Isoplanatic patch

The area of sky around a reference star over which high-resolution imaging is possible is called the isoplanatic patch (this will be discussed in more detail in Chapter 2). If the sky coverage of an imaging technique is substantially less than 100%, it will generally vary in proportion with the area of the isoplanatic patch. The diameter of the isoplanatic patch for an imaging technique thus has a very substantial impact on the applicability of that technique to astronomical imaging. A number of authors including Roddier et al. (1982b) have shown that the isoplanatic patch of fast frame-rate imaging techniques such as shift-and-add is expected to be substantially larger than that for adaptive optics. If the Lucky Exposures method selects exposures at times when the atmospheric conditions are particularly good, then this method would give an even larger isoplanatic patch than the shift-and-add method. In Chapter 5.5.2 I present results which demonstrate that the isoplanatic angle for Lucky Exposures observations can sometimes be as large as $30$ $as$ for I-band observations, a substantial improvement over typical values of $2$--$15$ $as$ predicted for speckle imaging and non-conjugate adaptive optics at wavelengths shorter than $1$ $\mu m$ (Roddier et al. , 1982a; Vernin & Muñoz-Tuñón, 1994; Roddier et al. , 1990; Marks et al. , 1999).