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

<strong>Tecnai</strong> on-line help Options 121Options Software version 2The following procedure is used:• Direct cross-grating calibration of a lower HM-STEM magnification (not necessarily the lowermostbecause this may be too low to resolve the grating squares effectively). If the image distortion is toohigh, the software will automatically adjust the distortions in an iterative procedure until the distortionsare less than 0.1% (vector lengths of the grating square dimensions) and 0.1° (angle betweenvectors).• Direct cross-grating calibration of a comparable Microprobe STEM magnification. If the imagedistortion is too high, once again the software will automatically adjust the distortions in an iterativeprocedure until the distortions are less than 0.1% (vector lengths of the grating square dimensions)and 0.1° (angle between vectors).These STEM calibration procedure will be run for a total STEM rotation angle of 0°. This means that thefree (user) STEM rotation is set to 0°, and the aligned value (which corrects for the mismatch betweenstage axes and STEM image axes) is also temporarily set to 0° (the latter value is reset at the end of theprocedure to the starting value). Effectively this means that the smallest distortions and most accuratemagnifications are achieved at the same STEM rotation angle.Note: Both types of STEM-mode magnifications are strongly dependent on the pivot points,perpendicular corrections and distortion adjustment. For STEM magnifications as accurate as possible,the same STEM settings should be used as during the calibration. It is therefore advised to adjust thepivot points and perpendicular corrections carefully before the calibration and store these settings in analignment file (after the calibration, because that may have included adjustment of the STEM distortion)and having users always use those alignments. If the STEM distortions initially were a long way off, stopthe procedure after the distortion adjustment and check the pivot points and perpendicular corrections,and then restart the procedure.The Microprobe STEM magnification calibration does not result in a calibration file. The calibration valuefor the Microprobe STEM magnification is directly set to the <strong>Tecnai</strong> software and also entered as acalibration value in the system branch of the microscope settings in the registry.9.1.7 Measurement on cross-gratingThe magnification calibration is done on the basis of a standard specimen, a cross-grating. Thisspecimen has squares on it with a spacing of 463 nm. The software will acquire an image of the crossgratingand analyse that to find the basic spacing. The analysis is done either using an auto-correlationor an FFT (at high and low magnifications, respectively) in which the two basic vectors will be outlined bycircles. Additional circles may be present further away from the center to show the position of themultiple of the base vector that is actually used for further refinement of the measurement.