Special Beam Tilt Calibration for Phase Plate » History » Revision 2
Revision 1 (Anchi Cheng, 03/29/2017 10:52 PM) → Revision 2/5 (Anchi Cheng, 03/29/2017 10:53 PM)
h1. Special Beam Tilt Calibration for Phase Plate Thin-carbon hole-free phase plate such as FEI Volta phase plate generate phase shift from the phase shift differential between the irradiated point from its surrounding. When we perform beam-tilt-based autofocus, the beam tilt angle need ideally outside of the information forming area, roughly 2 Angstroms resolution scattering distance. A 10 mrad beam tilt typically used in Leginon is suitable for the purpose with a few modifications. h2. Diagonal Beam Tilts The figure below shows the original beam tilt method and the modification for phase plate. !BeamTiltsOnPhasePlate.png! !BeamTiltOnPhasePlate.png! A phase plate is imaged in diffraction mode with a cross-grating replication in on-plane position in this figure. The magenta diffraction rings from gold are centered at a hypothetical patch position numbered N, shown as an orange spot. For reference, the positions of a few surrounding patch positions are also shown. The patch position is laid out such that a line connection N-1 and N is parallel to the straignt edge of the phase plate slot, its direction is highlighted by the sky-blue line. Upon a typical x +/- 10 mrad beam tilt, displayed by the red arrows, the diffraction pattern and its un-scatterred beam shifts on the phase plate. The white diffraction rings overlay in the figure corresponds to x+10 mrad. We observed that this is pointing towrads the previous row in the neighborhood of previously used N-19 patch position. x-10 mrad beam tilt would then point to the unused patch position of N+19. Since the distance between the patch position is approximatly the same magnitude the beam shift on phase plate with 10 mrad beam tilt, the beam-tilt autofocus inevitably affect the image quality when using N+19. Our solution to this is to beam tilt only towards used patch positions. N-1 and N-19 in the figure. The relationship between beam tilt magnitude and defocus is linear. Therefore the diagonal beam tilt schem shown in green arrows will allow maximum usage of the phase plate.