO is an instrument that achieves exactly that. Designed to measure only a single gas concentration, opposed to deriving it from a spectrum, it achieves high performance within a small design volume. The instrument enables instantaneous imaging of the gas distribution of the selected gas. An instrument demonstrator has been developed for NO2 detection. Laboratory measurements proved the measurement technique to be successful. An on-sky measurement campaign is in preparation. This paper addresses both the instrument design as well as the demonstrated performances.
The CHEOPS mission (CHaracterising ExOPlanet Satellite) is dedicated to searching for exoplanetary transits by
performing ultra-high precision photometry on bright stars already known to host planets. A 32cm diameter on-axis
Ritchey-Chrétien telescope is used for imaging onto a single cooled detector. With integration times up to 48 hours the
thermal stability of the telescope and its structure are key to the performance. Using a multi-lateration interferometer
setup TNO has successfully demonstrated the μm-level stability of the Structural Thermal Model (STM2) of the Optical
Telescope Assembly (OTA) in thermal vacuum. This OTA was later upgraded to become the Flight Model. Experiments
comprise thermal vacuum cycling, thermal vacuum stability testing where axial and lateral deformations are measured to
the nm-level sensitivity.
A novel design for a earth observation combined spectrometer and polarimeter is presented. The goal of the instrument is to measure both intensity (radiance) and the state of polarization. Some backgrounds for this instrument are presented but the main part of this article will be on the optical design and the ideas behind it.
The LISA Optical Stability Characterization project is part of the LISA CTP activities to achieve the required
Technonlogy Readiness Level (TRL) for all of the LISA technologies used. This activity aims demonstration of the
Telescope Assembly (TA), with a structure based on CFRP technology, that a CTE of 10-7 1/K can be achieved with
measures to tune the CTE to this level. In addition the demonstration is required to prove that the structure exhibits
highly predictable mechanical distortion characteristics when cooling down to -90°C, during outgassing in space and
when going from 1g environment to 0g.
This paper describes the test facilities as well as the first test results. A dedicated test setup is designed and realized to
allow monitoring dimensional variations of the TA using three interferometers, while varying the temperature in a
thermal vacuum chamber. Critical parameters of the verification setup are the length metrology accuracy in thermal
vacuum and the thermal vacuum flexibility and stability. The test programme includes Telescope Assembly CTE
measurements and thermal gradient characterization.
SPEX (Spectropolarimeter for Planetary Exploration) was developed in close cooperation between scientific institutes
and space technological industries in the Netherlands. It is used for measuring microphysical properties of aerosols and
cloud particles in planetary atmospheres. SPEX utilizes a number of novel ideas. The key feature is that full linear
spectropolarimetry can be performed without the use of moving parts, using an instrument of approximately 1 liter in
volume. This is done by encoding the degree and angle of linear polarization (DoLP and AoLP) of the incoming light in
a sinusoidal modulation of the intensity spectrum.
Based on this principle, and after gaining experience from breadboard measurements using the same principle, a fully
functional prototype was constructed. The functionality and the performance of the prototype were shown by extensive
testing. The simulated results and the laboratory measurements show striking agreement.
SPEX would be a valuable addition to any mission that aims to study the composition and structure of planetary
atmospheres, for example, missions to Mars, Venus, Jupiter, Saturn and Titan. In addition, on an Earth-orbiting satellite,
SPEX could give unique information on particles in our own atmosphere.
We present the Spectropolarimeter for Planetary EXploration (SPEX), a high-accuracy linear spectropolarimeter
measuring from 400 to 800 nm (with 2 nm intensity resolution), that is compact (~ 1 liter), robust and
lightweight. This is achieved by employing the unconventional spectral polarization modulation technique, optimized
for linear polarimetry. The polarization modulator consists of an achromatic quarter-wave retarder and
a multiple-order retarder, followed by a polarizing beamsplitter, such that the incoming polarization state is
encoded as a sinusoidal modulation in the intensity spectrum, where the amplitude scales with the degree of
linear polarization, and the phase is determined by the angle of linear polarization. An optimized combination
of birefringent crystals creates an athermal multiple-order retarder, with a uniform retardance across the field
of view. Based on these specifications, SPEX is an ideal, passive remote sensing instrument for characterizing
planetary atmospheres from an orbiting, air-borne or ground-based platform. By measuring the intensity and
polarization spectra of sunlight that is scattered in the planetary atmosphere as a function of the single scattering
angle, aerosol microphysical properties (size, shape, composition), vertical distribution and optical thickness can
be derived. Such information is essential to fully understand the climate of a planet. A functional SPEX prototype
has been developed and calibrated, showing excellent agreement with end-to-end performance simulations.
Calibration tests show that the precision of the polarization measurements is at least 2 • 10-4. We performed
multi-angle spectropolarimetric measurements of the Earth's atmosphere from the ground in conjunction with
one of AERONET's sun photometers. Several applications exist for SPEX throughout the solar system, a.o. in
orbit around Mars, Jupiter and the Earth, and SPEX can also be part of a ground-based aerosol monitoring
SPEX (Spectropolarimeter for Planetary EXploration) is an innovative, compact instrument for spectropolarimetry,
and in particular for detecting and characterizing aerosols in planetary atmospheres. With its ~1-liter volume
it is capable of full linear spectropolarimetry, without moving parts. The degree and angle of linear polarization
of the incoming light is encoded in a sinusoidal modulation of the intensity spectrum by an achromatic
quarter-wave retarder, an athermal multiple-order retarder and a polarizing beam-splitter in the entrance pupil.
A single intensity spectrum thus provides the spectral dependence of the degree and angle of linear polarization.
Polarimetry has proven to be an excellent tool to study microphysical properties (size, shape, composition) of
atmospheric particles. Such information is essential to better understand the weather and climate of a planet.
The current design of SPEX is tailored to study Martian dust and ice clouds from an orbiting platform: a compact
module with 9 entrance pupils to simultaneously measure intensity spectra from 400 to 800 nm, in different
directions along the flight direction (including two limb viewing directions). This way, both the intensity and
polarization scattering phase functions of dust and cloud particles within a ground pixel are sampled while flying
over it. We describe the optical and mechanical design of SPEX, and present performance simulations and initial
breadboard measurements. Several flight opportunities exist for SPEX throughout the solar system: in orbit
around Mars, Jupiter and its moons, Saturn and Titan, and the Earth.
A Darwin precursor breadboard, comprising both fine lateral and longitudinal metrology sensors was designed, built and partially tested. The lateral metrology sensor was designed and built by TNO TPD and more than meets the imposed requirements. The longitudinal metrology sensor consists of a dual wavelength interferometer with an integrated delay line for optical path stabilisation. Here TNO TPD supplied the delay line and implemented the optical path difference stabilisation control. Experiments under ambient conditions show that noise reduction up to five orders of magnitude is achievable.