Pre-hibernation performances of the OSIRIS cameras onboard the Rosetta spacecraft

¹ Centro di Ateneo di Studi e Attivitá Spaziali “Giuseppe Colombo”, University of Padova, via Venezia 15, 35131 Padova, Italy
e-mail: sara.magrin.1@unipd.it
² Department of Physics and Astronomy, University of Padova, vicolo dell’Osservatorio 3, 35122 Padova, Italy
³ CNR-IFN UOS Padova LUXOR, via Trasea 7, 35131 Padova, Italy
⁴ Department of Information Engineering, University of Padova, via Gradenigo 6, 35131 Padova, Italy
⁵ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁶ European Space Astronomy Centre, ESA, Villanueva de la Cañada, 28691 Madrid, Spain
⁷ LESIA, Observatoire de Paris, CNRS, UPMC Univ Paris 06, Univ. Paris Diderot, 5 Place J. Janssen, 92195 Meudon Pricipal Cedex, France
⁸ Université Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris, France
⁹ Laboratoire d’Astrophysique de Marseille, CNRS and Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France

Received: 18 March 2014
Accepted: 6 December 2014

Abstract

Context. The ESA cometary mission Rosetta was launched in 2004. In the past years and until the spacecraft hibernation in June 2011, the two cameras of the OSIRIS imaging system (Narrow Angle and Wide Angle Camera, NAC and WAC) observed many different sources. On 20 January 2014 the spacecraft successfully exited hibernation to start observing the primary scientific target of the mission, comet 67P/Churyumov-Gerasimenko.

Aims. A study of the past performances of the cameras is now mandatory to be able to determine whether the system has been stable through the time and to derive, if necessary, additional analysis methods for the future precise calibration of the cometary data.

Methods. The instrumental responses and filter passbands were used to estimate the efficiency of the system. A comparison with acquired images of specific calibration stars was made, and a refined photometric calibration was computed, both for the absolute flux and for the reflectivity of small bodies of the solar system.

Results. We found a stability of the instrumental performances within ±1.5% from 2007 to 2010, with no evidence of an aging effect on the optics or detectors. The efficiency of the instrumentation is found to be as expected in the visible range, but lower than expected in the UV and IR range. A photometric calibration implementation was discussed for the two cameras.

Conclusions. The calibration derived from pre-hibernation phases of the mission will be checked as soon as possible after the awakening of OSIRIS and will be continuously monitored until the end of the mission in December 2015. A list of additional calibration sources has been determined that are to be observed during the forthcoming phases of the mission to ensure a better coverage across the wavelength range of the cameras and to study the possible dust contamination of the optics.