Solar adaptive optics has to be considered a success story. The success of solar AO can be measured not only by the impressive imagery obtained with AO but most importantly by new scientific results, the use-rate of solar AO and the numerous scientific publications that were enabled by AO. Ground-based solar astronomy is experiencing a renaissance. New large aperture adaptive optics telescopes such as the ATST are currently under construction and will provide observations of the Sun with unprecedented resolution. These observations will allow us to put to the test and guide theoretical models and simulations and, thus, advance tremendously our physical understanding of solar phenomena. The successful development of solar AO and the demonstration of its scientific potential contributed in no small part to the community’s success in making the case for funding of large aperture ground-based solar telescopes.
This article summarized the difficult path that led to the successful implementation of solar AO at ground-based solar telescopes. It is fair to say that the rapid development of fast computer technology needed for the implementation of the correlating SHWFS as well as the commercial availability of deformable mirrors were major prerequisites in bring solar adaptive optics to fruition. As with any instrument a good understanding of potential error sources is crucial for estimating and optimizing the systems performance and, in turn, maximizing scientific productivity. The combination of AO and post facto image processing provides ground-based solar astronomers with a powerful tool that can rival or even surpass space based observations. AO enables diffraction limited observing at large aperture telescopes where similar size aperture telescopes from space are likely cost prohibitive.
New developments in the field of solar AO include the construction of extreme solar AO systems with thousands of actuators for the 4 m class telescopes, the very challenging development of MCAO and the possible use of GLAO. MCAO will mitigate one of the major limitations of conventional AO – the small corrected FOV. Compared to current small aperture solar telescopes the isoplanatic patch will be significantly smaller at 4 m solar telescopes. Some level of partial correction can be achieved over wide fields at small aperture solar telescopes with conventional AO. The correction level is sufficient to facilitate post-facto processing and diffraction limited imaging can be obtained over sizable FOVs even at visible wavelengths. This will be extremely challenging at 4 m solar telescopes. MCAO is, therefore, considered a vital technology for these next generation telescopes. The large 4 m aperture, in principle, provides 0.1” diffraction limited resolution at near infrared wavelengths. Because of the dependence of the Fried parameter the size of r0 increases by about a factor of four (4) for observations in the near infrared (1.6 µ). This means that the for a 4 m telescope and for near infrared wavelengths is roughly the same ratio as for a 1 m telescope operating at visible wavelengths and conventional AO performance is expected to be comparable. This may lead to more focus being placed on infrared instrumentation at these larger telescopes at least initially and until MCAO is fully operational at a level comparable to current conventional solar AO systems.
The implementation of MCAO at solar telescopes is aided by the fact that the multiple “guide stars” required to perform tomography of the turbulent volume are provided by the Sun’s omnipresent granulation pattern. The complexity of multiple laser guide stars needed for night-time MCAO is avoided. The initial success achieved with solar MCAO development is encouraging. Nevertheless, much work remains to be done before fully operational MCAO will be available at solar telescopes. GREGOR and NST will soon allow us to gain operational experience with solar MCAO. The use of GLAO may be of advantage for certain applications where sub-arcsec (but not diffraction limited resolution) and high photon flux is required. GLAO can be used in combination with post-facto reconstruction techniques, although at 4 m telescopes and at visible wavelengths achieving large, well corrected FOVs will be a serious challenge. GLAO also seems to be an attractive option for synoptic telescopes of moderate aperture (e.g., SOLIS).
Living Rev. Solar Phys. 8, (2011), 2
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