Planck Spacecraft with Light Path
Artist's impression of the Planck spacecraft. The baffle is removed to reveal the telescope and focal plane, with the light path also indicated. Light falling on the primary mirror (off-axis paraboloid) is reflected onto the secondary mirror (off-axis ellipsoid) after which it falls onto the focal plane, directly below the primary mirror.
Note the wave guides running from the back of the focal plane down into the service module.