Charge Transfer Inefficiency effect for high-precision radial velocity measurements

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Abstract
Charge Transfer Inefficiency (CTI) is a well known effect of charge-coupled devices (CCD). The charge transfer from one pixel to the next is not perfect and is quantified by the fraction of charge successfully moved (clocked) between adjacent pixels. The amplitude of this effect depends on the signal level inside the pixel. In the context of high-precision radial velocity measurements using cross-dispersed echelle spectrograph, this CTI effect on a CCD recording spectral orders may introduce associated spectral lines shifts. Indeed if the signal level recorded on the spectra is changing, the CTI amplitude will change affecting the associated centroid of all spectral lines. Such effect may introduced radial velocity shifts of several m s-1. We describe here CTI effect which is affecting the SOPHIE spectrograph installed on the 1.93-m telescope of Observatoire de Haute Provence (OHP). We calibrated the effect thanks to the Thorium-Argon lines and we applied a software correction on the spectra in order to assess the charge lost during the readout process on all pixels.

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