Electron optical design of the Phase Aberration Corrected Electron Microscope

Abstract

Thin biomolecular specimens are weak phase objects. Therefore image contrast is described by the so called Phase Contrast Transfer Function (PCTF) which depends on the phase introduced by the spherical aberration and defocus. The PCTF describes the signal transfer as a function of the spatial frequency. Due to the sine-type transfer function artefacts arise from contrast reversal and cancellation of low spatial frequencies. Substantial improvement of the quality of the images can be achieved by using a Phase Plate (PP) integrated in a CS-corrected microscope. The PP normally placed in the back focal plane of the objective helps to transfer lower frequencies while the CS corrector is essential for the transfer of higher frequencies. This offers the possibility of artefact free imaging over a wide range of spatial frequencies. In this paper we describe the electron optical design of the Phase Aberration Corrected Electron Microscope (PACEM), which is developed in collaboration with the Max- Plank-Institute for Biophysics in Frankfurt. First results will be presented.