Determination of ghost images for the Wide Angle Camera of the Rosetta ESA mission

Rosetta is a cornerstone mission of the European Space Agency (ESA); it has been launched in March 2004 and it will reach its primary target, the 67P/Churyumov Gerasimenko comet, in 2014. One of the Rosetta instruments is the OSIRIS camera system; it is composed of a Wide Angle Camera (WAC) and a Narrow Angle Camera (NAC). The NAC will observe at high resolution regions on the comet surface, while the WAC, 12°x12° Field of View (FoV), has been mainly designed for observing the weak coma features surrounding the bright comet nucleus. Being the expected contrast between the gas and dust jets radiance and the nucleus one of the order of 1/1000, a high contrast capability is required for the WAC camera. To meet this requirement, its optical design is off-axis and unobstructed, and to deeply study gas and dust emissions, the camera is equipped with 14 filters, each with its proper wavelength selection characteristics, in the range 230-720 nm. The theoretical achievable contrast capability is limited by multiple ghost images generated by the optical elements in the focal plane assembly, in fact the incoming beam is partially reflected on the surfaces of the filters, on the detector radiation protective glass cover and on the CCD detector itself. Given that the knowledge of ghosts position and intensity is essential for an adequate scientific data reduction, in this paper an analysis of the ghosts, the solutions adopted to limit image degradation and the impact on the WAC imaging performance are presented.