TNO TPD has an extensive experience in the realisation of highly stable passive and active opto-mechanical systems. Future missions of ESA like DARWIN  rely heavily on these systems such as metrology equipment, delay lines, nulling equipment etc.. The nulling equipment will be used in the Nulling mode of DARWIN in which it will detect Earth-like planets around nearby stars. In the Nulling mode the starlight is dimmed to a factor of 10-6 at the wavelength range of 6 to 18 μm. During the recent years TNO TPD has been strongly involved in the development of nulling equipment such as phase shifters and a nulling breadboard. Also new concepts for nulling have been generated. A prism based phase shifter has been developed that operates in the visible. Nulling depths were achieved near 1000 in a wavelength range of 550 to 750 nm (equivalent DARWIN nulling depth: 105). Limiting effects like vibrations are presently being eliminated by including an optical path difference control loop. With the nulling breadboard a null depth of 400,000 was achieved and a stable null has been realised over several hours. Further work in the near future will be amongst others the phase A study of a nulling instrument GENIE that will operate on the ESO VLTI.
TNO and DLR envision optical free-space communication between ground stations and geostationary telecommunication satellites to replace the traditional RF links for the next generation of Very High Throughput Satellites. To mitigate atmospheric turbulence, an Adaptive Optics (AO) system will be used. TNO and DLR are developing breadboards to validate Terabit/s communication links using an AO system. In this paper the breadboard activities and first results of the sub-systems will be presented. Performance of these subsystems will be evaluated for viability of terabit/s optical feeder links.
For the DARWIN mission the extremely low planet signal levels require an optical instrument design with utmost efficiency to guarantee the required science performance. By shaping the transverse amplitude and phase distributions of the receive beams, the singlemode fibre coupling efficiency can be increased to almost 100%, thus allowing for a gain of more than 20% compared to conventional designs. We show that the use of "tailored freeform surfaces" for purpose of beam shaping dramatically reduces the coupling degradations, which otherwise result from mode mismatch between the Airy pattern of the image and the fibre mode, and therefore allows for achieving a performance close to the physical limitations. We present an application of tailored surfaces for building a beam shaping optics that shall enhance fibre coupling performance as core part of a space based interferometer in the future DARWIN mission and present performance predictions by wave-optical simulations. We assess the feasibility of manufacturing the corresponding tailored surfaces and describe the proof of concept demonstrator we use for experimental performance verification.
Through the years many stable optical mounts have been designed, analyzed and tested at TNO. This paper gives an overview of the design principles used. Various examples are presented together with verification test results. <p> </p>The use of adhesives in combination with an iso-static mount design allows mounting of optical components in a limited volume with limited deformation of the optical surfaces due to thermal and mechanical loads. Relatively large differences in thermal expansion over large temperature ranges can be overcome using a simple and predictable design at reasonable costs. Despite adhesives have limited dimensional stability and loadability, stable optical mounts can be realized when proper design principles are used.
TNO has developed, built and tested the Wave Front Sensor (WFS) for ESA’s Gaia mission. The WFS will help Gaia create an extraordinarily precise three-dimensional map of more than one billion stars in our Galaxy. Part of ESA's Cosmic Vision programme, Gaia’s build is led by EADS Astrium and is scheduled for launch in 2012. The Wave Front Sensor will be used to monitor the wave front errors of the two main telescopes mounted on the GAIA satellite. These mirrors include a 5-degree of freedom (DOF) mechanism that can be used to minimize the wave front errors during operation. The GAIA-WFS will operate over a broad wavelength (450 to 900 nm) and under cryogenic conditions (130 to 200 K operation temperature). The WFS uses an all reflective, a-thermal design and is of the type of Shack-Hartmann. The boundary condition for the design is that the focal plane of the WFS is the same plane as the focal plane of the GAIA telescopes. The spot pattern generated after a micro lens array ( MLA) by a star is compared to the pattern of one of the three calibration sources that is included in the WFS, allowing in flight calibration. We show the robust and lightweight opto mechanical design that is optimised for launch and cryogenic operation. Details are given on its alignment and commissioning. The WFS is able to measure relative wave front distortions in the order of lambda/1000, and can determine the optimum position of the focal plane with an accuracy of 50 μm
The Global Ozone Monitoring Experiment-2 (GOME-2) represents one of the European instruments carried on board the MetOp satellite within the ESA’s “Living Planet Program”. Consisting of three flight models (FM’s) it is intended to provide long-term monitoring of atmospheric ozone and other trace gases over a time frame of 15-20 years, thus contributing valuable input towards climate and atmospheric research and providing near real-time data for use in air quality forecasting. <p> </p>The ambition to achieve highly accurate scientific results requires a thorough calibration and characterization of the instrument prior to launch. These calibration campaigns were performed by TNO in Delft in the Netherlands, in the “Thermal Vacuum Calibration Facility” of the institute. <p> </p>Due to refurbishment and / or storage of the instruments over a period of a few years, several re-calibration campaigns were necessary. These re-calibrations provided the unique opportunity to study the effects of long term storage and build up statistics on the instrument as well as the calibration methods used. <p> </p>During the re-calibration of the second flight model a difference was found in the radiometric calibration output, which was not understood initially. In order to understand the anomalies on the radiometry, a deep investigation was performed using numerous variations of the setup and different sources. The major contributor was identified to be a systematic error in the alignment, for which a correction was applied. Apart from this, it was found that the geometry of the sources influenced the results. Based on the calibration results combined with a theoretical geometrical hypothesis inferred that the on-ground calibration should mimic as close as possible the in-orbit geometry.
