The ESA formation Flying mission Proba-3 will y the giant solar coronagraph ASPIICS. The instrument is composed of a 1.4 meter diameter external occulting disc mounted on the Occulter Spacecraft and a Lyot-style solar coronagraph of 50mm diameter aperture carried by the Coronagraph Spacecraft positioned 144 meters behind. The system will observe the inner corona of the Sun, as close as 1.1 solar radius. For a solar coronagraph, the most critical source of straylight is the residual diffracted sunlight, which drives the scientific performance of the observation. This is especially the case for ASPIICS because of its reduced field-of-view close to the solar limb. The light from the Sun is first diffracted by the edge of the external occulter, and then propagates and scatters inside the instrument. There is a crucial need to estimate both intensity and distribution of the diffraction on the focal plane. Because of the very large size of the coronagraph, one cannot rely on representative full scale test campaign. Moreover, usual optics software package are not designed to perform such diffraction computation, with the required accuracy. Therefore, dedicated approaches have been developed in the frame of ASPIICS. First, novel numerical models compute the diffraction profile on the entrance pupil plane and instrument detector plane (Landini et al., Rougeot et al.), assuming perfect optics in the sense of multi-reflection and scattering. Results are confronted to experimental measurements of diffraction. The paper reports the results of the different approaches.
This paper presents the recent achievements in the development of ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), a solar coronagraph that is the primary payload of ESA’s formation flying in-orbit demonstration mission PROBA-3. The PROBA-3 Coronagraph System is designed as a classical externally occulted Lyot coronagraph but it takes advantage of the opportunity to place the 1.4 meter wide external occulter on a companion spacecraft, about 150m apart, to perform high resolution imaging of the inner corona of the Sun as close as ~1.1 solar radii. Besides providing scientific data, ASPIICS is also equipped with sensors for providing relevant navigation data to the Formation Flying GNC system. This paper is reviewing the recent development status of the ASPIICS instrument as it passed CDR, following detailed design of all the sub-systems and testing of STM and various Breadboard models.
Proba-V payload is a successor of the Vegetation instrument, a multispectral imager flown on Spot-4 and subsequently on Spot-5, French satellites for Earth Observation and defence. The instrument, with its wide field of view, is capable of covering a swath of 2200 km, which, in combination with a polar low Earth orbit, guarantees a daily revisit.<p> </p>The lifetime of Spot-5 expires in early 2013, and to ensure the continuity of vegetation data, BELSPO, the Belgian Federal Science Policy Office, supported the development of an instrument that could be flown on a Proba type satellite, a small satellite developed by the Belgian QinetiQ Space (previously known as Verhaert Space).<p> </p>The challenge of this development is to produce an instrument responding to the same user requirements as Vegetation, but with an overall mass of about 30 kg, while the Vegetation instrument mass is 130 kg. This development had become feasible thanks to a number of new technologies that have been developed since the nineties, when Vegetation was first conceived, namely Single Point Diamond Turning fabrication of aspherical mirrors and efficient VNIR and SWIR detectors.<p> </p>The Proba-V payload is based on three identical reflective telescopes using highly aspherical mirrors in a TMA (Three Mirrors Anastigmat) configuration. Each telescope covers a field of view of 34° to reach the required swath.<p> </p>One of the challenges in the development of the PROBA-V instrument is the efficient reduction of stray light. Due to the mass and volume constraints it was not possible to implement a design with an intermediate focus to reduce the stray light. The analysis and minimization of the in-field stray light is an important element of the design because of the large FOV and the surface roughness currently achievable with the Single Point Diamond Turning.<p> </p>This document presents the preliminary baffle layout designed for the Three Mirrors Anastigmatic (TMA) telescope developed for the Proba-V mission. This baffling is used to avoid 1<sup>st</sup> order stray light i.e. direct stray light or through reflections on the mirrors. The stray light from the SWIR folding mirror is also studied. After these preliminary analyses the mechanical structure of the TMA is designed then verified in term of vignetting and stray light.
Today more and more small Earth Observation satellites are under development. All of them are very ambitious and needs accurate on ground calibration. A typical case is PROBA V payload where to ensure the continuity of Vegetation data, an instrument responding to the same user requirements as Vegetation was build, but with an overall mass of about 30 kg, instead of the 130 kg of VGT. Because a very high level of performances is required these need to be verified and calibrated with a high level of accuracy. This paper presents the calibration facility developed for the testing of small Earth Observation payloads as PROBA V. The facility needs to address the geometrical and radiometric calibration of the payload. To achieve this, a 400 mm clear aperture off axis collimator with a dedicated focal plane is developed for the geometrical calibration and a 300 mm integrating sphere calibrated at the LNE is used for the radiometric calibration. To access all the Field Of View, the payload is placed on a rotating tip tilt table allowing rotation of +/- 180° for across track Field Of View scanning and +/-10° for along track scanning. The payload is surrounded by thermal shroud to provide the required thermal environment.
With the upcoming of TMA or FMA (Three or Four Mirrors Anastigmat) telescope design in Earth Observation system, stray light is a major contributor to the degradation of the image quality. Numerous sources of stray light can be identified and theoretically evaluated. Nevertheless in order to build a stray light model of the instrument, the Point Spread Function(s) of the instrument, i.e., the flux response of the instrument to the flux received at the instrument entrance from an infinite distant point source needs to be determined. <p> </p>This paper presents a conceptual design of a facility placed in a vacuum chamber to eliminate undesired air particles scatter light sources. The specification of the clean room class or vacuum will depend on the required rejection to be measured. Once the vacuum chamber is closed, the stray light level from the external environment can be considered as negligible. Inside the chamber a dedicated baffle design is required to eliminate undesired light generated by the set up itself e.g. retro reflected light away from the instrument under test. This implies blackened shrouds all around the specimen. The proposed illumination system is a 400 mm off axis parabolic mirror with a focal length of 2 m. The off axis design suppresses the problem of stray light that can be generated by the internal obstruction. A dedicated block source is evaluated in order to avoid any stray light coming from the structure around the source pinhole. Dedicated attention is required on the selection of the source to achieve the required large measurement dynamic.
