Design of the Gaia satellite: the M4/M'4 mirrors and focal plane # Incoming light from mirror M3 # Incoming light from mirror M'3 # Focal plane, containing the detector for the Astrometric instrument in light blue, Blue Photometer in dark blue, Red Photometer in red, and Radial Velocity Spectrometer in pink. # Mirrors M4 and M'4, which combine the two incoming beams of light # Mirror M5 # Mirror M6, which illuminates the focal plane # Optics and diffraction grating for the Radial Velocity Spectrometer (RVS) # Prisms for the Blue Photometer and Red Photometer (BP and RP)