Scherzer focus/defocus

Figure Bl.17.5. Examples of CTFs for a typical TEM (spherical aberration = 2.7 mm, 120 keV electron energy). In (a) and (b) the idealistic case of no signal decreasing envelope fimctions [77] are shown, (a) Pure phase contrast object, i.e. no amplitude contrast two different defocus values are shown (Scherzer focus of 120 mn imderfocus (solid curve), 500 mn underfocus (dashed curve)) (b) pure amplitude object (Scherzer focus of 120 mn underfocus) (c) realistic case mcluding envelope fimctions and a mixed weak...

Therefore, a structure image can be obtained from a weak phase object first by setting the defocus value at and then by cutting out those diffracted waves whose spatial frequencies are larger than u. These observation conditions derived from eq. (1.13) are called the Scherzer imaging conditions and the defocus value is called the Scherzer focus. 

Muscovite-2Mi viewed along [100], (b) Annite-2Mi viewed along [100]. (c) Annite-2Mi viewed along [010]. The parameters for the simulation are defocus = -42 nm (Scherzer focus) and specimen thickness = 2.5 nm. 

Defocus is conventionally given as a motion of the object, as one would focus an optical microscope.) Comparing Scherzer focus to geometric focus, the resolution is improved by a factor of about 2 if the included divergence angle is... 

If we use a TEM with a point resolution near 0.2 nm, how is the mica unit layer imaged Figure 2 shows several examples. They are simulated images (the simulation method is explained below), assuming that the specimen thickness is 2.5 nm and that the imaging is at Scherzer defocus (the focusing value calculated by [ -1.2 ], where... 

Adjust focus to obtain best quality HREM images. Typically, use the Scherzer defocus condition to take HREM images. The Scherzer defocus Af= 1.2 VCsk is a fixed value for a microscope without Cs corrector. For uncorrected TEM, typical Scherzer defocus values are around —50 nm at 200 kV. 

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