ESA GNC Conference Papers Repository

The fine guidance sensor for the Euclid european observatory
F.B. Boldrini, P. Fidanzati, E. Monnini, D. Procopio
Presented at:
Salzburg 2017
Full paper:

Euclid is a European Space Agency (ESA) optical/near-infrared survey mission dedicated to studying the geometry and the nature of the Dark Universe (dark matter and dark energy) with unprecedented accuracy. The Euclid instrument is a 1.2m diameter wide field (0.54 deg²) telescope with a high resolution (0.1 arcsec) visible imaging channel and a co-aligned near-IR channel for lower resolution broad-band photometry and spectroscopy. Euclid will be operated from a large-amplitude libration orbit around Lagrange point L2 of the Sun-Earth system. The science mission will be made up of a Wide Extragalactic Survey (WES), covering a large fraction of the extragalactic sky, and a Deep Survey (DS), covering a few tens of square degrees in repeated visits. The in-orbit operations phase, including Commissioning and the Nominal Science Operations, will last 6.25 years. Leonardo's Fine Guidance Sensor (FGS) is a star sensor that provides the Euclid Attitude and Orbit Control Subsystem (AOCS, developed by Thales Alenia Space Torino, who is also the Euclid Mission Prime) with the extremely accurate attitude measurements that are required to meet the demanding pointing performance for science observation. It is important to note that the FGS optics is provided by the Euclid telescope. The on-board FGS subsystem is composed by: The focal plane (FPA) that consists of two supporting plates (directly placed on the focal plane of the Euclid telescope), each one of which supports two CCDs (Field of View 0.11° x 0.11°). Two Proximity Electronics Modules (PEM) used to control the CCDs and acquire relevant images. Each PEM is dedicated to one of the two FPA plates, and includes two Proximity Electronic Channels (PEC), each dedicated to one CCD. The flex cables, connecting the CCD with the PEC, are shielded to minimize EMI interference with the VIS instrument (the Euclid main payload) detectors. A remote Electronic Unit (EU) that includes two redundant DC/DC converters and Processor Modules (PM) and a cross strapping system with 4 PEC power supplies, each one of which is needed to operate the corresponding connected PEC. The EUCLID FGS employs CCDs that operate in the visible region, at an operating temperature of approximately 150°K. This is due to the fact that the EUCLID telescope operates in the 500 to 900 nm bandwidth and that the FGS CCDs are mounted on the same plate that also support the VIS instrument CCDs. The complete PLM optical bench is thermally controlled at 150°K at satellite level. In the Euclid telescope the FGS is placed as close as possible to the VIS focal plane array, to minimize differential optical-thermo-elastic deformation and maintain the FGS LOS as stable as possible with respect to the VIS channel Line of Sight (LoS). To meet the Euclid telescope LOS stability requirements, it would be sufficient for the AOCS control loop to have the attitude error with respect to a reference attitude (i.e. a relative attitude as provided by the Relative Tracking Mode (RTM)). In addition, the inertial attitude measured by FGS is also needed to meet the Absolute Pointing Error (APE) requirement. As a result, the FGS is equipped with a dedicated star catalogue allowing to provide an estimation of the inertial attitude (ATM: Absolute Tracking Mode) . The FGS performance strongly depends on stars availability. In order to achieve the requested accuracy with such a small FoV, stars with magnitude up to visual magnitude 19 shall be used. The main performances of the FGS are summarized in the following Table: FGS performance The absolute attitude measurement accuracy of commercial STRs (around few arcsec) would be sufficient to meet the Euclid APE requirement, but it is referred to the local star-tracker reference frame. On the other hand, the APE budget has to take into account the relative deformation between the telescope reference frame (in which APE is defined) and the STR frame. The relative orientation of the two reference frames changes over time mainly because of the thermo-elastic deformation caused by attitude change and by the activation/deactivation of power delivered to the equipment. In the ESA Herschel mission the magnitude of this deformation was measured to be up to 70', a value that is widely incompatible with Euclid