A good understanding of the atmospheric properties above the telescope site and the resulting wavefront aberrations is crucial for the design and operation of an AO system. The importance of site survey and site characterization efforts cannot be overemphasized in this context. Key atmospheric parameters that determine the design and performance of an AO system include the Fried parameter r0, the Greenwood frequency , and the atmospheric turbulence profile . is the refractive index (n = refractive index) structure constant, which will be defined and discussed below. By using turbulence theory, these properties can be used to design AO systems and predict their performance. The most widely used model to describe atmospheric turbulence is the Kolmogorov model (Kolmogorov, 1941, 1991). Energy is introduced into the system by wind flows at a large scale (the outer scale) and cascades down to ever smaller scales until, finally, energy is dissipated at molecular scales (the inner scale). The inertial range is bound by the outer and the inner scale and marks the regime where the turbulent power of the temperature fluctuations as a function of spatial wave number can be expressed by a power law:
Similar power laws can be derived for other quantities such as the refractive index power spectral density in one dimension (Hardy, 1998; Tatarskii, 1967):
The equivalent in three dimensions leads to the well known power law (Roggemann and Welsh, 1996; Quirrenbach, 2002):
The refractive index structure constant is a measure for the strength of the turbulence and is related to the temperature structure constant by: .
At this point it is convenient to discuss the concept of the structure function, which was introduced by Kolmogorov to describe non stationary random processes such as turbulence. In general, structure functions are very useful in assessing the impact of turbulence on the image quality provided by an imaging system, independent of whether the aberrations are caused by atmospheric turbulence or optical imperfections. Today, structure functions are used to, for example, specify optical polishing tolerances.
In the context of atmospheric turbulence the refractive index structure function is assumed to be isotropic and is defined as:
Hence, if multiple turbulence layers (or a continuum) are present, Equation (5) has to be integrated along the line-of-sight. The zenith angle is introduced to account for changes in the length of the line-of-sight travel path with observing angle:
The Fried parameter r0 is a measure for the strength of the turbulence and is defined as (Fried, 1966a,b):flat in this case means the wavefront variance is less than 1 rad2. In that sense the Fried parameter can be interpreted as the smallest AO relevant scale of turbulence. Of course, the turbulent spectrum contains much smaller spatial scales. The Fried parameter is often used to quantify the seeing quality at an astronomical site. The statistical distribution as well as the temporal evolution of the Fried parameter ultimately determines the performance of an AO system at the site. The value of the Fried parameter depends on wavelength according to . This means r 0 has to be specified at a certain wavelength, typically 500 nm. The Fried parameter r0 is larger for longer wavelengths, which means the seeing is significantly better at infrared wavelengths. Hence, correcting the seeing with AO becomes easier at longer wavelengths.
The phase structure function can be expressed in terms of the Fried parameter resulting in a much simpler form of :
This phase structure function describes the performance of imaging systems in the presence of turbulence. Equation 8 is often referred to as the uncorrected, long exposure phase structure function. This simple form for the structure function is valid only in a statistical sense, i.e., when averaged over a sufficient amount of independent realizations of the atmospheric turbulence. Marino (2007) showed that, for daytime observations, exposures of only a few seconds can be regarded as long exposures. Solar astronomers tend to use such long exposure times when performing precision polarimetric measurements. High resolution spectroscopy sometimes requires exposure times of similar length. However, broad-band solar imaging is typically performed with very short exposures on the order of a few milliseconds with the intention to freeze the seeing. Image motion (tip and tilt) can essentially be removed in this way and the images contain speckle, i.e., structure with width of the diffraction limit . The fact that diffraction limited information is contained in the short exposure images can then be used to restore the true image by applying post-facto image reconstruction techniques. Löfdahl et al. (2007) give a review of various post-facto reconstruction in solar astronomy.
An analytical expression for the short exposure structure function can also be given (see, e.g., Hardy, 1998, p. 92):
The negative impact of atmospheric turbulence on imaging systems has been described in detail in textbooks (e.g., Hardy, 1998, Section 3). It is the de-correlation of the phase over distances larger than r0 that results in blurred images where the typical width of the blurred long exposure image is , i.e., the resolution is seeing limited.
The long exposure Optical Transfer Function (OTF) is defined as the ensemble average of the instantaneous OTFs over the entire exposure:
The OTF and the PSF are related through a Fourier transform. Figure 4 shows the OTFs and PSFs of the seeing limited long exposure (Equation 12), the aberration free, diffraction limited telescope and a typical AO corrected case. The seeing limited OTF does not transfer high spatial frequency information. The AO system is able to retain high spatial resolution information potentially up to the diffraction limit. However, the amplitudes are attenuated in particular, for high spatial frequencies where, depending on the particular observation, noise may begin to dominate before the theoretical diffraction limit.
The AO corrected PSF consists of two parts. A diffraction limited core and a seeing limited halo. The width of the core is , while the width of the halo is . The Strehl, which is defined as the ratio of the peak intensity of the observed PSF compared to the peak intensity of the ideal telescope PSF, of the AO corrected PSF shown in Figure 4 is S = 0.6. The Strehl ratio is also a measure for the energy contained in the core vs. energy in the halo. The AO corrected long exposure phase structure function will be revisited in detail in Section 6.3.
The time after which wavefront aberrations change significantly is another important parameter that determines and limits the performance of solar AO and also defines the meaning of a short or long exposure. Using the Taylor hypothesis of frozen in turbulence one can easily derive an estimate for . The assumption is that a turbulence screen is carried across the telescope aperture by wind at time scales much faster than the intrinsic evolution of the turbulence. This assumption has been experimentally verified (Poyneer et al., 2009) and can be used to implement predictive control of AO systems (e.g., Dessenne et al., 1998, 1999; Poyneer and Véran, 2008; Johnson et al., 2008) in order to improve performance. With this assumption the turbulence or seeing time constant can be estimated by the simple equation:
Living Rev. Solar Phys. 8, (2011), 2
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