The Gaia payload ensures maximum passive stability using a single material, SiC, for most of its elements. Dedicated metrology instruments are, however, required to carry out two functions: monitoring the basic angle and refocusing the telescope. Two interferometers fed by the same laser are used to measure the basic angle changes at the level of μas (prad, micropixel), which is the highest level ever achieved in space. Two Shack- Hartmann wavefront sensors, combined with an ad-hoc analysis of the scientific data are used to define and reach the overall best-focus. In this contribution, the systems, data analysis, procedures and performance achieved during commissioning are presented .
The ESA Gaia spacecraft has two Shack-Hartmann wavefront sensors (WFS) on its focal plane. They are
required to refocus the telescope in-orbit due to launch settings and gravity release. They require bright stars to
provide good signal to noise patterns. The centroiding precision achievable poses a limit on the minimum stellar
brightness required and, ultimately, on the observing time required to reconstruct the wavefront. Maximum
likelihood algorithms have been developed at the Gaia SOC. They provide optimum performance according to
the Crámer-Rao lower bound. Detailed wavefront reconstruction procedures, dealing with partial telescope pupil
sampling and partial microlens illumination have also been developed. In this work, a brief overview of the WFS
and an in depth description of the centroiding and wavefront reconstruction algorithms is provided.
Through the years many stable optical mounts have been designed, analyzed and tested at TNO. This paper gives an
overview of the design principles used. Various examples are presented together with verification test results.
The use of adhesives in combination with an iso-static mount design allows mounting of optical components in a limited
volume with limited deformation of the optical surfaces due to thermal and mechanical loads. Relatively large
differences in thermal expansion over large temperature ranges can be overcome using a simple and predictable design at
reasonable costs. Despite adhesives have limited dimensional stability and loadability, stable optical mounts can be
realized when proper design principles are used.
The Polarization-Based Collimated Beam Combiner efficiently produces pairwise interference between beams from multiple
telescopes. An important feature is achieving "Photometric Symmetry" whereby interference measurements have
no first-order sensitivity to wavefront perturbations (or photometric variations following spatial filtering) which otherwise
entail visibility measurements with increased error, bias, and nonlinearity in phase determination. Among other proposed
applications, this topology has been chosen as the basis for the design of the NOVA Fringe Tracker (NFT), a proposed 4
or 6 telescope second-generation fringe tracker for the VLTI. The NFT takes advantage of the photometric symmetry thus
achieved making it capable of tracking on stars resolved beyond the first visibility null, as well as interfering a telescope
beam with one which is 20 times brighter, a design goal set by ESO. By not requiring OPD modulation for interferometric
detection, the detector exposure time can be increased without performance reduction due to time skew nor is sensitivity
reduced by removing optical power for photometric monitoring, and use of two-phase interferometric detection saves one
half of the photons being diverted for detection of the other two (mainly) unused quadrature phases. The topology is also
proposed for visibility measuring interferometers with configurations proposed for the achievement of balanced quadrature
or 3-phase interferometric detection. A laboratory demonstration confirms >>100:1 rejection of photometric crosstalk in a
fringe tracking configuration where atmospheric OPD fluctuations were simulated using a hair dryer. Tracking with a 30:1
intensity ratio between the incoming beams was performed while rejecting large introduced photometric fluctuations.
