= Solution
A <scanning transmission electron microscope> makes an image by assigning a transmitted-electron signal to each position of a scanned probe. \b[Its essential chain is <electron> source, probe-forming optics, scan system, thin specimen, transmitted-electron detectors and synchronized acquisition.]
An <electron gun> and accelerating supply provide a stable beam of the chosen <kinetic energy>. <Condenser lens> optics and an aperture control probe current and convergence; the probe-forming <objective lens> focuses a small spot on the specimen. Alignment, focusing and <astigmatism> correction keep the probe usable across the field.
Scan coils deflect the probe in a raster. A stable specimen holder positions an electron-transparent region, and a high-vacuum column limits gas scattering and contamination. A <bright-field transmission electron microscopy> detector collects the central transmitted beam; an annular detector can select <annular dark-field electron imaging>, including <high-angle annular dark-field imaging>. Different angular selections give different contrasts, rather than a universal map of height.
Detector electronics, scan timing and a computer assign the collected signal to the corresponding image pixel. Spectrometers are useful additions but not essential to basic image formation. The post-specimen optics can adjust detector collection geometry; they do not set scan-image <magnification> in the same way as the projection lenses of conventional transmission imaging.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2018/paper-1-microscopy.png]
{title=Original schematics of scanned transmitted-electron imaging and contact-mode force feedback; panels show different instruments}
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