Fast and accurate defocus for improved tunability of cryo-EM experiments

Current data collection strategies in electron cryo-microscopy (cryo-EM) record multiframe movies with static optical settings. This limits the number of adjustable parameters that can be used to optimize the experiment. Here, we propose a method for fast and accurate defocus (FADE) modulation during movie acquisition. It uses the objective lens aperture as an electrostatic pole that locally modifies the electron beam potential. The beam potential variation is converted to defocus modulation by the typically undesired chromatic aberration of the objective lens. The simplicity, electrostatic principle and low electrical impedance of the device will allow fast switching speeds that will enable per-frame defocus values in cryo-EM movies. Researchers will be able to define custom defocus “recipes” and tailor the experiment for optimal information extraction from their sample. The FADE method could help to convert the microscope into a more dynamic and flexible optical platform that delivers better performance in cryo-EM single particle analysis and electron cryo-tomography.