Tecnai on-line help manual -- Options - UT Southwestern

<strong>Tecnai</strong> on-line help Options 79Options Software version 25.11 Focussed diffraction and shadow imageThe shadow image is often a handy tool during tilting. In normal operation it is unfortunately alwaysnecessary to refocus the diffraction pattern (for SAED: Selected Area Electron Diffraction) or the beam(for CBED: Convergent Beam Electron Diffraction) when the shadow image has been used. Not only isthis cumbersome, it can also require changing to different conditions before changing back to thediffracting conditions required. Smart Tilt allows the operator to set a focus and a defocus condition (onthe Intensity for CBED, on the diffraction lens for SAED) and then toggle rapidly and accurately betweenthem.Shadow image: an explanationThe shadow image essentially is a mixture of diffraction and image information. In a focussed diffractionpattern (either SAED or CBED), there is no image information. The diffraction information in the patternis therefore at its optimum, but it is difficult to determine where exactly the information is coming from. Inthe shadow image all diffraction disks (even the SAED pattern no longer has real spots) show a smallimage containing the information contributing to the diffraction disk. This image information makes it thuspossible to see where the diffraction pattern is coming from or to track and correct for sidewaysmovement during tilting.In the shadow image Fresnel contrast can be visible. This Fresnel contrast (not the diffraction contrast -an area scattering strongly will remain dark in the transmitted beam) depends on whether the shadowimage is underfocus or overfocus, both in SAED and CBED. Going from under- to overfocus results in aFresnel contrast reversal as well as a flip of the image visible in the disks by 180°. In order to getconsistent results, one should always choose one or the other.