Example of Data Collection, Evaluation, and Processing

Using the procedure illustrated in Fig. 4, we have determined several subnanometer-resolution structures of icosahedral particles (Zhou et al, 2000, 2001, 2003 Viang et al, 2001a, 2003 Liang et al, 2002). In this section, we discuss each of the steps in our procedure and mention the specific modular programs (in italics) employed. 

Most of the subnanometer-resolution studies used field emission gun cryomicroscopes because of the advantages mentioned above (Table I). When the LaBg gun is used, the spatial coherence of the instrument is relatively poor. Therefore, a high-resolution image must be recorded with a small defocus value to minimize the dampening due to the partial coherence envelope function (Zhou and Chiu, 1993). Consequently, these images have low contrasts for their low-resolution features, making it difficult to locate the particles and to determine their initial orientation 

Before digitization, micrographs are first examined visually to discard those with apparent image astigmatism or drift. So far, aU the image data used in subnanometer-resolution reconstructions have been recorded on 

A potentially difficult problem is ice thickness variations across a micrograph, which would result in defocus difference at different regions 

Orientation and Center Determination by the Focal Pair Method 

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