Leginon

h1. 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.