Optical Design

Coelostat at the VTT

Two Coelostat mirrors (diameter 80cm) on top of a  10 floor high building feed the sunlight on the primary mirror of the VTT. The latter has a diameter of 70 cm and a focal length of 46 m. The main advantage of the Coelostat system is the stationary (non-rotating) solar image in the laboratory. Further down the optical path another mirror defects the light to the primary focus. The whole optical path between Coelostat and the latter is evacuated in order to avoid internal seeing. 


Since 1998 an  adaptive optic system is been developed at the KIS. Its main components  are a Shack-Hartmann wavefront sensor with 36 sub-apertures, a deformable mirror and a high-speed digital camera. A closed-loop bandwidth of about 130 Hz is achieved. Since 2003, the AO system is permantly installed and available to all focal plane instruments.  

Some numbers:

  • Diameter of primary mirrorl: 70 cm 
  • Focal length: 45.640 m  
  • Aperture ratio f/D: 65.7
  • Resolving power at  543.4 nm: 0.196" 
  • Image scale in primary focu: 4.59"/mm