As Zenit camera is called a compact camera, with which the stars are photographed in the vicinity of the zenith, to thereby determine the perpendicular direction.
Zenit cameras are important instruments of the modern Astro Geodesy and can (measurement of latitude, longitude and their amendments ) except Lot and geoid determination are also used to measure time and Sternörtern. They are available in two versions:
- Photographically: camera with focal length of 30 to 100 cm, highly sensitive photographic plates, precision rotary table for 90 ° - or 180 ° rotations about a vertical axis.
- Digital camera: focal length 10-50 cm and CCD sensor instead of the photographic plate. The axis of rotation must be due to shorter design and exposure time not be so precisely manufactured, which enables the construction of easily transportable devices.
After the first exposure (for example, 5-10 seconds ) is rotated by 180 ° and made further exposure (s). Measure the short star trails gives the axis of rotation at the starry sky, and thus the perpendicular direction, if it is vertical. Your any small tilt is detected with dragonflies or Lotsensoren.
The older, photographic design is now scarcely used because the measurement of the photographic plates with a comparator takes about 1-2 hours. The measurement data consist in the image coordinates of all stellar tracks ( about 100 stars). With digital sensors and methods of digital image processing, the measurement of the star coordinates is performed automatically.
With digital zenith cameras, the accuracy of the measured perpendicular direction, depending on the effort between 0.05 " and 0.1 ", whereby local geoid and Quasigeoidbestimmungen be made possible with millimeter accuracy.
At the University of Hannover, Vienna University of Technology and ETH Zurich Zenit cameras were developed and verified using automated CCD. They will help in the coming years to achieve the desired results since 1995 centimeter - geoid in the countries of Central Europe.