There are some obvious limitations that restrict the resolution of the reconstruction: one is because the observed ''preferred orientation" in reality encompasses a finite orientation range (see section 7.5 for the likely size of the angular range). Another, related limitation, stems from the need to classify particles on the basis of their 0° appearance—a task which may have ambiguous results (see section 4.11 in chapter 4). A third limitation has to do with the fact that the azimuthal angles (as well as the subsequent classification) are determined from the images of particles (at 0°) that have already been exposed to the electron beam and may have been damaged. All three limitations are in the end mitigated by the use of a refinement method, according to which each projection is allowed to vary its orientation with respect to the reconstruction from the entire data set (see section 7.2).
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