Giuseppe Sarri

Gaia Data Release 1 - On-orbit performance of the Gaia CCDs at L2

The European Space Agencys Gaia satellite was launched into orbit around L2 in December 2013 with a payload containing 106 large-format scientific CCDs. The primary goal of the mission is to repeatedly obtain high-precision astrometric and photometric measurements of one thousand million stars over the course of five years. The scientific value of the down-linked data, and the operation of the onboard autonomous detection chain, relies on the high performance of the detectors. As Gaia slowly rotates and scans the sky, the CCDs are continuously operated in a mode where the line clock rate and the satellite rotation spin-rate are in synchronisation. Nominal mission operations began in July 2014 and the first data release is being prepared for release at the end of Summer 2016. In this paper we present an overview of the focal plane, the detector system, and strategies for on-orbit performance monitoring of the system. This is followed by a presentation of the performance results based on analysis of data acquired during a two-year window beginning at payload switch-on. Results for parameters such as readout noise and electronic offset behaviour are presented and we pay particular attention to the effects of the L2 radiation environment on the devices. The radiation-induced degradation in the charge transfer efficiency (CTE) in the (parallel) scan direction is clearly diagnosed; however, an extrapolation shows that charge transfer inefficiency (CTI) effects at end of mission will be approximately an order of magnitude less than predicted pre-flight. It is shown that the CTI in the serial register (horizontal direction) is still dominated by the traps inherent to the manufacturing process and that the radiation-induced degradation so far is only a few per cent. We also present results on the tracking of ionising radiation damage and hot pixel evolution. Finally, we summarise some of the detector effects discovered on-orbit which are still being investigated. (Less)

Gaia astrometric CCDs and focal plane

ESAs Gaia astrometry mission is due for launch in 2011. The astrometric instrument focal plane will have an area of up to 0.5m2 and will contain more than 100 CCDs. These will be operated in Time Delay and Integration mode in order to track and observe sources whilst the telescopes continuously scan the sky. Gaias target for astrometric precision of a few millionths of an arc second, places extreme demands on focal plane thermo--mechanical stability and electronics performance. The CCDs themselves are large area, back illuminated, full--frame, four phase devices. They require maximum efficiency for observing the majority of (faint) objects, yet must simultaneously be able to handle very bright objects that will regularly cross the field of view. Achieving the final astrometric precision will also require excellent noise performance and MTF. In addition to demanding excellent performance from each CCD, they will need to be produced in large numbers which raises production and yield issues. When analyzing Gaia data it will be essential to understand and calibrate CCD behaviour correctly, including the expected performance degradation due to radiation damage. This is being addressed through comprehensive testing and the development of CCD models.

The Gaia focal plane

The astronomic mission Gaia is a cornerstone mission of the European Space Agency, due for launch in the 2011 time frame. Requiring extremely demanding performance, Gaia calls for the development of an unprecedented large focal plane featuring innovative technologies. For securing the very challenging Gaia Focal Plane Assembly (FPA) development, technology activities have been led by EADS Astrium from 2002 to 2005. After EADS Astrium selection for the development of the Gaia satellite, the program started in early 2006. The all-Silicon Carbide FPA hosts all the mission scientific functions for Astronometry, Photometry and Radial Velocity Spectrometry, encompassing 106 large scientific CCDs operated in TDI mode with windowing readout. With a sensitive area of about half a square meter, the FPA includes more than 935 millions of 10 μm x 30 μm pixels. To fulfill all the requirements, the CCDs feature a specific design with a Silicon Carbide package and on-chip functions such as TDI dynamic gain control and pulsed charge injection. Main development issues are related to the mass production of CCDs, and extremely low noise and miniaturized focal plane electronics. Finally, the major challenge of the overall focal plane mechanical and thermal accommodation is to allow full modularity while providing perfectly stable temperature and efficient thermal decoupling between the CCDs area (160 K - 170 K) and electronics operated in standard temperature conditions.

Gaia Radial Velocity Spectrometer

This paper presents the specification, design, and development of the Radial Velocity Spectrometer (RVS) on the European Space Agency’s Gaia mission. Starting with the rationale for the full six dimensions of phase space in the dynamical modelling of the Galaxy, the scientific goals and derived top-level instrument requirements are discussed, leading to a brief description of the initial concepts for the instrument. The main part of the paper is a description of the flight RVS, considering the optical design, the focal plane, the detection and acquisition chain, and the as-built performance drivers and critical technical areas. After presenting the pre-launch performance predictions, the paper concludes with the post-launch developments and mitigation strategies, together with a summary of the in-flight performance at the end of commissioning.

GAIA payload module description

The European Space Agency has approved Gaia, the sixth cornerstone mission of its Scientific Programme, and awarded the satellite development contract to EADS Astrium SAS at beginning of 2006 for its Implementation Phase. The Gaia mission will provide unprecedented stellar position and radial velocity measurements. This will be used to produce a three-dimensional map of about one billion stars in our Galaxy and beyond. The most prominent feature of the Gaia satellite is the high precision payload, which is fully developed by EADS Astrium SAS. The paper is devoted to the payload description.

