Real-time detection and single-pass minimization of TEM objective lens astigmatism

Abstract

Minimization of the astigmatism of the objective lens is a critical daily instrument alignment task essential for high resolution TEM imaging. Fast and sensitive detection of astigmatism is needed to provide real-time feedback and adjust the stigmators to efficiently reduce astigmatism. Currently the method used by many microscopists is to visually examine the roundness of a diffractogram (Thon rings) and iteratively adjust the stigmators to make the Thon rings circular. This subjective method is limited by poor sensitivity and potentially biased by the astigmatism of human eyes. In this study, an s2 power spectra based method, s2stigmator, was developed to allow fast and sensitive detection of the astigmatism in TEM live images. The “radar”-style display provides real-time feedback to guide the adjustment of the objective lens stigmators. Such unique capability allowed us to discover the mapping of the two stigmators to the astigmatism amplitude and angle, which led us to develop a single-pass tuning strategy capable of significantly quicker minimization of the objective lens astigmatism.

Defocus and magnification dependent variation of TEM image astigmatism

“…As shown here and in our previous work (Yan et al, 2017), s 2 stigmator can help achieve accurate correction of objective lens astigmatism at any imaging condition using a single pass tuning strategy. Understanding the principle of this single-pass tuning strategy will help further unveil the underlying theory of astigmatism variations with imaging parameters.…”

“…The analysis described above explains the relationship of the objective lens astigmatism and the correction fields generated by the stigmators, and how the stigmators can be controlled to optimally compensate the objective lens astigmatism. The vector summation model described here further refines our previous model (Yan et al, 2017).…”

“…For quantitative measurement of astigmatism variations, astigmatism needs to be accurately corrected at a wide range of imaging conditions, which is very challenging for the current method relying on visual examination. We have recently published a closed-form algorithm, s 2 stigmator , with a single-pass tuning strategy (Yan et al, 2017), that allows fast and sensitive detection of astigmatism using TEM live images and guides the users to reliably and efficiently adjust the two objective lens stigmators to correct astigmatism. Fig.…”

“…We have recently published a method, s 2 stigmator (Yan et al, 2017), with a single-pass tuning strategy, that allows rapid and sensitive detection of astigmatism using TEM live images and can reliably and efficiently guide the user to manually adjust the two stigmators to correct astigmatism. This exciting method opens up possibilities to minimize astigmatism with real-time feedback at a wide range of imaging conditions that are not available by visual examination.…”

Flaviviruses have imperfect icosahedral symmetry

“…The magnification used was 18,000× in superresolution counting mode, which corresponded to a pixel size of 0.81 Å per pixel. The objective lens astigmatism was corrected with s2stigmator (35). Frames were recorded every 0.2 s for 8 s, with a dose rate of ∼8 electrons per physical pixel per second.…”