Experiments with the goal to provide a ground layer correction by using a wide field SHWFS were performed at the DST (Rimmele et al., 2010c). Modeling of wide field SHWFS was performed by Wöger and Rimmele (2009). As the WFS FOV is increased from the small field of the conventional AO to larger and larger field size the correction applied with the DM is an average over an increasingly larger number of field directions, i.e., a large number of isoplanatic patches. The field averaging of wavefront information is essentially done optically and by the correlation algorithm. However, this simple implementation of GLAO has its limitations as is seen in Figure 54, which shows the variance of residual image motion overlaid on the speckle reconstructed granulation. The WFS FOV was 42” × 42”. The residual image motion variance is fairly uniform across the field but residual field dependence is still visible.
It has been demonstrated that a wide field WFS can increase the sensitivity of the correlating WFS and, thus, is expected to work for worse seeing conditions (Owner-Petersen et al., 1993). As pointed out by Rimmele et al. (2010c) GLAO may be a tool to improve telescope efficiency. Bad seeing periods during which conventional AO provides very low Strehl or simply does not work at all can still be utilized for certain science projects that require subarcsecond but not diffraction limited resolution. Such observations would gain from the available photon flux of a large aperture. In particular, in the near infrared a modestly sized GLAO system could potentially open up a new window for high cadence polarimetry.
GLAO might also be an attractive option for synoptic solar telescopes such as SOLIS and a future larger aperture GONG network. Depending on the site characteristics the 50 cm SOLIS telescope could potentially operate close to its diffraction limit for the full solar disk with a relatively modest GLAO system.
Living Rev. Solar Phys. 8, (2011), 2
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