The power of these techniques, in particular when combined with AO, is exemplified by just a few images and movies. Figure 27 shows a speckle reconstructed (Wöger and von der Lühe, 2008) time sequence of g-band images of a plage region observed near the limb. Uniform diffraction limited resolution is achieved over a 2’ × 2’ FOV. The algorithm takes into account the adaptive optics correction by utilizing the AO telemetry data in order to achieve high precision photometry (Wöger et al., 2008). Some nice examples of MOMFBD reconstructed images and movies can be downloaded from http://www.iac.es/galeria/svargas/movies.html (see also Vargas Domínguez et al., 2008). The movie shown in Figure 28 shows evolution of a pore that develops a penumbra (Schlichenmaier et al., 2010). This speckle reconstructed sequence covers a period of 4 hours and 40 minutes with occasional gaps. This movie is a nice case study of penumbra formation. Figure 29 shows a MOMFBD processed movie of chromospheric structure and is an impressive example of highly dynamic chromospheric fibrils seen in H (image and movie from van Noort and Rouppe van der Voort, 2006).
These are just a few examples of many that can be found in the literature that demonstrate how new scientific results can be achieved from a combination of AO and post-facto processing. Real-time instead of post-facto processing of the AO data is of advantage and is now considered for implementation at the ATST (Wöger et al., 2010). Using GLAO, once operational, instead of conventional AO may be the better choice for some of these reconstruction methods since GLAO provides uniform and potentially subarcsec seeing across the FOV, i.e., there is no field dependence of AO performance which complicates the reconstruction algorithm.
The MOMFBD reconstructed movie of chromospheric H structure of Figure 29 has been obtained with a relatively narrow filter passband of 12.8 pm and an exposure time of 15 ms. At red, and in particular infrared wavelengths, the longer seeing time constant allows for increasing the maximum exposure time the still freezes the seeing. With efficient instrumentation such as IBIS (Cavallini, 2002; Righini et al., 2010), CRISP (Scharmer et al., 2008), and the GREGOR Fabry–Pérot interferometer (GFPI) (Denker et al., 2010) reconstruction techniques can be applied to very narrow-band images. A recent example is shown with Figure 30, which displays speckle reconstructions of a sunspot region observed with the IBIS instrument tuned to the core of the Ca ii 8542 Å line. The passband at this wavelengths is about 4 pm and the exposure time was 30 ms. The large FOV of 240” × 240” was constructed by mosaicing the 90” × 90” FOV of the IBIS. This example demonstrates that, in principle, reconstruction techniques that are typically associated with broad-band imaging can be used to perform spectroscopy with high spectral resolution. Cadence, i.e., temporal resolution becomes the issue since multiple (50 in the example presented here) exposures are required at each spectral position the filter is tuned to. If polarimetry is performed even more images have to be collected. In principle, adding many short exposures is equivalent to a single long exposure in terms of S/N as long a the photon noise dominates over read noise for each exposure, and the read-out time is small relative to the exposure time. This means that with appropriate detectors this potential drawback can be overcome. Remaining issues the high data storage and processing requirements. However, given the rapid and continuous advances with these technologies these issues are not expected to be a limitation. Nevertheless, the capability to post-process and improve long exposure AO images may also be of interest for some applications.
Living Rev. Solar Phys. 8, (2011), 2
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