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

<strong>Tecnai</strong> on-line help Alignments 17<strong>Tecnai</strong> 12 Software version 2If the diffraction pattern is not focused properly, there are a number of consequences:• The camera length can be wrong• The diffraction will be rotated away from its proper orientation• The pattern may be distorted• Alignments such as beam shift pivot points can be wrong• The scanning magnification can be wrong due to misaligned pivot pointsDue to the absence of a clear criterion, we end up with a chicken-and-egg situation (what was first, thechicken or the egg?). For example, if it can be assumed that the shift pivot points are correct, then it iseasy to establish the correct diffraction focus by wobbling a beam shift and minimising diffraction-patternmovement. However, the pivot points can only be aligned correctly if the diffraction pattern is focusedproperly.In order to resolve this situation, we have determined Intensity settings for the different modes (LM, HM-TEM and Nanoprobe) for a parallel beam. These Intensity settings are preset in the alignmentprocedures, making it easy to find diffraction focus (the spot-pattern condition). After the alignment havebeen done (camera length focus), the diffraction focus can also be found by simply pressing theEucentric focus button (this resets the variable diffraction focus to zero). With this method forestablishing diffraction focus, the SA aperture is not (and should not be) used.3.6 The diffraction shadow imageWhen the diffraction pattern is focused properly at the back-focal plane, the pattern is either a spotpattern in SAED or a disk pattern in CBED (with a fully focused beam). In either case the diffractionpattern contains no image information at all (the magnification of the image in the diffraction pattern isinfinite). It is also possible to defocus the diffraction pattern slightly (see below). When this is done, the(by now expanded) spots or disks do contain image information (the magnification is no longer infinite)and so we have obtained a mixture of diffraction and image information. This is called a shadow image.The shadow image (in this case from SAED diffraction) can be understood from the diagram below.When a parallel illuminates an area of the specimen, all transmitted (bright-field) beams converge in asingle cross-over in the back-focal plane of the objective lens. In this cross-over we cannot distinguishbetween beams coming from the different parts of the specimens, because all beams go through asingle point (ideally). However, above or below the back-focal plane, the beams do not go through asingle point but - in three dimensions - they form a disk and each point in the disk corresponds to anarea of the specimen.