Automatized alignment of the focal plane assemblies on the PLATO cameras

peer reviewedPLATO (PLAnetary Transits and Oscillation of stars) is a medium-class space mission part of the ESA Cosmic vision program. Its goal is to find and study extrasolar planetary systems, emphasizing on planets located in habitable zone around solar-like stars. PLATO is equipped with 26 cameras, operating between 500 and 1000nm. The alignment of the focal plane assembly (FPA) with the optical assembly is a time consuming process, to be performed for each of the 26 cameras. An automatized method has been developed to fasten this process. The principle of the alignment is to illuminate the camera with a collimated beam and to vary the position of the FPA to search for the position which minimizes the RMS spot diameter. To reduce the total number of measurements which is performed, the alignment method is done by iteratively searching for the best focus, decreasing at each step the error on the estimated best focus by a factor 2. Because the spot size at focus is similar to the pixel, it would not be possible with this process alone to reach an alignment accuracy of less than several tens of microns. Dithering, achieved by in-plane translation of the focal plane and image recombination, is thus used to increase the sampling of the spot and decrease the error on the merit function

Analysis of a stray-light facilities for applications in earth observation instruments

Description of the development of a facility for measuring stray light for earth observation instruments. Performance analysis by simulation (ASAP), and applications on instruments like ProbaV or Solar Orbite

Why Stray-light in optical instruments matters and what to do about it

Stray-light is a critical concern for space instruments. This presentation presents the underlying phenomena and presents guidelines for what to do against i

Contrôle de lumière parasite dans les instruments spatiaux : au-delà des limites conventionnelles

Stray light is a critical problem for space telescopes. It these aspects could be solved, we could develop space instruments with far better performances. This would open the door to new applications and scientific discoveries, from monitoring Earth atmosphere composition better than ever to observing fainter objects in the galaxy. This work aspired to solve this problem. It started by the application of state of the art methods of control by design, then new approaches were proposed to overcome the conventional limits. One of them consists in removing complex stray light features by post-processing, where an improvement by two orders of magnitude is demonstrated. Next, a major limitation of traditional approaches is the inability to measure and identify individual stray light contributors in an instrument. Ultrafast time-of-flight characterization is introduced to solve this decades-old problem, enabling unprecedented understanding and control of stray light in space telescopes

Contrôle et analyse de lumière parasite dans un télescope anastigmate à trois miroirs

peer reviewedOff-axis three-mirror anastigmat (TMA) telescopes provide excellent correction of aberrations over a large field-of-view in one direction. In a push broom configuration, this lightweight and compact optical configuration enables high-performing imaging. Moreover, passive multi-spectral acquisition can be achieved using a filter stack positioned at the detector vicinity. Stray light is a typical limiting factor for instrument performance and its control is specific to the type of optical configuration. We describe the stray light control and analysis methods in an off-axis TMA. The design intends to control first-order scattering from non-optical surfaces and to block straight shots. This is achieved through usage of elements such as apertures and baffles, both internal and external to the instrument. It will be demonstrated that the aperture stop is a critical element, whose stray light contribution can be controlled with V-groove vanes. Impact of mirrors roughness, a manufacturing physical limitation, is evaluated by modeling the bidirectional scattering distribution function and computing the stray light distribution at the detector for point-like source illumination. We show that the scattering on the different mirrors broaden differently the spot size, as the successive mirrors focus or expand the scattered rays differently. Finally, the ghost reflections inside the filter stack are evaluated and we show that there is no cross-talk between the different channels

Advanced course on stray-light in space optical instruments

Advanced course (20 hours) on stray-light given for engineers and Phd at the China Academy of Space Technology in Beijin

A 100 M GROUND RESOLUTION GLOBAL DAILY COVERAGE EARTH OBSERVATION MISSION

PROBA-V has been successfully launched on 7th May 2013 and is providing a global monitoring in the continuity of the SPOT-VEGETATION mission. The progress in terms of ground resolution between Spot VGT and PROBA-V is a factor 3 (1 km to 1/3 km ground resolution product). The User Community requirements for the next generation of global monitoring are a 100 m ground resolution product. This means an additional factor 3 improvement, but in a short time frame (5 years). After success of the PROBA-V mission, the Belgian Science Policy (BELSPO) initiates a PROBA-V Successor feasibility study. This study was undertaken by VITO and CSL to identify potential tracks to achieve a follow-on mission which is expected to be relevant for the User Community. The mission analyses for each of these tracks was evaluated. Today the PROBA-V mission lifetime is expected to expire by mid of 2018. Since the interest for global land monitoring is expected to continue in the future, this study proposes mission requirements and a shortlist of optimal mission scenarios for a follow-on mission in this short time frame. The goal of such a new PROBA-V mission is clear: it should ensure the data continuity of global vegetation monitoring, while taking the opportunity to further improve the data quality. Data continuity is essential for understanding long term trends of land use that may affect the global equilibrium of the planet (in the context of scarcity for land or food, natural disasters, climate change). As for added value, a fine example is the improvement of spatial resolution when comparing PROBA-V with the spatial resolution in SPOT-VEGETATION products. An improvement in spatial resolution towards a full 100m product is considered by the user community as the main target for a PROBA-V follow-on mission

Time-resolved characterization of stray light

Traditional approach to stray light characterization is intrinsically limited. While the stray light level in an optical instrument can be measured, it is not possible to derive from experimental measurements the origin of the different features. Consequently, when unexpectedly high stray light is present, it is extremely difficult to find how to improve the system. In this paper, we introduce a new method where a pulsed laser and an ultra-fast sensor is used. As different stray light contributors have different optical path lengths, they reach the detector at different times and resolving them temporally allows to measure them separately. Their origin can be retrieved by using the optical path length as a mean of identification. We present the conceptual study and the experimental proof of concept of this new method. We were able to characterize individually the different stray light components in an imaging system and determine their origin. We show how the measurements allow to reverse engineer the instrument properties and even verify sub-system requirements