Dave Mitchell's DigitalMicrograph™ Scripting Website

Function

Method to null microscope column distortion by using the objective stigmators to produce distortion-free diffraction patterns.

Version

version:20211223, v1.0

Author

D. R. G. Mitchell

Acknowledgements

Comments

Column defects and residual magnetism in the column/viewing chamber can result in elliptical distortion in diffraction patterns. This can be significant enough (>1%) to be visible to the naked eye and makes measurement of diffraction pattern distortions due to strain and compositional effects impossible without post-acquisition correction.

The scripts and method described here enable pre-acquisition correction of the distortion at the microscope. It involves using the objective stigmators to apply a distortion field to counter that occurring further down the column. Applying this method resulted in distortion of an as-installed new microscope being reduced from 1.6% to 0.3%, the latter being the precision of the technique. The method involves two scripts. One allows manual control of the objective stigmators and storage/recall of optimum values for diffraction and imaging. A second script enables automatic acquisition of a matrix of diffraction patterns while varying the objective stigmators. Measurement of these matrices of diffraction patterns enables the objective stigmator values producing a minimum in distortion to be identified. A detailed manual is provided.

This method has been described in a journal article: D. R. G. Mitchell, A script-based method for achieving distortion-free selected area electron diffraction, Microscopy Research and Technique, 85 (7), (2022), 2708-2713.

System Requirements

Tested under GMS 2.3 and GMS 3.2 but should work with earlier versions. Both scripts have hardware dependencies. They will only run on an instance of DigitalMicrograph which is connected to a microscope. All modern microscope types should be supported. Obj. Stig. Scanner requires a Gatan camera to be present. This has been tested with an Ultrascan and with a Rio16, but should be compatible with all reasonably modern Gatan cameras.

Known Issues

WARNING: Diffraction patterns can move when the objective stigmation values are changed by a large amount. The possibility of the (000) beam moving out from behind the beam stop and irradiating the camera is present. Read the relevant section in the Manual to avoid this happening or to control it when it does.

Supported

Yes

Included Files

Zip archive containing two scripts and a software manual (PDF).

Source Code