7.1 Short exposure

Adaptive optics provides only partial correction and the correction becomes increasingly worse when science observations are performed at short wavelengths (e.g., g-band, Ca ii K). In addition, for conventional AO the correction is optimal only within the isoplanatic patch and deteriorates as one moves away from the AO lock point. Post-processing can correct for residual phase aberrations and, in principle, restore the correct amplitudes. Furthermore, post processing can provide uniform image quality across the entire FOV. It is beyond the scope of this article to review the various techniques used to reconstruct short exposure AO images. The reader is referred to Löfdahl et al. (2007Jump To The Next Citation Point) for an in-depth discussion of post-facto reconstruction techniques used in solar astronomy. In addition, individual methods are discussed by: speckle interferometry (Wöger et al., 2008Jump To The Next Citation Point; Wöger and von der Lühe, 2008Jump To The Next Citation Point), phase-diversity and phase-diverse speckle (Löfdahl and Scharmer, 1994; Seldin and Paxman, 1994; Paxman et al., 1996; Seldin et al., 1999; Löfdahl et al., 2007; Valenzuela et al., 2010), multi-frame-blind-deconvolution (MFBD) (van Kampen and Paxman, 1998; Löfdahl, 2007; Scharmer et al., 2010), and Multi-Object-Multi-Frame-Blind Deconvolution (MOMFBD) (van Noort et al., 2005).

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Figure 27: mpg-Movie (6275 KB) Speckle reconstructed g-band movie of granulation and g-band bright points near the solar limb. The FOV is 2’ × 2’. The lower left corner zooms in on an area with magnetic bright points (courtesy of F. Wöger, NSO).

The power of these techniques, in particular when combined with AO, is exemplified by just a few images and movies. Figure 27Watch/download Movie 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 External Linkhttp://www.iac.es/galeria/svargas/movies.html (see also Vargas Domínguez et al., 2008). The movie shown in Figure 28Watch/download Movie shows evolution of a pore that develops a penumbra (Schlichenmaier et al., 2010Jump To The Next Citation Point). 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 29Watch/download Movie shows a MOMFBD processed movie of chromospheric structure and is an impressive example of highly dynamic chromospheric fibrils seen in Hα (image and movie from van Noort and Rouppe van der Voort, 2006Jump To The Next Citation Point).

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Figure 28: mpg-Movie (13829 KB) Speckle reconstructed g-band image sequence that captures the formation of a sunspot penumbra (from Schlichenmaier et al., 2010).

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Figure 29: mpg-Movie (12914 KB) MOMFBD reconstructed image and movie of chromospheric Hα fine structure (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 Hα structure of Figure 29Watch/download Movie 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 30View Image, 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.

View Image

Figure 30: Line-center intensity of an active region at disk center in the Ca ii 854.2 nm line. The extended 240” × 240” field-of-view was constructed by mosaicing the 90” × 90” FOV of the Interferometric Bidimensional Spectrometer (IBIS). Speckle interferometry was applied to the sequences of images obtained at each of the nine mosaic positions, and the reconstructed images were stitched together to produce the final mosaic (courtesy of K. Reardon, Florence).

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