Fitting a square beam in a square camera: novel condenser apertures for low-dose transmission electron microscopy

In transmission electron microscopy (TEM) cameras are square or rectangular but beams are round. With a beam size chosen to fill the camera at a given image magnification, the circular lobes of the beam will extend beyond the camera’s field of view and irradiate areas that are not acquired on the camera, damaging and precluding them from future acquisitions if the sample is beam sensitive. In this paper we present development of condenser aperture plates for TEM that have square and rectangular apertures which improve the efficiency of sample area usage by 44% or greater in low dose TEM applications. We demonstrate that the use of these apertures is compatible with high-resolution cryogenic (cryo) TEM by reconstructing sub 2 Å apo-ferritin models from a datasets recorded with both square and rectangular apertures. Moreover the design of our aperture plates should improve the flexibility of 2 condenser systems for cryo-TEM acquisitions with multiple shots per hole by tailored matching of beam sizes to camera sizes at each magnification.