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Figure 5
Assessment of resolution by means of the point-spread function. The effective resolutions of a reconstruction can be evaluated by comparison of the point-spread functions calculated for a simulated perfect data set and the experimental data (Unger & Schertler, 1995BB29). Using 7.5 Å in plane resolution and a maximum specimen tilt of 30°, the zx sections for (a) the simulated perfect data set and (b) the experimental data are shown. A grid with 40 Å spacing in x and z was superimposed on the sections to provide an internal scale. The elongation of the spheres along z, which is characteristic for the presence of a `missing cone' of data, can clearly be seen. The spheres were scaled to the same maximum density and sections were contoured with a single positive contour set at half height of the peak to satisfy the Raleigh criterion of resolution. At this height, the peak measures 5 Å across in the direction of x. A conversion factor of 1.5 correlates the nominal resolution of 7.5 Å with the width at half height (i.e. nominal resolution/width at half height = 7.5 Å/5 Å). This conversion factor can be used for the determination of the effective resolution from the half-width of the point-spread functions. Accordingly, 17.2 Å is the best possible vertical resolution (i.e. along z) at 7.5 Å in-plane resolution and a 30° maximum tilt for the specimen (= 1.5 × 11.5 Å). Applied to the experimental data (i.e. set all amplitudes to unity, set all phases to 0° and use FOMs obtained by LATLINE as weight in Fourier inversion), an effective vertical resolution of 20.4 Å is obtained.

Journal logoBIOLOGICAL
ISSN: 1399-0047
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