The Wide Angle Camera for the Rosetta Mission

The Wide Angle Camera (WAC) for the Rosetta mission had to fulfil many scientific requirements: Field of View of 12x12 sq deg and focal length of 140 mm, excellent optical throughput in the range 240 - 900 nm after 10 years in space, Encircled Energy of 80 ratio of 10E+4 in order to detect faint gaseous emission features around a bright nucleus, minimum exposure times of 10 msec with photometric accuracy better than 5x102, scattered light rejection for sources out of the FoV (e.g. Sun) and in the FoV (e.g. cometary nucleus), a cover to close off the cometary dust, an optical bench capable to maintain the optical alignment in a passive way and to support the shutter, the baffle, a double filter wheel and the Focal Plane Assembly. To these initial requirements several other constraints were added in the course of the design, in particular a very complex thermal profile, a massive shielding of the front FoV in order to protect the CCD from the cosmic radiation, and a very strict total mass envelope. These requirements called for an unconventional optical design, with 2 aspherical mirrors in an off-axis configuration (the primary mirror being convex), and a carefully studied 3-stage baffle. Both the shutter and the front cover provided extremely challenging technological goals, for the mechanical and for the electronics aspects of them. The paper describes all the main elements of the WAC. At present, the Structural Thermo Model has been delivered after successful completion of vibration and vacuum tests. The STM optical bench configuration has been slightly revised for the flight Model in order to provide more attenuation to the internal baffle