needs. Thermo-elastic issues in Euclid are tackled by placing the Fine Guidance Sensor inside the payloads cavity and by having the FGS detectors as close as possible to the VIS instrument. The aforementioned design allows the FGS to achieve a relative attitude measurement accuracy of 03' over 700sec with 99.7% confidence level (i.e. 0.0142'/sqrt(Hz) up to 0.5Hz) around the transverse axes and; 1' over 700sec with 99.7% confidence level (i.e. 1'/sqrt(Hz) up to 0.5Hz) around the perpendicular axis (boresight). The absolute measurement accuracy achieved by FGS is 0.6' at 99.7% confidence level and 8.7' at 99.7% confidence level respectively around the transverse and boresight axes. In order to achieve such performance, the FGS has a very narrow Field Of View (FOV) of 0.11x0.11 degrees with a strong impact on stars availability, that imposes the requirement to detect stars as faint as magnitude 19. This value was chosen as limiting magnitude considering both the FGS FoV and the statistics of star population per unit of area brighter than a specific magnitude. Another star-catalogue related challenge is the not perfect astrometric and photometric knowledge of some objects, which automatically excludes them from the useful set of targets that may be used for attitude calculation. The FGS attitude reconstruction algorithm requires at least 3 targets to be present on each detector for attitude determination. Nevertheless, a nominal set of 9 stars is used to improve the attitude accuracy. Commercial Autonomous Star Tracker are equipped with star catalogues containing stars up to the magnitude 8, that are sufficient taking into account their larger FoV. Considering the star coverage at that magnitude, on average, the FGS would have less than one star in the FoV and could not determine attitude. Another innovative aspect of the Euclid FGS is the complexity and the management of the star catalogue. Due to the limiting magnitude (Magnitude 19) and to the extensive sky area to be covered, a dedicated Euclid Star Catalogue is being developed for the FGS by merging the information of existing catalogues. The accuracy of the star position information of the catalogue is a major contributor to the FGS attitude accuracy. As a result, a stringent requirement - < 0.4 arcsec per axis (1sigma) - on the position accuracy is imposed to the selection of the guide stars candidates. In addition, possible disturbing neighbor stars (for position and magnitude) shall be avoided and therefore stars with a neighbor within 2 arcsec and brighter than 2 mag shall be discarded. The Euclid FGS Star Catalogue will be the database of all the known available stars on which FGS can rely for ATM during the entire mission. Because of its dimension (estimated > 1.5GB) it cannot be stored in the FGS on-board memory. For this reason an advance catalogue management system was developed; the master static catalogue will be stored on-ground (Input Star Catalogue, representative of the whole sky) and portions of it will be uploaded periodically on-board the FGS: i.e. the On-board Star Catalogue is a subset of the complete star catalogue, specifically generated for each observation field and session. Another of Euclid's peculiarities relate to the solutions that were developed to solve electromagnetic noise issues due to the proximity between the VIS Instrument and the FGS at the focal plane level. To avoid impacts on VIS, which is the Euclid main instrument, the FGS assembly includes a pair of EMI shields between FPA and PEM. This solution was taken to limit the electromagnetic noise generated by the high frequency signal passing in the four FGS CCD flex cables and which, if injected on the VIS instrument readout chain, could impact its performance. In the Euclid FGS the detector type and technologies, as well as the operating temperatures, are driven by the telescope's design and by the other instruments assembled on the telescope focal plane. However, the FGS configuration can be adapted to suit other missions with different payload instruments, environmental conditions and functionalities. For example, FGS may have its own optics imaging the detectors, starting from a common path in the telescope, feeding the FGS optics through a beam splitter. This paper will describe the Euclid FGS characteristics and performance and potential adaptations for other applications requiring sub-arcsecond attitude knowledge performance to accomplish their mission objectives.