Due to the worse daytime seeing conditions and the fact that much of the science is done at visible wavelengths, solar AO systems require a large number of corrective elements in spite of the so-far relatively small (compared to night-time telescopes) apertures of solar telescopes. New generation solar telescopes such as the 4 m ATST require a much larger number of DOF, and the AO systems for the ATST and EST approach the complexity of what is referred to as extreme AO. In addition, the corrected FOV of a high order solar AO system implemented at a 4 m telescope is significantly smaller than what solar astronomers are accustomed to from their experience with AO at smaller existing telescopes. This issue will be addressed in much more detail in Section 6.1.3.
The small value of r0 at visible wavelengths and with daytime seeing conditions require solar AO systems to achieve a very high closed loop bandwidth. The incoming wavefront varies rapidly in time. Figure 7 plots as a function of temporal frequency the Power Spectral Density (PSD) of Zernike coefficient Z4 (astigmatism) and Z24 as measured with the low order NSO AO system (Rimmele, 2000). A break point in the PSD occurs at about 10 Hz for Z4 and 20 Hz for Z24. The frequency at which the break occurs is the Greenwood frequency and increases with the radial mode number. This demonstrates the well known fact that higher order systems require higher bandwidth as well. The spectrum contains signal power out to at least 200 Hz at which point noise becomes dominant. The high Greenwood frequency or more accurately the high temporal frequency content of the wavefront fluctuations leads to required sampling rates of > 2 kHz and closed loop bandwidths for high order solar AO systems in excess of 100 Hz (Rimmele, 2004a).
A major challenge for solar AO was the development of a suitable wavefront sensor. Wavefront sensors used for night-time AO system cannot be directly used for solar AO systems because point sources that are used as guide stars (natural or laser) for night-time AO systems are not available when observing the Sun. A solar AO system has to be able to lock on extended targets such as pores, sunspots or a substructure of a sunspot and solar granulation. Solar granulation, in particular, is a challenging target to track on since the granulation pattern is of low contrast and changes on time scales of about 1 min.
Laser guide stars are not a practical solution for solar AO since either extremely bright lasers would be needed to project a laser spot against the bright background of the solar disk or very special narrow-band filters (e.g., magneto-optical filters for sodium) would have to be used (Beckers, 2008). The complexity and cost of this approach has so far prevented any serious efforts in this direction. A possible application for laser guide stars in solar astronomy may be observations of the very faint corona. The brightness of the corona is only a few millionths of the disk brightness and natural guide stars, i.e., coronal structure bright enough to track are not available. The future use of laser guide star AO may therefore be considered for coronal observations to be performed with the 4 m Advanced Technology Solar Telescope.
Living Rev. Solar Phys. 8, (2011), 2
This work is licensed under a Creative Commons License.