I want to take SAED,did a selected-area aperture locate in the image plane of the objective lens or in the back focal of the intermediate lens ?
The SAED aperture is located in the image plane of the objective lens. This aperture isused to select the area on the image (sample) from where the diffraction pattern is desired. The intermediate lens makes the back focal plane of the objective lens its object so as to focus the diffraction pattern on to a second intermediate image plane, which is then projected on to the screen using a projector lens.
The SAED aperture is always located in the image plane of the objective lens, for reasons described well by Dr. Bhattacharyya. The location of this aperture concerning the intermediate lens depends on the setting of the intermediate lens - In other words on which imaging condition is selected. By changing the strength (the focal length) of the intermediate lens you switch from imaging to diffraction condition and vice versa.
In imaging mode the object plane of the intermediate lens is set to the object plane of the objective lens and thus, the aperture is in the object plane of the intermediate lens.
In diffraction mode the object plane of the intermediate lens is set to the back focal plane of the objective lens and thus, the aperture should be in the (front) focal plane of the intermediate lens.
I added a figure to visualize. It stems from Williams & Carter "Transmission Electron Microscopy" - a wonderful textbook.
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