Dave Mitchell's DigitalMicrograph™ Scripting Website

Function

Analyses the front-most FFT (Thon ring pattern of an amorphous film) to determine the Cs and Defocus values.

Version

version:20211011, v1.0

Author

D. R. G. Mitchell

Acknowledgements

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Comments

This script will measure the Thon ring positions in a FFT of a thin amorphous film to determine the Cs and Defocus values. FFTs acquired on FEG instruments, especially cold FEGs, using thin amorphous films (eg 30nm of carbon), will produce good results. Images of the thin film should be acquired with coma and objective astigmatism corrected. This script assumes that these aberrations are not present.

Create a FFT from a captured image, using the FFT function in DigitalMicrograph. The Reduced FFT method may also be used, but for small defoci, only a limited number of Thon rings may be present - and the FFT method will provide more. Clicking on the Source FFT button in the script dialog will analyse the front-most FFT. The position of the dark and bright rings are measured and a graphical display is produced. If rings are numbered in the correct order in the series eg 1,2,3 . . or 2,3,4 . . or 3,4,5 . . etc) then a straight line fit will occur and the slope of that fit is then used to derive the values of Cs and Defocus. If the numbering sequence is not correct a parabolic fit results. The numbering sequence can be easily changed by clicking on the n+ or n- button, to find the sequence yielding a straight line. Ring positions which are wrong or just poor fits can be easily excluded from straight line fit by unchecking them in the dialog and the fit will update dynamically.

System Requirements

Tested on GMS 2.32, but should work on all versions of GMS.

Known Issues

I dabbled with this script over many years and rather lost interest in high-resolution TEM, preferring instead STEM for everything except the most beam-sensitive materials. I was never completely sold on the accuracy of this script. The numbers it produced were somewhat similar to what I expected from my instrument (within around 20%), but not that accurate. The microscope was a probe-corrected TEM/STEM and perhaps the presence of the corrector had some bearing on that.

Supported

I had plans to develop tools to analyse and correct objective astigmatism. However, I lost interest in TEM and I always had more interesting things to work on. As such, I will no longer develop this script. However, if users find errors or bugs, they are welcome to let me know and I will fix them. If somebody who is a die-hard hi-res TEM guy/gal uses this script and finds it is actually producing sensible data - please let me know.

Included Files

Main script.

Source Code