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

<strong>Tecnai</strong> on-line help Options 159Options Software version 212 TrueImage Focus Series AcquisitionThe Automated Experiments Control Panel with TrueImageFocus Series Acquisition active.12.1 IntroductionThe TrueImage suite of software provides the tools for the PAL-MAL focus series reconstruction ofHRTEM images. The software consists of two parts, one for the acquisition and the other for thereconstruction. The acquisition on the <strong>Tecnai</strong> microscope is covered by the Focus series acquisitionautomated experiment. This software is the only reliable way for acquiring HRTEM through-focus serieswherein the focus steps are kept the same throughout the series. In order to do so, the focus seriesacquisition software controls the objective-lens current at the level of the DACs (Digital-to-AnalogConverters that control the actual current through the objective lens). Using manual control via the Focusknob is not sufficiently accurate for this purpose because the optics software focus step size is not linkeddirectly to the control of DACs.The objective-lens current is set by two (coarse, fine) or three (coarse, fine, superfine) DACs which arestacked on top of each other, the number depending on the type of microscope and objective lens.During focus series acquisition the resetting of the finest DAC (as happens when the DAC runs out ofrange) must be avoided because the compensation of the reset by the coarser DAC is not accurateenough. The software ensures there is enough range on the finest DAC for recording the whole focusseries. In order to do so, it may reset the finest DAC before the series is recorded. When that happens,the software gives a message, the user must refocus (to be sure the starting focus is still correct), andpress the Next button to continue.Because of the DAC control, the step available for the focus is always in integer values of the DAC step,and not a value in physical units. The software indicates the physical focus step associated with thesmallest DAC step. The step you select is then a multiple of that value (e.g. if you want steps of 5 nmand the smallest step is 1.6 nm, you choose as step size 3 which the closest to 5/1.6). Anotherconsequence is the possibility of a small mismatch between the indicated focus on the microscope andthe actual focus in the series (the former is controlled by the focus knob while the latter is solelydependent on the DAC settings). Typical step sizes normally used for focus series acquisition range from~2 nm (300kV U-TWIN) to ~6nm (200kV S-TWIN).