8.1 The Swedish Solar Telecope (SST) AO system

The SST is a 97 cm aperture vacuum telescope located on La Palma, Spain (Scharmer et al., 2003). Adaptive optics is an essential component of the telescope. The SST was built after the power of solar AO had been demonstrated and consequently the DM is integrated into the optical path that feeds the instrumentation. A separate correlation tracker is used to measure the tip/tilt component, which is corrected by a tip/tilt stage located close to the DM. The optical layout of the system is very simple, compact, and efficient (Scharmer et al., 2000). The 37-element bimorph mirror is combined with a 37 subaperture correlating Shack–Hartmann wavefront sensor. The system corrects about 35 modes. The lenslet array with a hexagonal arrangement provides a subaperture size of d = 13.8 cm. The off-the-shelf CCD camera reads the Shack–Hartmann images with a frame rate of 955 fps, which limit the bandwidth of the system to about 65 Hz (0 dB crossover of error rejection). The data is processed by an off-the-shelf general purpose computer. Processing of subapertures, reconstruction, and servo control, as well as other control tasks, are all performed by this commercially available computer, which allows for easy upgrade of hardware since the software can be easily ported to faster computers as they become available.

The large aperture of the NSST combined with the 37-element AO system and post-facto processing techniques have produced many of the highest resolution images and movies of the Sun so far, a few examples of which were shown in previous sections. However, the 35 mode correction provided by the SST AO system has to be considered a low order correction that provides good results under excellent seeing conditions. An upgrade of the SST AO system to a 85-element bimorph DM is already in progress and will enable diffraction limited observations for a wider range of seeing conditions and, thus, further increase the scientific productivity of the SST.

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