When not merely obscured in our field of view, things which we cannot see – that are essentially invisible – often may be either impossibly distant or impossibly small (among other factors). That’s why we have telescopes and microscopes.
At cosmic scales, imaging a black hole was like seeing something spanning “the size of a period at the end of a sentence if you were looking at it from Earth and that period was in a leaflet left on the moon.”
At microscopic scales, can we see atoms? What’s changed in the last ~50 years?
So, how small can we go in imaging atoms? Beyond the direct imaging of conventional electron microscopes, there’s reconstructive imaging based on pattern processing – for highly ordered atomic lattices. An enhancement which extracts higher resolution geometries from scattered electrons .
• Phys.org > “Researchers see atoms at record resolution” by David Nutt, Cornell University (May 21,2021)
(quote) In 2018, Cornell researchers built a high-powered detector that, in combination with an algorithm-driven process called ptychography [the method is time-consuming and computationally demanding], set a world record by tripling the resolution of a state-of-the-art electron microscope [for ultra-thin samples that were a few atoms thick].
Now a team, again led by David Muller, the Samuel B. Eckert Professor of Engineering, has bested its own record by a factor of two with an electron microscope pixel array detector (EMPAD) that incorporates even more sophisticated 3D reconstruction algorithms.
The resolution is so fine-tuned, the only blurring that remains is the thermal jiggling of the atoms themselves.
 Historically, much like using X-ray crystallography to construct atomic and molecular structure.
- Electron ptychographic reconstruction
- Speckle patterns (coherent interference patterns)
- Phase velocity
• Ultimate why? > Comment 2-14-2018 re a prize winning photo of a single atom.