Atmospheric compensated imaging

In addition to excellent imaging properties, high-resolution telescope systems
require a large entrance pupil to improve the diffraction-limited
resolution. As a result of wind, air pressure and temperature differences, turbulences arise in the optical path which can vary the refraction index via the beam cross section spatially and with regards to time. This results in an enlargement of the minimum possible diffraction spot which leads to a loss of resolution. Adaptive optics can resolve these limitations.
A mobile demonstration system was developed with the background of providing verification of the principle of compensation of local turbulence. In this system, a fast Shack-Hartmann wavefront sensor (bandwidth 1 kHz, 16 x 16 microlens array) and an adaptive membrane
reflector with 52 actuators are used. Using this system, the compensation of
local turbulence was successfully demonstrated. However, it appeared that the compensation of volume-distributed turbulence on the laser test range was far less successful. This is due to the high level of requirements on the spatial resolution and the bandwidth of the wavefront sensor and the adaptive reflector. At the same time, the isoplanatic angle restricts the effective usable image area. Multi-conjugate adaptive optical methods or iterative optimisation algorithms offer a new approach to better fulfil these requirements in the future. A complete compensation of volume distributed turbulence is, however, not possible using the approaches available at present.