Derivative-free optimization for autofocus and astigmatism correction in electron microscopy

A simultaneous autofocus and two-fold astigmatism correction method for electron microscopy is described. The method uses derivative-free optimization in order to find a global optimum of an image variance, which is an image quality measure. The Nelder-Mead simplex method and the Powell interpolation-based trust-region method are discussed and compared for an application running on a scanning transmission electron microscope.