The Gaia mission is already delivering spectacular data to scientists and, by around 2022, will provide us with a very accurate three-dimensional map of our Milky Way galaxy. The Gaia spacecraft, launched on 19 December 2013, has been making scientific observations of the universe since July 2014. TNO developed, built and tested the Wave Front Sensor for this mission.
The Gaia-WFS is used to keep the ‘camera’ (telescopes) in sharp focus so that the mission can generate accurate data while data from the Gaia-WFS are being used to monitor that the telescopes do not misalign during the mission. The instrument is also being used for diagnostics.
Gaia, part of the Cosmic Vision programme of ESA and built by Airbus DS/Mersen-Boostec, enables the creation of a precise three-dimensional map of about one billion stars throughout our galaxy and beyond, mapping, in the process, their motions that provide information about the origin and evolution of the Milky Way. Comprehensive photometric classification provides details about physical properties of each star observed: luminosity, temperature, gravity and elemental composition. This massive stellar census will provide the basic observational data to tackle an enormous range of important problems related to the origin, structure and evolutionary history of our Milky Way.
This Wave Front Sensor (WFS) system developed by TNO measures the wave front errors of the two Gaia telescopes mounted on the Gaia spacecraft. The optical aberrations of this system itself have to be low. The required accuracy for the WFS is lambda/1000. Gaia is active over a broad wavelength and in cryogenic conditions (450 to 900 nm wavelength band and 130 to 200 K operating temperature). Invar developed a temperature independent solution to comply with these conditions.
The telescope projects images of the stars onto the Focal Plane Array (FPA). The WFS picks up a small part of this field and then projects the telescope pupils onto a Micro Lens Array (MLA) whereby each micro lens images part of the pupil on the same FPA. Monitoring the spot positions on the FPA array provide the basis for measuring the wave front quality of the Gaia telescope (for the respective field of view). The spot positions of a star can be compared to the images of three calibration sources built into the WFS.