Three mirror anastigmat (TMA) telescope designs  had been implemented in different projects ranging from the narrow Field-Of-View large instruments as Quickbird (2° FOV)  to smaller telescopes as JSS 12° FOV developed for RapidEye mission .<p> </p>This telescope configuration had been also selected for the PROBA-V payload, the successor of Vegetation, a multispectral imager flown on Spot-4 and subsequently on Spot-5 French satellites for Earth Observation and defence. PROBA-V, small PROBA-type satellite, will continue acquisition of vegetation data after the lifetime of Spot-5 expires in 2012.<p> </p>The PROBA-V TMA optical design achieves a 34° FOV across track and makes use of highly aspherical mirrors. Such a telescope had become feasible due to the recently developed Single Point Diamond Turning fabrication technology. The telescope mirrors and structure are fabricated in aluminium and form an athermal optical system.<p> </p>This paper presents the development of the compact wide FOV TMA, its implementation in PROBA-V multispectral imager and reviews optics fabrication technology that made this development possible. Furthermore, this TMA is being used in combination with a linear variable filter in a breadboard of a compact hyperspectral imager. Moreover, current technology allows miniaturization of TMA, so it is possible to use a TMA-based hyperspectral imager on a cubesat platform.
PROBA V is an ESA mission devoted to the observation of the Earth’s vegetation, providing data continuity with the Spot 4 and 5 vegetation payloads. Thanks to the heritage of the Proba series, the satellite’s platform is smaller than a cubic metre, accommodating the main payload, i.e. the Vegetation Instrument (VI), and some technology demonstrators. The VI extremely wide viewing swath, together with a polar low Earth orbit, enables daily revisits during 2.5 years, with a possible extension to 5 years. The mission, whose satellite is developed by Belgian QuinetiQ Space, is actually in Phase D and the targeted launch is early 2013 with the VEGA launcher. <p> </p>The Vegetation Instrument is a high spatial resolution pushbroom 4 spectral bands imager composed of three distinct Spectral Imagers (SI). Each SI has 34° Field Of View (FOV) across track, and the total FOV of the VI is 102°, covering an Earth swath of 2260 Km with ground sampling distance down to 96 m at Nadir for VNIR bands. <p> </p>The spectral bands are centred around 460 nm for the blue, 655 nm for the red, 845nm for the NIR and 1600 nm for the SWIR. The imaging telescope is built from a Three-Mirrors Anastigmat (TMA) configuration, including two highly aspheric mirrors. The optics is manufactured from special grade aluminium by diamond turning. The material being identical to the whole structure, no defocus or stresses build up with temperature variations in flight. <p> </p>This paper gives an overview of the VI performances, and focuses on the results of the optical tests and on-ground calibrations.
With the development of new spectrometer concepts, it is required to adapt the calibration facilities to characterize correctly their performances. These spectro-imaging performances are mainly Modulation Transfer Function, spectral response, resolution and registration; polarization, straylight and radiometric calibration. <p> </p>The challenge of this calibration development is to achieve better performance than the item under test using mostly standard items. Because only the subsystem spectrometer needs to be calibrated, the calibration facility needs to simulate the geometrical “behaviours” of the imaging system. <p> </p>A trade-off study indicates that no commercial devices are able to fulfil completely all the requirements so that it was necessary to opt for an in home telecentric achromatic design. The proposed concept is based on an Offner design. This allows mainly to use simple spherical mirrors and to cover the spectral range. The spectral range is covered with a monochromator. Because of the large number of parameters to record the calibration facility is fully automatized. <p> </p>The performances of the calibration system have been verified by analysis and experimentally. Results achieved recently on a free-form grating Offner spectrometer demonstrate the capacities of this new calibration facility. <p> </p>In this paper, a full calibration facility is described, developed specifically for a new free-form spectro-imager.
The Centre Spatial de Liège in Belgium (CSL) has developed a stray light test facility for In Field and Out of Field of
View stray light characterization of small Earth observation satellites. The first tested satellite is PROBA V, a small ESA
satellite, developed by a Belgian consortium, dedicated to replace the SPOT VGT on SPOT missions. The test results
demonstrate that the stray light performance of both PROBA V and the test facility are excellent and are in line with the
model predictions. The new facility is designed for in-field and far field stray light characterization: intensities dynamic
range up to 10<sup>8</sup>:1 for in-field and up to 10<sup>10</sup>:1 for far field stray light in the visible to SWIR spectral ranges. Moreover, from previous stray light tests performed at CSL, vacuum conditions are needed for reaching the 10<sup>-10</sup> rejection requirement mainly to avoid air/dust diffusion. To fulfill these requirements the stray light facility is built in one of CSL vacuum chamber located in a class 100. The large dynamic range required is achieved by using a high radiance point source allowing small diverging collimated beam. A lot of care is taken in the design of the collimator focal plane to
provide a highly purely collimated luminance. Previous articles have presented the principle, the concept and a detailed analysis of the facility for stray light characterization of EO satellites. This paper goes a step forward with the presentation of actual test facility description and results obtained on PROBA V EO satellite. The achieved results are put in parallel to the modeled computed values.