Star background
Star background as a measurement method
The recording technique using a star background is used in astronomy , space travel and satellite geodesy . The celestial object you are looking for or to be measured is photographed against the background of the star field that is in the predicted direction.
The stars only serve as control points , with whose known coordinates the transformation of the camera orientation into the celestial reference system is carried out. A distinction is made between the astrometric methods ( affine mapping , 2D polynomials, long turner , etc.) and the photogrammetric methods, which model the optical mapping.
Sometimes the traces of the stars, which are caused by the rotation of the earth with longer exposure times , are also an aid to discovering new, weak celestial bodies such as asteroids , because the recorded traces of moving bodies are slightly pivoted against those of the fixed stars .
The camera can either be fixed ( Altaz or ballistic camera ) or track the daily movement of the starry sky ( equatorial mounting of telescopes or astrographs ). With the latter method, which depicts the stars as points, weak objects can be detected with the blink comparator , which can compare two consecutive images.
Star background for comparison
The more distant stars of the Milky Way system are also referred to as the star background if they serve as a reference for measuring closer stars. With their relative movement, for. B. Friedrich Wilhelm Bessel 1838 for the first time to prove an annual parallax of a nearby "foreground star", namely the fast runner 61 Cygni .
Similarly, galactic astronomy uses the background of distant galaxies in order to be able to measure and analyze the movement of closer star systems .
Occasionally, background stars or galaxies can also be used to gain important insights indirectly, for example through the gravitational lens or microlens effect , which a dark mass moving past in the foreground causes on the apparent brightness of the objects behind. This is how dark matter was discovered in isolated cases and in 2003 an exoplanet .