小中大从来没调过，Tecnai 的帮助文件这样写的。
5.12 Coma-free alignment
Purpose: Make sure that the beam is along the optical axis of the objective lens.
Importance: ESSENTIAL for obtaining reliable and accurate high-resolution data.
Method: The microscope will slowly wobble the beam in the x or y direction with a set amount of beam tilt (determined by the coma-free amplitude alignment). Adjust the coma-free center until the images for both tilts have the same apparent defocus.
Procedure
The alignment procedure consists of three steps:
• The first step is a preparation step to center the beam and focus the image.
• In the two following steps, is wobbled (first step in the x, second step the y direction), and the coma-free center is adjusted until the two 'wobble' images have the same defocus.
Description
There are several possible methods for aligning the objective lens (but all affect the same parameter - the tilt of the electron beam):
• Current center - by wobbling the objective lens and minimizing image displacements.
• Voltage center - by wobbling the high tension and minimizing image displacements (not implemented on Tecnai).
• Coma-free alignment - by wobbling the incident beam and minimizing focus difference.
The former two methods came about in an age when it was important to minimize the effects of objective-lens (current center) and high-tension (voltage center) instabilities in order to achieve high resolution. Nowadays these instabilities are so small that they are no longer of major concern. Meanwhile it had been found by Zemlin et al. (1978; Ultramicroscopy 3, 49) and later Smith et al. (1983; Ultramicroscopy 11, 263) that neither current nor voltage center is sufficient alignment for high-resolution imaging. One difficulty is often the precision (reproducibility) with which the current or voltage center can be aligned. The main problem is, however, that for most instruments neither center lies along the true objective-lens optical axis. A misaligned objective lens will lead to different phase shifts (the Contrast Transfer Function) for the equivalent hkl and -h-k-l diffracted beams, which has a major effect on apparent symmetry in high-resolution images.
Only coma-free alignment is sufficiently accurate for centering of the objective lens for high-resolution imaging. In coma-free alignment, the beam is wobbled slowly between a -x and +x tilt. Because of the presence of spherical aberration, these beam tilts lead to an apparent defocus (overfocus). This defocus is relative to the true optical axis and not to the 'unwobbled' case. In order to align the objective lens, the beam tilt is then adjusted to make the defocus of the two wobble directions identical. The adjustment is done for the x and y tilt directions.
Note 1: It is important to remember that it is the apparent defocus that must be the same for the two images. This doesn't mean that the images become identical. After all, the beam still goes through the specimen (best an amorphous carbon foil or amorphous edge of a hole in the specimen) in two different directions and the difference in the line-up of atoms in the amorphous material can still lead to differences in the images themselves.
Note 2: Because coma-free alignment and the rotation center affect the same parameter (the tilt of the incident beam), there is no point in iterating rotation center, then coma-free alignment and then rotation center again (with the last alignment the coma-free alignment is undone). These alignments differ in the method, not in their result.
Note 3: The effect of coma (the lens error introduced by beam tilt) looks very similar to that of astigmatism. The final astigmatism correction should be done after coma-free alignment, but it may be necessary to do an initial astigmatism correction before.
