The market for nano- and microsatellites is developing rapidly. There is a strong focus on 2D imaging of the Earth’s surface, with limited possibilities to obtain spectral information. More demanding applications, such as monitoring trace gases, aerosols or water quality still require advanced imaging instruments, which are large, heavy and expensive. In recent years TNO has investigated and developed different innovative designs to realize advanced spectrometers for space applications in a more compact and cost-effective manner. This offers multiple advantages: a compact instrument can be flown on a much smaller platform (nano- or microsatellite); a low-cost instrument opens up the possibility to fly multiple instruments in a satellite constellation, improving both global coverage and temporal sampling (e.g. to study diurnal processes); a constellation of low-cost instruments may provide added value to the larger scientific and operational satellite missions (e.g. the Copernicus Sentinel missions); and a small, lightweight spectrometer can also be mounted easily on a high-altitude UAV (offering high spatial resolution). Last but not least, a low-cost instrument may allow to break through the ‘cost spiral’: lower cost will allow to take more risk and thus progress more quickly. This may lead to a much faster development cycle than customary for current Earth Observation instruments. To explore the potential of a constellation of low-cost instruments a consortium of Dutch partners was formed, which currently consists of Airbus Defence and Space Netherlands, ISISpace, S and T and TNO. In this paper we will illustrate this new design approach by using the most advanced design of a hyperspectral imaging spectrometer (named ‘Spectrolite’) as an example. We will discuss the different design and manufacturing techniques that were used to realize this compact and low-cost design. Laboratory tests as well as the first preliminary results of airborne measurements with the Spectrolite breadboard will be presented and discussed. The design of Spectrolite offers the flexibility to tune its performance (spectral range, spectral resolution) to a specific application. Thus, based on the same basic system design, Spectrolite offers a range of applications to different clients. To illustrate this, we will present a mission concept to monitor NO2 concentrations over urban areas at high spatial resolution, based on a constellation of small satellites.
The use of diffusers in space-based spectrometers leads to the creation of speckles, in the entrance slit. Due to these speckles, unwanted modulations in the measured spectral data are observed. We explain the origin of these unwanted modulations, which we refer to as speckle-induced spectral features or, in short, spectral features. A single value is introduced that can be used to indicate the effect of the speckle-induced spectral features, the spectral feature amplitude (SFA). Measurement and modeling of the SFA parameter on aluminum diffusers are presented.
The presence of structures, as observed in real spectral data from earth observing satellites, that are due to the on-board diffusers are discussed. The structures are shown to be caused by the speckles in the entrance slit of the spectrometer, created by the diffusers. A dedicated setup for the study of these spectral features will be presented together with results on different types of diffusers, i.e. surface diffusers and volume diffusers. Finally, methods to reduce the amplitude of the spectral features will be presented. It will be shown that QVD (quasi volume diffuser) and Spectralon (a volume diffuser) are far better in reducing the unwanted spectral features than an aluminum surface diffuser. Overall it can be stated that QVD is the "best" diffuser where the validity of this statement depends on the type of its use.
Earth observation satellites are commonly equipped with an on-board diffuser. Their use will be pointed out and the errors associated with their use will be explained. The origin of the errors, the speckles formed by scattering on the diffusers, will be dealt with in detail and solutions to minimize these errors will be presented. The use of diffusers, and the types of diffusers will be discussed in combination with their ability to minimise the contrast of the spectral features.
With the future growing size of telescopes, new, high-resolution, affordable wavefront corrector technology with low power dissipation is needed. A new adaptive deformable mirror concept is presented, to meet such requirements. The adaptive mirror consists of a thin (30-50 μm), highly reflective, deformable membrane. An actuator grid with thousands of actuators is designed which push and pull at the membrane’s surface, free from pinning and piston effects. The membrane and the actuator grid are supported by an optimized light and stiff honeycomb sandwich structure. This mechanically stable and thermally insensitive support structure provides a stiff reference plane for the actuators. The design is extendable up to several hundreds of mm's. Low-voltage electro-magnetic actuators have been designed. These highly linear actuators can provide a stroke of 15 micrometers. The design allows for a stroke difference between adjacent actuators larger than 1 micron. The actuator grid has a layer-based design; these layers extend over a large numbers of actuators. The current actuator design allows for actuator pitches of 3 mm or more. Actuation is free from play, friction and mechanical hysteresis and therefore has a high positioning resolution and is highly repeatable. The lowest mechanical resonance frequency is in the range of kHz so a high control bandwidth can be achieved. The power dissipation in the actuator grid is in the order of milliwatts per actuator. Because of this low power dissipation active cooling is not required. A first prototype is currently being developed. Prototypes will be developed with increasing number of actuators.
