GaiaData Release 1

Abstract

The European Space Agency's 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.

TheGaiamission

“…The focal plane assembly of Gaia (for a detailed description, see Kohley et al 2012;Crowley et al 2016b) is common to both telescopes and has five main functions: (i) metrology (wave-front sensing [WFS] and basic angle monitoring [BAM]; Sects. 3.3.3 and 3.3.4); (ii) object detection in the sky mapper (SM; Sect.…”

“…5;Crowley et al 2016b) are backilluminated, full-frame devices with an image area of 4500 lines along-scan and 1966 columns across-scan; each pixel is 10 µm × 30 µm in size (corresponding to 58.9 mas × 176.8 mas on the sky), balancing along-scan resolution and pixel full-well capacity (around 190 000 e − ). All CCDs are operated in time-delayed integration (TDI) mode to allow collecting charges as the object images move over the CCD and transit the focal plane as a result of the spacecraft spin.…”

“…Davis et al 2001). Owing to the (so far) benign nature of the current solar cycle 24, however, the accumulated radiation dose has been significantly lower than expected, with only ∼4% of the predicted end-of-mission (five-year) dose having materialised so far during the first two years of operations (for a detailed description, see Crowley et al 2016b). Radiation damage is hence not expected to strongly affect the long-term performance of Gaia.…”

“…3.1), dedicated payload calibrations are periodically performed (Crowley et al 2016b) in close collaboration with the DPAC payload experts group. These calibrations include the following.…”

GaiaData Release 1

“…"CCD transits" refer to the crossing by a source of a particular CCD in the focal plane. Thus the focal plane transit of the astrometric field typically consists of 10 transits across individual CCDs, while a photometric instrument transit (BP or RP) consists of only one CCD transit, and a transit across the RVS instrument consists of three CCD transits (see Gaia Collaboration 2016b; Crowley et al 2016b, for more details on the focal plane layout and functionalities, and the in-flight performance of the Gaia CCDs). This distinction is important when it comes to the difference between the number of measurements (CCD transits) collected for a source and the number of times it was observed (focal plane transits) by Gaia.…”