The obj. stigmatism should be corrected as much as possible, and it should be checked for every netative stain low-dose image that you are taking, as staining material might change field in local area. This is a bit hard by hand. Even with lastest version of SerialEW, this can be done by software, it is still not easy and time efficient. However, slight stigmated image is not critically bad, as it can be corrected as part of CTF correction computationally.
Here are steps to correct Obj lens stigmatism, manually:
Go to a relatively high mag., such as 100,000X, and focus the image.
If possible, acquire continous CCD image with live FFT so Thon rings can be seen.
Press the button, highlight , and select proper stepsize.
Adjust the stigmatism using until it becomes minimum at all defocus levels. (It shows up more at close to focus.)
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