Date: 05 Apr 2008
Depicts: Composite view of WMAP satellite sky positions
Copyright: Sebastien Bouquillon (SYRTE/Obs. de Paris), Ricky Smart (INAF/OATo, Torino) and Alexandre Andrei (Observatorio Nacional, Rio de Janeiro)

Three images taken on 5 April 2008 at time intervals of a few minutes were added up to create this composite frame, showing the NASA's Wilkinson Microwave Anisotropy Probe (WMAP) satellite as it moved across the sky. The obervations were made with the ESO 2.2m telescope at La Silla, Chile, that allowed the imaging of WMAP's position by the sunlight reflecting of the satellite's sunshield.

The exercise was set up to test the ground-based optical tracking concept envisaged for Gaia, that like WMAP will be located at the second Earth-Sun Lagrange point L2.

Before superposition, the three images (actually black-and-white images) were artificially coloured red, green and blue. For the stars, these three colours added up to neutral white. In contrast, the WMAP satellite shows up as the string of coloured points - since it is the only object having moved between the times the three images were taken. In addition to WMAP and a number of stars, a faint galaxy is visible as a slightly fuzzy blob at top centre of the picture.

The exposures were 60 seconds each in the V band. Alexandre Andrei got a preliminary brightness of V=19.4 for WMAP, using the IRAF software, calibrating with 5 UCAC-2 stars, and applying a R-to-V magnitude correction. The WMAP ephemeris predicted an apparent magnitude for La Silla, at the time of observation, of V=18.7.