2.3 Solar AO challenges: difference between night and day

In basic design solar AO systems are quite similar to night-time AO systems. However, compared to night-time AO, solar AO faces a number of different challenges and solar AO systems are in some aspects technically more challenging than night-time AO (Rimmele, 2004aJump To The Next Citation Point). The main challenges are the poor and time varying daytime seeing, the fact that solar astronomers mostly observe at visible wavelengths (down to 380 nm), and the solar wavefront sensor, which has to work on low-contrast, extended, time-varying objects such as solar granulation. Due to heating of the ground by direct sunlight, the near-ground turbulence layer is much stronger during the day and typical Fried parameters are of order 10 cm (500 nm) at an excellent site and at a typical telescope height of 20 – 40 m above ground. The entrance aperture of the Dunn Solar Telescope at Sacramento Peak, NM was placed at a height of 40 m in order to get above a large fraction of the near-ground turbulence. Nevertheless, the Fried parameter fluctuates significantly on short time scales (seconds) and often drops to values of just a few centimeters (Figure 6View Image). In comparison, night-time seeing conditions generally provide significantly larger and less fluctuating Fried parameters. In addition, most night-time AO systems operating on large aperture night-time telescopes operate at infrared wavelengths where the Fried parameter is again larger. Although some night-time AO systems are already operating at visible wavelengths (Fugate, 2003) and efforts to implement visible AO at large aperture night-time telescopes are in progress (Bouchez et al., 2010).

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.

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Figure 6: Fried parameter as a function of time as measured at the DST. The seeing during the daytime can fluctuate significantly and with short time scales (from Marino et al., 2004).

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 7View Image 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, 2000Jump To The Next Citation Point). 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, 2004aJump To The Next Citation Point).

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Figure 7: Corrected and uncorrected modal PSD for Zernikes (Noll, 1976Jump To The Next Citation Point) Z4 and Z24 derived from the NSO low order AO system. Z24 is not corrected with this system due to its low order of correction of about 20 modes. The seeing contains frequencies well above 100 Hz (from Rimmele, 2000Jump To The Next Citation Point).

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.


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