Beam Tilt Imager¶

To achieve high resolution, the electron beam is best to be parallel and aligned with the
optical axis of the lenses at the specimen. This is particularly important if better than 5
Angstrum resolution is required.

• Parallel beam can be practically achieved for low dose condition in the Nanoprobe
  mode of FEI microscope. In Microprobe mode that is more commonly used, a smaller C2
  condensor aperture will provide more parallel beam although never truly parallel. The
  current rotation center alignment that listed above provides a reasonable average beam
  tilt for given illumination area at the specimen. Since smaller C2 condensor aperture
  lets thourgh smaller percentage of the beam through, the beam intensity is lowered.
  Therefore, a larger spot size (smaller spot number) need to be used to give similiar
  illumination.

For example, at NRAMM, we improved our parallelity of the same illumination area in
the final exposure by switching from 100 um C2 aperture with C1 spot 5 to 50 um C2
aperture with C1 spot 3.

• The average beam tilt (theta) realtive to the optical axis achieved by rotation
  center alignment can be further refined by the so-called coma-free alignment. There are
  tools in Leginon that can help you do so found in the application "Alignment"

If the image acquired at on-axis average beam tilt (theta=0) has minimal astigmation
in its power spectrum (or diffractogram), the more the beam is tilted away from the
optical axis, the stronger astigmation would be observed. The "Beam Tilt Imager" node in
the application generate Diffractogram tableaus by user-defined additional beam tilt and
number of tilt directions to be measured. The objective is to adjust the average beam
tilt value so that the the diffractograms of the tilts in the opposite but equal value
relative to the average beam tilt look similar.

• The tableau is displayed in a normal Cartisian coordinate. with x+ towards right
  and y+ toward top.

• Reduce the additional tilt angle if the astigmation is too large for
  comparison.

• Move the beam tilt towards the direction of the tilted diffractogram that has a
  astigmation closer to the untilted diffractogram shown at the center.