The Delft Testbed Interferometer (DTI) will be presented. The
basics of homothetic mapping will be explained together with the
method of fulfilling the requirements as chosen in the DTI setup.
The optical layout incorporates a novel tracking concept enabling
the use of homothetic mapping in real telescope systems for
observations on the sky. The requirements for homothetic mapping
and the choices made in the DTI setup will be discussed. Finally
the first results and the planned experiments will be presented.
In the accurate radiometric calibration of earth observation instruments, diffusers are used as “white-references”. In the frame of on-ground calibration campaigns of instruments such as SCIAMACHY, GOME2 and OMI (all using on-board diffusers), which took place at TNO TPD, a modulation of the reflectance signal in the spectral domain was discovered. This modulation appears when two spectra, recorded under slightly different conditions, are compared. This modulation, referred to here as Spectral features, reduces the accuracy of spectrometers as used in earth observation satellites. The spectral features are caused by speckle phenomena in the entrance slit of the spectrometer. The work reported here describes the origin of the spectral features, the measurements performed in order to reproduce and characterize this effect and the result of simulations performed on speckle patterns thanks to a dedicated analysis tool. A new set-up dedicated to spectral features measurements is also described.
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.
A test-bench simulating the GAIA metrology system has been developed by TNO-TPD. The lines-of-sight of the two telescopes in the GAIA instrument are separated by an angle (called `basic angle') of about 106 degrees. The basic angle stability should be within 10 microarcsecond rms over the satellite revolution period of 3 hours, or should be at least known with this accuracy. The basic angle is monitored by a high-precision interferometric laser metrology system.
An imaging spectrometer has ben designed as a new instrument for remote sensing of the atmosphere. The design has been focused on applications in the field of weather forecast, climate research and atmosphere chemistry research. The push-broom type of instrument consists of a wide field of view telescope, in combination with three spectral channels. The instrument operates without a scanning mirror. Each channel contains a grating, imaging optics and a state-of- the-art 2D CCD detector. The channel separation is performed by a dichroic filter. A sunlit diffuser is applied for the in-flight radiometric calibration and the spectral calibration of the instrument. The polarization sensitivity is minimized by the application of a polarization scrambler. The basic instrument module consists of a UV-channel, a visible channel and a NIR-channel. The main new features of the instrument are a fast and high-resolution coverage of the atmosphere, in combination with high-resolution spectral measurements. This performance is realized in a rather compact instrument. The modular instrument configuration enables an easy adaptation of the instrument for different spectral bands.
A key technique for optical aperture synthesis instruments is the cophasing of the beams from the individual apertures. A cophasing system includes a number of delay lines for the adjustment of the optical path lengths. The design of a high-resolution delay line is based on the mission requirements of a space-borne interferometer. A cat's-eye type of retroreflector is the selected option for the optics. Three delay lines, including actuators and control electronics, have been built and have been tested in a dedicated setup, which includes a star simulator and a cophasing interferometer. The test results support the feasibility of the presented delay line concept.
The Optical Aperture Synthesis Technologies project is aimed at the development of technologies for space interferometry. One of the key technologies is the path length stabilization (or cophasing) of light beams from a guide star. A setup has been developed for testing a system for controlling the cophasing (a delay line) in combination with a system for measuring the cophasing (a cophasing interferometer). The light from a simulated guide star passes a delay line system and it is imaged by the cophasing interferometer on a detector. From the captured (white light) fringe patterns the optical path difference (OPD) is determined in real time. The OPD data are used to drive the delay lines in a control loop system for fast stabilization of the optical paths to a sub-wavelength accuracy. The major design drivers for the testbed development were the optimal mechanical and thermal stability (nanometer level), the control loop bandwidth, the OPD stabilization accuracy and the wavefront quality. The instrumentation (optomechanical breadboard and control system) that has been developed for this project is described, and a first set of test results.