The HSOB GAIA: a cryogenic high stability cesic optical bench for missions requiring sub-nanometric optical stability

Global astrometry, very demanding in term of stability, requires extremely stable material for optical bench. CeSiC developed by ECM and Alcatel Alenia Space for mirrors and high stability structures, offers the best compromise in term of structural strength, stability and very high lightweight capability, with characteristics leading to be insensitive to thermo-elastic at cryogenic T°. The HSOB GAIA study realised by Alcatel Alenia Space under ESA contract aimed to design, develop and test a full scale representative High Stability Optical Bench in CeSiC. The bench has been equipped with SAGEIS-CSO laser metrology system MOUSE1, Michelson interferometer composed of integrated optics with a nm resolution. The HSOB bench has been submitted to an homogeneous T° step under vacuum to characterise the homothetic behaviour of its two arms. The quite negligible inter-arms differential measured with a nm range reproducibility, demonstrates that a complete 3D structure in CeSiC has the same CTE homogeneity as characterisation samples, fully in line with the GAIA need (1pm at 120K). This participates to the demonstration that CeSiC properties at cryogenic T° is fully appropriate to the manufacturing of complex highly stable optical structures. This successful study confirms ECM and Alcatel Alenia Space ability to define and manufacture monolithic lightweight highly stable optical structures, based on inner cells triangular design made only possible by the unique CeSiC manufacturing process.

Gaia Data Release 2: Calibration and mitigation of electronic offset effects in the data

The European Space Agency Gaia satellite was launched into orbit around L2 in December 2013. This ambitious mission has strict requirements on residual systematic errors resulting from instrumental corrections in order to meet a design goal of sub-10 microarcsecond astrometry. During the design and build phase of the science instruments, various critical calibrations were studied in detail to ensure that this goal could be met in orbit. In particular, it was determined that the video-chain offsets on the analogue side of the analogue-to-digital conversion electronics exhibited instabilities that could not be mitigated fully by modifications to the flight hardware. We provide a detailed description of the behaviour of the electronic offset levels on microsecond timescales, identifying various systematic effects that are known collectively as offset non-uniformities. The effects manifest themselves as transient perturbations on the gross zero-point electronic offset level that is routinely monitored as part of the overall calibration process. Using in-orbit special calibration sequences along with simple parametric models, we show how the effects can be calibrated, and how these calibrations are applied to the science data. While the calibration part of the process is relatively straightforward, the application of the calibrations during science data processing requires a detailed on-ground reconstruction of the readout timing of each charge-coupled device (CCD) sample on each device in order to predict correctly the highly time-dependent nature of the corrections. We demonstrate the effectiveness of our offset non-uniformity models in mitigating the effects in Gaia data. We demonstrate for all CCDs and operating instrument and modes on board Gaia that the video-chain noise-limited performance is recovered in the vast majority of science samples.

Laser metrology for high stability optical bench characterization

In the framework of activities dedicated to the GAIA mission, Alcatel Alénia Space has developed a High Stability Optical Bench (HSOB) made with CeSiC. A key criterion was the stability of its geometry, requiring it to be confirmed by tests. The idea was to use a metrologic system able to work in a primary vacuum environment with sub-nanometric resolution and reproducibility compliant with the expected performances of the bench. In answer to these requirements, SAGEIS-CSO has proposed a system consisting of a frequency stabilised laser, an optical coupler, two optical sensor heads named MOUSE I and a detection unit. The delivered information is a longitudinal displacement measurement between the sensor and its joint retroreflector. Verification of the resolution by tests has given a value of about 15 pm in a 15 Hz pass band. Displacement repeatability versus temperature has been checked before tests campaign and in the final configuration, giving a value between ±1.4 nm/K and ±1 nm/K. These excellent performances allowed the sensor to qualify the stability of the HSOB.

The Gaia Astrometry Mission: Status after CDR Completion

Gaia is an ESA science cornerstone mission, which relies on the proven principles of ESAs Hipparcos mission to solve one of the most difficult yet deeply fundamental challenges in modern astronomy: to create an extraordinarily precise three-dimensional map of about one billion stars aiming at star magnitudes down to 20 throughout our Galaxy and beyond. In the process, it will map the motion of the stars, which encodes the origin and subsequent evolution of the Galaxy. Through comprehensive photometric classification, it will provide the detailed physical properties of each star observed: characterizing their luminosity, temperature, gravity, and elemental composition. This massive stellar census will provide the basic observational data to tackle an enormous range of important problems related to the origin, structure, and evolutionary history of our Galaxy. To achieve Gaia’s challenging mission goals, the spacecraft contain a number of novel design solutions. Astrium SAS in Toulouse has been awarded the contract to build the Gaia spacecraft in Spring 2006. Since then the program has successfully completed the design and development phase and is now in the integration and verification phase of the flight model. This paper describes the Gaia mission, focusing on the main spacecraft design features as well as on the status of the Gaia mission.

Manufacturing of a 3D complex hyperstable Cesic structure

Global astrometry requires extremely stable materials for instrument structures, such as optical benches. Cesic®, developed by ECM and Thales Alenia Space for mirrors and high stability structures, offers an excellent compromise in terms of structural strength, stability and very high lightweight capability, with a coefficient of thermal expansion that is virtually zero at cryogenic T°. The High-Stability Optical Bench (HSOB) GAIA study, realized by Thales Alenia Space under ESA contract, aimed to design, develop and test a full-scale representative of the HSOB bench, made entirely of Cesic®. The bench has been equipped with SAGEIS-CSO laser metrology system MOUSE1, a Michelson interferometer composed of integrated optics with nm-resolution. The HSOB bench has been submitted to a homogeneous T° step under vacuum to characterize 3-D expansion behavior of its two arms. The quite negligible interarm differential, measured with a nm-range reproducibility, demonstrates that a complete 3-D structure made of Cesic® has the same CTE homogeneity as do characterization samples, fully in line with the stringent GAIA requirements (1ppm at 120K). This demonstrates that Cesic® properties at cryogenic temperatures are fully appropriate to the manufacturing of complex highly stable optical structures. This successful study confirms ECMs and Thales Alenia Spaces ability to design and manufacture monolithic lightweight highly stable optical structures, based on inner-cell triangular design made possible by the unique Cesic® manufacturing process.