[[To Read]] [[flexible-stem-with-simultaneous-phase-and-depth-contrast.pdf|Flexible STEM paper]] [Online link](https://www.cambridge.org/core/journals/microscopy-and-microanalysis/article/flexible-stem-with-simultaneous-phase-and-depth-contrast/131F0EAA69FD8D9665E7E685EC81B1BE) References: [[Random scan patterns]] ![[urn-cambridge.org-id-binary-20220222144506628-0637-S1431927621012861-S1431927621012861_fig2.jpg]] > The raster scan is the most natural way to fill a Cartesian plane, with a fast scan in one direction and a slow step in the other. The raster scan also corresponds conveniently to storage of data arrays in a computer by row and column. One should only synchronize the sampling in order to generate a 2D image similar to the read-out of a camera. The raster scan is not, however, a natural way to steer an electron beam. Both the magnetic deflectors and the electronic amplifiers that drive them have a minimal response time, which means that the actual beam location lags behind the control signal that determines the recorded pixel position. At the end of each line, the beam must come rapidly to a halt and reverse direction. This causes very strong scan distortions near the edges of the frame, where severe damage often accumulates. The displayed field is normally cropped to a smaller region where the scan is properly linear. A significant fraction may have to be discarded, and the displayed area may also shift horizontally depending on the scan speed. > >A more natural way to scan would be to minimize changes in the probe acceleration. For example, a circular scan is entirely smooth, with sine and cosine functions driving orthogonal directions. The probe lag is equivalent then to a phase delay on both. By slowly reducing the amplitude, we obtain a shrinking spiral or set of concentric circles. A variety of spiral scan schemes has been explored previously (Sang et al., [Reference Sang, Lupini, Unocic, Chi, Borisevich, Kalinin, Endeve, Archibald and Jesse2016](https://www.cambridge.org/core/journals/microscopy-and-microanalysis/article/flexible-stem-with-simultaneous-phase-and-depth-contrast/131F0EAA69FD8D9665E7E685EC81B1BE#ref33)). Alternatively, the plane may be covered by sweeping a large circle slowly along a line. The Hilbert pattern is another attractive scanning option to reduce distortion by shortening the flyback paths (Velazco et al., [Reference Velazco, Nord, Béché and Verbeeck2020](https://www.cambridge.org/core/journals/microscopy-and-microanalysis/article/flexible-stem-with-simultaneous-phase-and-depth-contrast/131F0EAA69FD8D9665E7E685EC81B1BE#ref41)). Maximal flexibility is achieved by preparing an array of scan coordinates in advance. The Cartesian pixel grid is recovered by interpolation between the sampled points taking the phase delay into account.