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Modern Solar Facilities – Advanced Solar Science, 107–113 F. Kneer, K. G. Puschmann, A. D. Wittmann (eds.) c  Universitätsverlag Göttingen 2007 Solar adaptive optics T. Berkefeld Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany Email: berke@kis.uni-freiburg.de Abstract. We give an overview of solar adaptive optics (AO) systems, its differences to nighttime AO systems and some basic design considerations for the wavefront sensor and the wavefront reconstruction . As an example, we present the Kiepenheuer Adaptive Optics System (KAOS), lessons learned from four years of operation, and the various AO-related projects that use or will use KAOS. 1 Introduction Ground based observations of the Sun in the visible spectral range are severely limited by wavefront aberrations caused by the Earth’s atmosphere. Typically, a spatial resolution of only 1–1.5 arcseconds can be achieved, corresponding to the diffraction limit of a 10 cm telescope . However, in order to answer fundamental questions that occur when trying to understand the Sun, a spatial resolution approaching the diffraction limit of meter-class telescopes is required. For very short exposures this can be achieved by image reconstruction techniques such as phase diversity (van Noort et al. 2005; Tritschler & Schmidt 2002) or speckle imaging (v.d.Lühe 1989; Mikurda & v.d.Lühe 2006). But for high accuracy observations, e.g. spectropolarimetric measurements, long exposure times make a realtime wavefront correction mandatory. Considering observing efficiency, AO works in two ways: • For a given spatial resolution, AO increases the amount of observing time. • For a given observing time, AO improves the spatial resolution. At first, image stabilisation systems were introduced (Schmidt & Kentischer 1996; Ballesteros et al. 1996), but only by using a full compensation of the wavefront can the theoretical resolution of the telescope be restored. Although adaptive optics has been used for nighttime astronomy for more than ten years (Rousset et al. 1990), its application in solar astronomy was delayed for several years due to three reasons: • Wavefront sensors must be able to track an arbitrary structure on an extended source, rather than an isolated point source. • The Sun heats the ground, so the daytime seeing is usually worse than the nighttime seeing. • Solar observations are mostly done in the visible, therefore the AO must correct in this regime. 108 T. Berkefeld: Solar adaptive optics Table 1. Present solar AO systems. DST, Sac. Peak SST, La Palma VTT, Tenerife BBSO, Big Bear first light 1998 2000 2002 2004 telescope diameter [m] 0.76 0.98 0.70 0.65 # corrected modes 60 27 27 40 # actuators 97 35 35 97 0 db bandwidth [Hz] 130 ? 100 130 Presently four solar AO systems are used regularly (in the order of first light): The AO at the Dunn Solar Telescope (DST) of the National Solar Observatory, Sacramento Peak (Rimmele & Radick 1998), the AO at the Swedish solar telescope (SST), La Palma (Scharmer et al. 2000), the AO at the German Vacuum Tower Telescope (VTT) of the Kiepenheuer-Institute, Tenerife (v.d.Lühe et al. 2003), and the AO at the Big Bear Solar Observatory (BBSO) (Rimmele et al. 2004). Table 1 shows the basic parameters of these four systems. During the last years the existence of a good working AO system has become an indicator for the competitiveness of a telescope (e.g quality of the post focus instrumentation). Section 2 gives an overview of solar wavefront sensing, in Section 3 the basic wavefront reconstruction is explained and in Section 4 the KAOS system is shown as an example for solar AO systems. 2 Solar Shack-Hartmann wavefront sensing As already mentioned, the wavefront sensor (WFS) in solar AO systems has to track on arbitrary structures. Up to now, only crosscorrelating Shack-Hartmann (SH) WFSs are used. The differences to nighttime SH sensors are the larger field of view (FOV), the faster and larger CCD/CMOS WFS cameras and the way the shift vector is obtained. In order to achieve a good correlation function, the FOV should be as large as possible, but it should not exceed the isoplanatic patch (∼ 10 in the visible). The pixel scale should match the resolution of one subaperture. Crosscorrelating the image of a reference subaperture with the other subapertures leads to correlation functions that are more star-like and can be treated like stars in a stellar WFS (parabolic/gauss fit around the brightest pixel). Both FFT-based crosscorrelation and direct crosscorrelation methods are used. Solar WFS cameras typically have ∼ 250x250...

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