Driven by the quest for ever higher spatial resolution observations of the Sun, the development of solar adaptive optics has excelled tremendously during the last 5 – 10 years. Several solar AO systems have been deployed at major ground-based solar telescopes and are now routinely operated. These AO systems have facilitated observations of structure in the solar atmosphere at a resolution that is at or near the diffraction limit of those telescopes. It is worthwhile to briefly summarize the main scientific drivers for observations of the Sun at the highest possible resolution and motivate the need for solar adaptive optics. The solar atmosphere is highly structured and dynamic. Understanding the physics of the small scale structure observed on the Sun in many cases is crucial to understanding important scientific questions such as:
The two important scales that determine the structuring of the solar atmosphere are the pressure scale height and the photon mean free path. Both are of order 70 km or 0.1” in the solar photosphere and quickly become even smaller at deeper layers of the atmosphere. An angular resolution of better than 0.1” is required to resolve these fundamental scales. Structures as small as a few tens of kilometers on the solar surface corresponding to a few tens of milli-arcseconds on the sky have been predicted by sophisticated MHD models of the solar atmosphere (Cattaneo et al., 2003; Vögler and Schüssler, 2007; Nordlund and Stein, 2009; Nordlund et al., 2009).
Current high-resolution solar telescopes, such as the Dunn Solar Telescope (DST), the Swedish Solar Telescope (SST), the Vacuum Tower Telescope (VTT) are in the one-meter class and utilize AO up to > 95% of the observing time to achieve the diffraction limit at visible and NIR wavelengths. Solar AO has revitalized ground-based solar astronomy at existing telescopes.
Figure 1 shows a sunspot image obtained at the DST using AO and a fast imaging camera that takes short exposure images in rapid sequence. The sequence of images has been post processed with a speckle reconstruction algorithm that compensates for the effects of residual wavefront errors that the AO was not able to correct. The images were recorded with a g-band filter with a passband centered at 430 nm. At this short wavelength, AO becomes very challenging and post-facto reconstruction becomes a necessity. Figure 1 illustrates the degree of fine-scale structuring of the solar atmosphere. Structures, such as penumbral filaments and g-band bright points that mark the sites of magnetic fields are seen at scales of about 0.12”. The apparent size of these structures is near the diffraction limit of the telescope and the granulation pattern that is visible in the photosphere and covers the entire surface of the Sun is seen.
Figure 2 shows several g-band images of sunspot fine-structure obtained at the 97 cm Swedish Solar Telescope (SST) on La Palma (Scharmer et al., 2007). Adaptive optics and post-processing techniques were used to reach a resolution that again is near the diffraction limit. Due to the larger aperture, the diffraction limit of the SST at the wavelength of g-band is 0.1” or about 70 km on the Sun, i.e., the resolution is higher than that of the DST. With this increase in aperture and, hence, resolution Scharmer et al. (2002) were able to clearly identify dark cores in penumbral filaments. This discovery of dark cores has contributed significantly to the development of a better physical understanding of penumbral fine-structure and sunspots in general. Close inspection of Figure 1 shows that penumbral dark cores are visible in this image as well, but only the increase in resolution provided by the larger aperture of the SST enabled the discovery of dark cores as a feature with significance to sunspot physics and magneto-convection in general. The fact that ever more details that allow us to advance our physical understanding of solar magnetic fields are revealed with even modestly increased resolution demonstrates the importance of fully resolving solar features. The measured sizes of many small-scale magnetic features are close to the limit set by diffraction, implying they are not adequately resolved by present solar telescopes.
What kind of resolution is needed to fully resolve the important physical processes? Sophisticated theoretical models and simulations, including radiative energy exchange and cooling, provide fundamental insights. Figure 3 shows simulated observations with a 4 m aperture telescope used by the ATST project science team in order to define imaging requirements for the Telescope-AO system (Rimmele, 2005). The numerical simulation of granular convection (Nordlund and Stein, 2009) is coupled with radiative transfer calculations for the Fe i line 630.2 nm. narrow-band intensity maps and line-of-sight magnetograms are shown over a 8” × 8” FOV. The magnetic fields generated by dynamo action near the surface are small scale, mixed-polarity fields. In order to simulate AO observations of these features the data were convolved with an AO Point Spread Function (PSF). The performance of an AO system varies with seeing conditions. The Strehl ratio measures how close the imaging performance provided by the AO is to that of the ideal diffraction limited telescope. The theoretical diffraction limited PSF has a Strehl of S = 1 and can not be achieved in practice. This performance measure will be discussed in detail in Section 6. In this simulation the Strehl ratio varies from S = 0.001 (seeing limited, virtually no AO correction) to S = 0.55 (good AO correction). These realistic simulations clearly demonstrate that large aperture telescopes with a high performance AO system that obtains high Strehl ratios are required in order to obtain meaningful measurements.