TNO developed a Wave Front Sensor (WFS) instrument for the GAIA mission. This Wave Front Sensor will be used to monitor the wave front errors of the two main telescopes mounted on the GAIA satellite, which may be corrected by a 5-degree of freedom (DOF) mechanism during operation. The GAIA-WFS will operate over a broad wavelength (450 to 900 nm) and under cryogenic conditions (130 to 200 K operation temperature). The WFS uses an all reflective, a-thermal design and is of the type of Shack-Hartmann. The boundary condition for the design is that the focal plane of the WFS is
the same plane as the focal plane of the GAIA telescopes. The spot pattern generated after a micro lens array (MLA) by a star is compared to the pattern of one of the three calibration sources that is included in the WFS, allowing in flight calibration. We show the robust and lightweight opto mechanical design that is optimised for launch and cryogenic operation. Furthermore we give details on its alignment and commissioning. The WFS can measure wave front
distortions in the order of lambda/1000, and determines the focal plane with an accuracy of 50 μm.
We present the design of a new testbed experiment to demonstrate nulling interferometry using polarization properties.
This three-beam set-up is perfectly symmetric with respect to the number of reflections and transmissions
and should therefore allow a high rejection ratio in a wide spectral band.
The primary goal of DARWIN is to detect earth-like extrasolar planets and to search for biomarkers. This is achieved by means of nulling interferometry, using three free-flying telescopes and a Beam-Combiner (BC) hub. DARWIN will be able to perform astrophysical imaging with high spectral and spatial resolution. Should one of Darwin's telescope flyers fail, then Darwin's capability of detecting earth-sized exo-planets is dramatically reduced. However, with only two telescopes the imaging mode can continue operating with minimal performance degradation, thus ensuring mission success. This work describes a trade-off study between four conceptual three-beam BC's, that are capable of performing both as a nuller and as an imager. A proposed breadboard design will demonstrate end-to-end Fringe-Tracking (FT) and Optical Path-Length (OPL) control. The BC concept is based on a pupil-plane (Michelson) beam combination scheme. Pupil-plane imaging BC's offer a large overlap in terms of optical layout with the nulling BC concept, making it possible to develop a combined nulling- and imaging BC. This means that a reduced number of optical components can be used compared to a scheme with separate BC's. The BC concept inherently compensates for unequal OPL's, which in ground-based interferometers is compensated for by long stroke Optical Delay Lines (ODL's).
Darwin is a space based interferometry mission<sup>1</sup> of the European Space Agency (ESA) with the aim to detect and characterise earth-like planets outside our solar system. The current Darwin baseline consists of four spacecrafts (3 telescopes). Destructive interference of the starlight is required to allow detection of much fainter planet signals. The nulling ratio required is 10<sup>5</sup>.
For Darwin high requirements are set upon the wavefront quality of the beams. In order to be able to have destructive interference with a contrast factor of 10<sup>5</sup>, a wavefront quality of λ/1400 (λ=6 micrometer) is needed. With current and/or foreseen technology, it is not possible to produce the optical elements with sufficient quality to meet this requirement. This means it is vital to develop wavefront filter devices for Darwin.
Most promising for this purpose are single mode fibres. For visible and near-infrared light commercially available single mode fibres are available, however they do not extend yet to wavelengths above 4 micrometer. To overcome this shortcoming new single mode fibres are developed (i.e. by Astrium and TNO/ University of Rennes) for the Darwin wavelength range (6-20 μm). To characterize and test these fibres a system is designed allowing to determine the possible star light suppression with the fibre. This system is called "Darwin Infrared Nulling Interferometer Demonstrator" (DINID).
The system is designed using the in-house knowledge from previous nulling set-ups in the visible and near-infrared wavelength range. It will permit to test fibres around 4 and 9 micrometer and includes an optical path difference control in order to compensate drifts.
This paper describes the basis on which the set-up is designed.
The Achromatic Phase shifter breadboard (APS) is designed for broadband (550-750 nm) nulling. This system has already been described in detail previously<sup>1,2</sup>, this paper describes improvements to the breadboard currently implemented and the corresponding results obtained. The breadboard improvements concern the following four points: A) Residual vibrations are compensated by active OPD control; B) Beam overlap can be optimized by an extra alignment mechanism; C) A more versatile detector/preamplifier is used; D) Adjustments of the phase shifter are now computer controlled.
The paper describes the set-up and the results of these improvements on the nulling performance of the breadboard with the goal to achieve a 10<sup>4</sup> nulling ratio and to validate the computer simulations of the achromatic phase shifter. The validated value for nuldepth observed is currently 4600+/-400, and ratio's above 10,000 have been found, when measuring with a higher bandwidth. Nulldepths like these have to our knowledge not been reported before for relative spectral bandwidths of this magnitude. The program continues to optimize the setup and improve the results.
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 10<sup>6</sup> times stronger than the planet signal. Nulling interferometry should decrease the broadband star signal by about this factor of 10<sup>6</sup>. 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 3.5.10<sup>5</sup> for polarized laser light and just below 10<sup>3</sup> for polarized visible broadband light in the wavelength range of 530-750nm.