Wavelet image analysis in aberration-corrected scanning transmission electron microscopy

The correction of aberrations on the VG HB501 instrument of the SuperSTEM facility at Daresbury Laboratory relies on the numerical analysis of a series of electron Ronchigrams of an amorphous specimen. In particular, cross-correlation techniques are used during the auto-tuning procedure to measure the shifts between images recorded at different tilt angles, hence deducing the experimental values of the aberration to be corrected. The use of wavelets and a newly defined wavelet cross-correlation function provide the basis for a powerful new method for image registration that could simplify and systematize the tuning procedure. The method was tested on simulated Ronchigrams of amorphous samples and used in conjunction with edge detection algorithms to measure the movements of fringes observed in Ronchigrams of crystalline samples.