The first adaptive optics experiments with the Sun were performed at the Dunn Solar Telescope by Hardy in 1979 – 1980 (Hardy, 1980). Hardy used a shearing interferometer for a wavefront sensor and a 21 actuator continuous faceplate DM. Wavefront sensing targets were stars and sunspots. This was one of the first on-sky adaptive optics experiments and success was limited.
The National Solar Observatory AO program aimed to develop a solar AO system for the DST (Dunn, 1987; Dunn et al., 1989; Dunn, 1990) using an in-house built 61 actuator continuous faceplate DM (Dunn et al., 1992) and a focal plane LCD mask WFS (von der Lühe, 1988). This wavefront sensor concept can be traced back to the well known Focault knife-edge test (Darvann and Dunn, 1987), which also places a mask (knife-edge) in a focal plane and visualizes phase aberrations as intensity fluctuations in a pupil plane. The sensor measures wavefront gradients, and for small wavefront errors its output has been shown to be equivalent to that of the Shack–Hartmann sensor (Rimmele and Radick, 1998). For high contrast objects that are limited in spatial extent (star, pore, small sunspot, planet) a straight knife edge can be used as focal plane mask as is demonstrated with Figure 8.
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Granulation, however, requires a rather complicated focal plane mask, an example of which is shown in Figure 9. The mask is derived from the following equation:
Since granulation evolves on time scales of minutes the mask has to be continuously updated; this can be implemented using a programmable LCD screen. Such a sensor was implemented at the DST but, in particular when used on granulation, had serious signal-to-noise issues. This approach does not divide the pupil into subapertures and therefore does not suffer from the limitations such as subaperture diffraction. However, the focal plane mask introduces diffraction effects in the pupil plane and in this way limits the resolution with which the wavefront can be resolved. The concept was recently investigated further with a laboratory setup (Schmidt and von der Lühe, 2007) but so far has not been successfully implemented at a solar telescope.
A Shack–Hartmann based solar AO system was developed by Lockheed (Acton and Smithson, 1992; Acton and Dunn, 1993) and tested at the DST. The system was based on a custom built 19 element segmented mirror combined with a Shack–Hartmann sensor. The SHWFS used analog quad-cell detectors to sense image shifts, which limited its application to small high contrast objects, i.e., solar pores and thus severely limited the system’s scientific use. In addition, due to the complexity of the system, it could be characterized as an optical experiment rather than a science instrument. Figure 10 shows the segmented DM and the quad-cell SHWFS of the Lockheed AO system. A particular challenge of the segmented mirror approach is phasing the segments. Expertise developed for the Lockheed AO system has since been useful to segmented mirror telescope projects such as Keck and JWST.
These early solar AO efforts were forced to custom-develop all components, such as DM, reconstructor and control hardware, and WFS. Many of these components were not available commercially, and development (either in-house or through development contracts) was extremely expensive, time consuming and plagued by frequent setbacks. A viable and practical solution to the solar wavefront sensor problem was also lacking. A breakthrough in solar AO came with the development of the correlating Shack–Hartmann wavefront sensor and its implementation in the NSO low-order adaptive optics system. The NSO low-order solar AO system was the first fully operational solar AO system that was also capable of tracking on granulation. The design of this 24 subaperture solar AO system is described in detail by Rimmele and Radick (1998) and Rimmele (2000).
This system was successfully tested in 1998 at the DST and was operated on a routine basis at the DST for a number of years. The low-order solar AO system achieved diffraction limited imaging with high Strehl ratios (up to 0.6) in good seeing conditions (r0 (500 nm) > 12 cm). The success was made possible by rapid development of computer technology that allowed the implementation of the compute and data transfer intensive correlation algorithm described in more detail in Section 4.
The NSO low-order AO system made use of components that at the time had just become commercially available. An off-the-shelf XINETICS DM with 97 (Ealey and Wellman, 1994) actuators and sophisticated control electronics could be implemented. A correlating Shack–Hartmann wavefront sensor with 24 subapertures was developed based on Digital Signal Processor (DSP) technology. The correlating Shack–Hartmann wavefront sensor uses the same principle that had been used for quite some time to provide tip/tilt correction at solar telescopes with a device called Correlation Tracker (von der Lühe et al., 1989). The challenge for low-order AO system development was implementing 24 correlation tracker channels running in parallel, at high update rates and with low latency. In 1998 the NSO low-order solar AO was the first system to demonstrate that AO can work on granulation and represented an important and timely milestone in making a compelling case for the ATST through the US decadel review process. A solar AO system was installed at the 50 cm Swedish Solar Telescope in 1999 (Scharmer et al., 2000, 2003). Following the successful implementation of these systems other solar AO systems were developed at major solar telescopes (von der Lühe et al., 2003; Scharmer et al., 2003; Keller et al., 2003) some of which are still in operation. All solar AO systems currently in operation are based on the correlating Shack–Hartmann wavefront sensor. Section 8 summarizes the characteristics of these systems.
