Embedded Files
This page is adapted from our interactive focus page which since July 1996 has been providing electron detectives with an on-line way to hone their pattern-recognition skills at correcting focus and astigmatism. The pre-calculated-image exercises are already mobile-friendly, although we are now looking into HTML5/JS strong-phase-object routines that might allow for real time exploration as well. These might let one translate and tilt the specimen, as well as change magnification, in addition to adjusting microscope contrast transfer!
A wave-optical (e.g. light or electron) imaging system is focused when it forms an image of wave intensity passing through the "object plane" which is of interest. For example, you may want your digital camera to focus on a flower in the foreground, or on some mountains in the distance. In electron phase-contrast imaging (HR-TEM) of very thin specimens, one sometimes focuses on an object plane downstream from the specimen, where the specimen's "wake" gives rise to detectable intensity differences on the film. The figure at left shows how the electron image of thin holey carbon film might vary as the focus is changed.
Images have astigmatism when waves that approach the image plane from different directions carry their focus from different object planes. This happens, for example, if the lens of our eye has axial asymmetry. The figure at right shows how the electron image of an amorphous film might change as the astigmatism varies. Traditionally, astigmation (making focus the same in all directions) and focusing (choosing the correct focus) have been challenges faced by most electron microscopists.
A more comprehensive challenge at characterizing unknown nano-structures on-line, with help from the focus-optimization skills discussed here, may be found on our JS/HTML5-canvas electron microscope specimen explorer page here.
Now for the challenge! How many steps will it take you to correct astigmatism and find Scherzer defocus on one of the specimens below? Even better, try this out on our new 256x256 JS/HTML5-canvas challenge here. If you send us a couple of sentences telling us how many steps it took you, and the strategy employed in getting it done, we'll try to post your note for the potential benefit of those looking for strategies which will work for them.
Specimens in preparation...
Holey Carbon Film: 32x32, 64x64, and 128x128.
Amorphous Specimen: 32x32, 64x64, 128x128.
Polycrystalline Specimen: 32x32, 64x64, 128x128.
Atom-Thick Graphite 2D-PolyCrystal: 32x32, 64x64, 128x128.
Then practice astigmatism correction, with power spectrum information to better see what is happening. Begin with a corrected image, then make it worse and then better. Image sizes in preparation include 32x32, 64x64, and 128x128. This can also be done (with the δf buttons) on our new 256x256 JS/HTML5-canvas simulator here.
First, practice focusing with power spectrum information available to better see what is happening. Begin at exact "Gaussian focus". Image sizes we are calculating include 32x32,64x64, and 128x128. Even better now, try out our new 256x256 JS/HTML5-canvas simulator here. On the larger size images, you might also ask if there isn't something in the holey carbon film specimen in addition to the holes...
Page updated
Google Sites
Report abuse