8.2 Kiepenheuer Institute Adaptive Optics System (KAOS)

The Kiepenheuer Institute Adaptive Optics System has been operating at the 70 cm Vacuum Tower Telescope (VTT) on Tenerife, Spain, since 2002 (von der Lühe et al., 2003). The system is very similar in design and implementation to the SST system in that it uses a correlating SHWFS with 36 subapertures (d = 10 cm) with a hexagonal arrangement and a 35 element bimorph mirror. The FOV of the subaperture images is 12” × 12” sampled with 24 × 24 pixels. Using more pixels per subaperture increases the tracking stability (Berkefeld, 2007Jump To The Next Citation Point). Significant effort has been spent to automate many system calibration functions, which is essential for making the system operational to the non-expert user and enabling remote system support. The system frame rate was increased in 2006 in order to increase the system bandwidth to approximately 100 Hz. The minimum seeing needed to achieve diffraction limited observations is 0.8”. The processor unit is a general purpose Sun Fire V880. Figure 34View Image shows the optical implementation of KAOS into the German VTT on Tenerife. Figure 35View Image (left) shows quantitatively what level of correction can be achieved with KAOS as a function of the Fried parameter. The uncorrected and corrected power spectral density is plotted as well in Figure 35View Image (right) and indicates that a bandwidth of about 100 Hz is achieved. The residual rms wavefront error is 0.1 waves and a Strehl of S = 0.7 is achieved for seeing of about r0 = 20 cm. In comparison, the high order DST AO76 system achieves a Strehl of S = 0.7 for seeing of r0 = 8 cm. Figures 36Watch/download Movie and 37Watch/download Movie show uncorrected and corrected granulation movies, respectively, that demonstrate the ability of KAOS to compensate for atmospheric turbulence and fixed aberrations of the optical system.
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Figure 34: Opto-mechanical implementation of KAOS at the VTT on Tenerife (from Rimmele, 2004a).
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Figure 35: Performance of KAOS. Left: Residual wavefront errors are plotted vs. the Fried parameter r0 (from Berkefeld, 2007Jump To The Next Citation Point). Right: Corrected and un-corrected temporal PSD of wavefront errors. The crossover occurs at a bandwidth of about 100 Hz (from Berkefeld et al., 2007).

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Figure 36: mpg-Movie (1536 KB) KAOS: uncorrected granulation movie (courtesy of T. Berkefeld, KIS).

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Figure 37: mpg-Movie (2162 KB) KAOS: corrected granulation movie (courtesy of T. Berkefeld, KIS).

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