On-orbit performance of the Gaia CCDs at L2

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. Finally, we summarise some of the detector effects discovered on-orbit which are still being investigated.