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On September 14th, ESA will release data that has been obtained during two years of the Gaia-mission. The goal of this mission is to compose a three dimensional map of our Galaxy containing information of an unprecedented quality on the position of over a billion stars. TNO has contributed to the success of this mission by designing, developing and testing on-board instruments that monitor both the angular stability of the GAIA-telescopes as well as the wavefront quality. The measured angle of the two telescopes is used to determine the exact 3D position, a measurement technique known as parallax.
The payload module of Gaia contains one main instrument that carries out three separate functions: astrometric, photometric and radial velocity measurements. The light that reaches the instrument is collected by two separate telescopes which are set at a fixed angle of 106.5°. To ensure the required precision of the parallax measurements this angle does not only need to remain stable, but even more importantly, it must be known to an extremely high degree. For this purpose, TNO developed a metrology system called the Gaia-BAM: the Basic Angle Monitor. The Gaia-BAM sends two pairs of parallel laser bundles to the two telescopes, which will create two interference patterns. If the angle between the telescopes change, these patterns will also show a shift. On the basis of this shift, the Gaia-BAM determines the basic angle between the telescopes with a precision of 0.5 microarcseconds. This compares to measuring the size of Euro coin on the moon, as seen from Earth.
During the commissioning phase, it was found that the variations in the basic angle were several orders of magnitude larger than was predicted. Since the high precision parallax measurements require the basic angle to be known at levels far below the accuracy of Gaia itself, this could have had serious consequences for the mission. At this moment, the Gaia-BAM proved itself to be crucial for the success of the mission. The Gaia-BAM showed that its measurements are stable and consistent and even better than designed for and is thereby providing the data that is essential in correcting the unforeseen large variations in the basic angle.
The contribution of Gaia-BAM is also being recognized by Anthony Brown, Chairman of the Gaia Data Processing and Analysis Consortium Executive (DPACE): “The Gaia satellite shows variations in the angle between the lines of sight of its two telescopes that would reduce the accuracy of the data. However thanks to the BAM these can be measured and corrected for, making this first data release already a success. In future data releases this information will continue to play a prominent role in the processing of the Gaia measurements."
Since the position and the motion of a star depends for a large part on the conditions at its birth, and can therefore lead to new insights regarding the formation of our Galaxy, this has for a long time been of interest for astronomers. In 1989 ESA already launched Hipparcos, a predecessor of Gaia also mapping our Galaxy. During the four years in which Hipparcos was collecting data and sending this back to earth, data of almost 120.000 stars was obtained. The collected data played a role in discoveries such as the refinement of the age of our Universe by several billion years. The technological advancements since the days of Hipparcos have led to several substantial improvements on the Gaia-mission, such as a hundred times higher precision, the possibility to measure multiple stars at once with a billion pixel camera. These advancements allow Gaia to map over a billion stars, covering approximately 1% of the stars in our Galaxy, during its time in orbit. On a single day, Gaia can measure as many as 270 million stars.
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