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Sub-Angstrom TEM of materials requires focal-series reconstruction (FSR) or electron holography to retrieve the electron wave at the specimen exit-surface to very high resolution. As a consequence, we need to measure the microscope information limit. With a sub-Angstrom information limit, the one-Angstrom microscope (OAM) project at the NCEM has achieved sub-Angstrom resolution by FSR. We present a new method of estimating the information limit of the microscope, based on energy-spread measurements with an image filter.

Sub-Angstrom TEM of materials at intermediate voltages requires a sub-Angstrom information limit for the microscope. With a Scherzer resolution of 1.7 Angstrom, but a sub-Angstrom information limit, the one-Angstrom microscope (OAM) project at the NCEM is able to generate resolution below 0.8 Angstrom. Microscope information limit comes from damping of transfer by the temporal coherence. A major term contributing to temporal coherence is energy spread in the electron beam. We derive a new expression for the energy spread, and show how it can be measured from the result that is obtained using a standard electron spectrometer.