Can anybody explain me this phenomenon? When correcting objective astigmatism (OA), the image (the particles) gets deformed. I must say that the values for correction OA are large.
Thank you
A non-physics explanation;
Take an air filled round balloon. Draw a particle shape on it and the grab the balloon at opposite ends and pull. The balloon distorts as does what you drew. Grab the balloon at opposite ends at 90 degrees to what you initially grabbed and pull. Again the shape distorts 90 degrees to the initially distortion. The same thing happens when you adjust the objective astigmatism in a TEM; you are distorting the beam (and therefore particles projections within it) in both the X and Y axis'.
There are some good books about how a TEM functions that have all the formulas I have omitted.
Dear Mihaela Folea , you are right, when you apply too much OA correction, the beam becomes aberrated with an excess of astigmatism and therefore your particles appear distorted in shape. It is like if you try a pair of glasses corrected for a person with astigmatism, you can´t see properly because they are overcorrected for your eyes. Now imagine that a person with visual astigmatism goes to the optician to get correction glasses, but he/she get glasses that are over-correcter (or undercorrected) for her/his vision, therefore he/she will see badly. The same is true for a TEM, (or SEM or any beam microscope). There are two main types of astigmatism, vertical and horizontal and combinations of both of them. Imagine you have vertical astigmatism, you should correct it applying horizontal astigmatism.
It can also happen that your TEM have also other type of aberrations, like spherical aberration or comma or higher order aberrations (often less relevant). Some TEM have a special set of magnetic lenses intended to correct high order aberrations, it could be an hexa or octa-pole lens.
For astigmatism correction you often have a couple of knobs in your TEM console, for vertical and horizontal correction. If your particles are round and they appear as ellipsoids or rods, you should tune the knobs till they appear round. This opperation must be done with care and if you cannot recover the beam shape, the best idea is to contact with a technician or call the technical service of the TEM manufacturer.
Hope it helps.
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