The NSO low-order AO system was the first to demonstrate adaptive optics on granulation and scientific utility of solar AO. Hence, some early results from this system are shown here, even though, this system has since been surpassed by higher performing systems installed at solar telescopes such as the SST on La Palma, the German VTT on Tenerife, and the DST in New Mexico.
Figure 11 shows observations of a sunspot obtained with the low-order adaptive optics system at the DST. The observations were performed using a CCD camera behind the Universal Birefringent Filter (UBF), which has a passband of about 250 mÅ. The images shown were obtained by co-adding 12 individual 1.5 s exposures resulting in a 18 s effective exposure time. The top row images show a narrow-band filtergram and the corresponding line-of-sight magnetic field map obtained by analyzing the circular polarization states. As expected, the bright points surrounding the sunspot seen in the intensity map are co-located with magnetic field elements. The size of these bright points is on the order of the diffraction limit of 0.2” at 630 nm: demonstrating that diffraction limited resolution has been achieved in these long exposure data. Similarly, diffraction limited resolution is achieved in the intensity (bottom left) and velocity (bottom right – bright: upflow; dark: downflow) map, respectively, taken with the UBF tuned to the wings of an Fe i line at 557.6 nm.
In the above examples the high contrast sunspot structure was used as a wavefront sensing target. However as discussed above, the noise of a correlating Shack–Hartmann wavefront sensor increases as the image contrast of the object decreases. Figure 12 shows a narrow-band filtergram of solar granulation recorded with an effective exposure time of 30 s. The AO was locked on a 10” × 10” FOV in the center of the image. These images demonstrate that the diffraction limit is still achieved when the low contrast granulation is used as a wavefront sensor target.
The low-order AO system has produced a number of impressive results. However, the low-order system was not well matched to median seeing conditions at Sacramento Peak. An early site survey determined that the median seeing at Sac Peak is r0 (500 nm) = 8.7 cm (Brandt et al., 1987). The ATST site survey (Hill et al., 2004, 2006) performed a much more extensive and systematic measurement of the Fried parameter at Sac Peak and determined a lower median r0 of less than 5 cm for the Sac Peak daytime seeing. The Fried parameter fluctuates on time scales of seconds during the highly variable daytime seeing conditions. This results in large variations in the Strehl ratio, which are mostly due to the wavefront errors in the uncorrected higher order modes (see Figure 13).
The variations in Strehl ratio also make the interpretation of spectral and polarimetric data very difficult. Difference images are often used to produce magnetograms (left circular – right circular polarization) or dopplergrams (blue wing – red wing of a spectral line), examples of which are seen in Figure 11. In general, those images are not taken simultaneously and variations in Strehl between the, e.g., LCP and RCP images result in spurious magnetic signals. For many science applications time sequences of high resolution images or spectra a needed in order to study the highly dynamic solar atmosphere. For these applications consistent and good image quality is needed for all images/spectra in the time sequence. Correcting more spatial modes mitigates this problem to some extent. Provided that seeing fluctuations are not too severe a high order AO system is more likely to provide sustained high Strehl ratios in variable daytime seeing conditions (see Figure 13). This motivated the development of a high order solar AO system – the AO76 system – for the DST, which is discussed in Section 5.
For completeness it should be mentioned that curvature wavefront sensors that have been implemented with great success in night-time AO systems (Roddier, 1988, 1990, 1991; Roddier et al., 1992; Graves et al., 1998) have also been proposed for solar AO. Although some effort has gone into the development of curvature sensing techniques for solar adaptive optics application (Kupke et al., 1994, 1998; Molodij et al., 2002) those efforts have not yet led to practical implementation of this concept, which is likely due to fundamental signal-to-noise problems with this approach when applied to an extended object like solar granulation (Fienup et al., 1998).
Living Rev. Solar Phys. 8, (2011), 2
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