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What Is CTF (Contrast Transfer Function)?

April 26, 2017contrast CTF function transfer

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What Is CTF (Contrast Transfer Function)?

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(Phase) Contrast Transfer Function or CTF is frequently referred to in (HR)TEM. To put it simple, CTF allows to evaluate and compare performances (point-to-point resolution and information limit) of different microscopes. CTF (Contrast Transfer Function) is the function which modulates the amplitudes and phases of the electron diffraction pattern formed in the back focal plane of the objective lens. It can be represented as: Clearly, it will be a complicated curve which will depend on: Cs (the quality of objective lens defined by spherical aberration coefficient) l (wave-length defined by accelerating voltage) Df (the defocus value) k (spatial frequency) The figure below shows a typical CTF plotted for an imaginary 200 keV microscope: CTF for a 200 keV microscope at “extended” Scherzer defocus. (Envelope functions ARE NOT applied). CTF for the same microscope at “extended” Scherzer defocus. (Envelope functions ARE applied). Important points to notice: CTF is oscillatory: there are “pas