It is this iterative interaction between theoretical modeling and observations with a resolution that is comparable to that of the models (order 10 km) that is vital in arriving at a physical understanding of the fundamental astrophysical processes observed on the Sun. New large aperture telescopes are needed to resolve these features and put models to the test. The development of high-order solar AO that is capable of delivering high Strehl in the visible will be absolutely essential for next generation solar telescopes.
Several new solar telescope efforts are currently under way. Telescopes of the 1.5 m class such as the 1.5 m aperture GREGOR (Volkmer et al., 2003, 2006; Volkmer, 2008; Volkmer et al., 2010) on Tenerife and the 1.6 m aperture New Solar Telescope (NST) (Goode, 2006; Goode et al., 2010) are currently in their commissioning phase. The 4 m Advanced Technology Solar Telescope (ATST) (Rimmele et al., 2006a; Wagner et al., 2008; Rimmele et al., 2010b) is in its construction phase and is expected to be fully commissioned in 2018. In order for these telescopes to achieve their scientific goals complex adaptive optics systems are an essential and integral component of the optical system that feeds the solar instrumentation.
Accurate and precise measurements of physical parameters, such as magnetic field strength and direction or plasma velocity, require spectroscopy and polarimetry at high spatial, but also high spectral (R > 300 000), resolution and high polarimetric sensitivity. A sufficient number of photons has to be collected to achieve the required sensitivity, which leads to long exposure times since even the Sun turns into a faint object when observed at this ultra high spectral and spatial resolution. Short exposure observations that allow to fully freeze the seeing and, thus, retain diffraction limited information in many but not all cases are limited to broad-band imaging and are of somewhat limited utility for the precise quantitative scientific analysis mentioned above. However, with highly efficient telescope systems and instrumentation (e.g., CRISP; Scharmer et al., 2008) that have high throughput, and use slightly compromised spectral resolution, sufficiently short exposures can be achieved to allow at least partially if not fully freeze the seeing and, thus, provide short exposure, narrow-band observations. This type of approach is particularly useful at the red end of the visible spectrum and at near infrared wavelengths where the seeing time constant is longer. Frame selection and post-facto image processing can be applied to short exposure, narrow-band images leading to impressive results. High signal-to-noise ratio can in principle be achieved by accumulating post-processed short exposure filtergrams, although a very high duty cycle is required to ensure the required temporal resolution.
Even for short exposure imaging applications AO correction provides a significant advantage in that only small, residual wavefront errors have to be post-facto corrected, which leads to much higher signal-to-noise of the reconstructed images.
Solar adaptive optics can also provide diffraction limited long exposure spectroscopic and polarimetric observations of the solar atmosphere. With a well designed and optimized AO system the exposure time can be chosen to provide optimal sensitivity of the measurement and does no longer have to be limited by the desire to freeze the seeing. It should be mentioned that evolution of the solar structures also limits the length of the exposure interval. The optimal choice of method and observing parameters will always have to be made based on the specific scientific problem at hand and the instrumentation available.
This review paper summarizes the current state of solar AO technology and attempts to give a sense of the impact of AO on the field of high resolution solar astronomy. Section 2 summarizes basic AO principles. In order to understand AO technology a basic understanding of the problem – atmospheric turbulence – is pre-requisite. Many of the challenges of solar AO are common to astronomical AO in general. Section 2.3 discusses the solar AO specific challenges in comparison to night-time adaptive optics systems. The long and difficult path toward developing operational and scientifically productive solar AO systems is summarized in Section 3. The vital role the development of the correlating Shack–Hartmann wavefront sensor played in making solar AO a successful technology is described in Section 4. A number of highly successful solar AO systems are now operated at major solar telescopes. Due to the author’s bias, the DST solar AO system was selected as an example to discuss implementation details (Section 5). In using AO and interpreting AO data it is important to understand the limitations of solar AO or AO in general. AO performance is not perfect as is the case for many optical systems, including space borne telescopes. Section 6 details performance limitations of adaptive optics in the context of developing an AO residual wavefront error budget. Error budgets provide important guidance for the design of an AO system. Performance limitations can be overcome to some extent by estimating the AO PSF and subsequent application of post-facto deconvolution techniques (Section 7). An overview of operational solar AO systems is given in Section 8. Future solar AO developments, including the development of Multi-conjugate AO are discussed in Section 9.
Living Rev. Solar Phys. 8, (2011), 2
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