9.3 Ground-Layer Adaptive Optics (GLAO)

Ground-Layer Adaptive Optics is an attractive option for solar AO (Rimmele et al., 2010cJump To The Next Citation Point). During the daytime most of the turbulence is located near the ground. Up to 90% of the turbulence can be located within the first 100 – 200 m while the r0 of the upper atmosphere can be rather large. Fried parameters of 20 – 40 cm are possible at those higher layers. Excluding the ground layer and assuming an overall r0 in the visible of 10 cm (20 cm) the Haleakala turbulence profile shown in Table 1 would yield an effective r0 for all higher layers of 22 cm (42 cm). Because of the λ −56 dependence of r0 the best utility of GLAO at large aperture telescopes may be at near infrared wavelengths. This means that with a simple high order ground layer correction it is theoretically possible to achieve subarcsecond resolution over a very wide field. A number of science objectives can be addressed with a resolution of 0.25 – 0.5” that potentially might be provided by GLAO. At this reduced resolution (with GLAO the diffraction limit can not be reached at visible wavelengths) a large aperture solar telescope, such as the 4 m ATST, would provide a tremendous photon flux and, thus, a significant cadence and/or sensitivity advantage. Sensitive vector magnetic field measurements of active regions that currently take on the order of an hour and, hence, miss much of the very dynamic nature of active region evolution could be performed in a few minutes provided efficient instrumentation is used.

Experiments with the goal to provide a ground layer correction by using a wide field SHWFS were performed at the DST (Rimmele et al., 2010cJump To The Next Citation Point). 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 54View Image, 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.

View Image

Figure 54: Maps of the variance of residual image motion measured with GLAO mode at the DST.

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