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The projected-potential in this challenge contains both thin amorphous-material and perforation-edges that face in more than one direction, both of which can be quite useful. The perforation-edges may be used to directionally "even out" the edge Fresnel-fringe, which is dark/bright in over/under-focus conditions, while the amorphous material offers up "speckle contrast" that should appear isotropic (directionally unbiased), and in fact featureless near Gaussian focus, when astigmatism is eliminated.
A variation on this, especially for isotropic specimens, might involve adjusting astigmatism so that a wide range of recognizable focus-conditions can be recorded in a single image. One might in other cases want to record an image with instrument contrast-transfer-function (CTF) zeros that allow one to determine the CTF as accurately as possible.
This web link points to a 2D "focus & astigmatism challenge", implemented in javascript but inspired by our now ancient on-line focus & astigmatism adjustment pages. The challenge is this:
Minimize astigmatism and optimize focus given a TEM image but no live-image power-spectrum. This is an irreplaceable skill for researchers using microscopes which do not calculate and display an image power spectrum for the operator in real time.
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