It is important to know what size of the obj. aperture you are using. You don't want to use too small size to actual cut off useful high resolution signal. Meantime, you don't want to use the aperture size too large, as the non-usable high resoltion beam becomes noise to your image. This reduces signal to noise ration unneccesarily.
The position of the aperture could affect the obj. lens stigmatism. Therefore, you want to do this step before you finally check Obj lens stigmatism.
Here are the steps to insert and center Objective lens aperture:
Make sure the large screen is down, to prevent CCD from damage.
Switch to diffraction mode by pressing the diffraction "D" button.
Adjust camera length to ~1m using magnification knob.
Adjust the (C2) and knobs to see the shape edge of the obj. aperture.
Adjust the related mechanical screws on aperture holder to choose the proper size of the aperture and center it to the central beam on diffraction pattern.
Switch back to image mode by pressing button again.
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