Modern TEMs are equipped with electronic detectors (such as charge coupled device, or CCD, detectors), in addition to a retractable fluorescent viewing screen, to capture TEM images in a digital format. The main user-controlled settings include the sample stage position, magnification, objective lens current, beam current (spot size), and the choice of which apertures and detectors to use when acquiring data. Due to the complexity of the instrumentation, most of the components are automatically computer controlled with only a few key parameters controlled by the microscopist. A vacuum system is used to maintain the required vacuum levels throughout the column. The microscope column consists of a series of electromagnetic lenses and apertures to focus the electron beam onto the sample and magnify the TEM image onto the viewing screen (or detectors). Typical accelerating voltages range from 80 kV up to 300 kV. At the top of the column is the electron gun which couples to a high voltage source used to set the kinetic energy of the electron beam.
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