Leginon

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!

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.