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(4) Condenser lens astigmatism correction
- Press STIG to activate stigmator control.
- Press COND soft key so that it is highlighted (the default is OBJ).
- Condense the beam.
- Use MULTIFUNCTION knobs to correct the astigmatism. The beam should be round
when changing the setting of the INTENSITY knob, or the filament image will be
sharpest if filament is under-saturated.
- To see the sharp image of filament, three conditions, under-saturated, crossover, and
free from astigmatism, must be met.
- Press STIG to de-activate stigmator control.
(4) Pay attention to filament image while slowly increase filament current to saturate (prefer
slightly under-saturate) position. Emission current should be 5~8 µA.
(5) Check aperture center by spreading the beam, make sure that beam spreads without sweep.
(C) Alignment of Imaging System:
(i) Voltage Center Alignment
To align the imaging system, the AC component is superimposed on the accelerating voltage
in order to move the image radially. The center of the radial movement (called “voltage
center”) is then matched with the center of the fluorescent screen. The main purpose of this
step is to center the beam tilt so there won’t be a tendency to have a two-dimensional image
smeared out in one direction. This is very important in doing high-resolution work.
(1) Z-alignment (eucentric height adjustment)
- Rough focus the sample at 10,000 to 20,000X.
- Find an identifiable object and center it on the screen
- Tilt the sample. This may cause the center of the field of view to move towards the edge
of the projection screen if the specimen is not at eucentric height position.
- Set the specimen height at the point where there is no movement of the specimen upon
tilting by adjusting the z control (screw on the goniometer just below the sample holder).
- Bring the tilt mechanism to 0
o.
(Hint: if autofocus is way off, z-alignment is bad. Focus the image with the z-alignment
from autofocus (use screw on the goniometer just below the sample stage))
(2) Pivot point X and Y
- Move to center of sample (i.e., any hole or away from the sample).
- Press PIVOT PT X soft key in ALGN.
- Condense beam.
- Use MULTIFUNCTION knobs to bring two circles together.
- Press PIVOT PT Y soft key in ALGN.
- Condense beam.
- Use MULTIFUNCTION knobs to bring two circles together.
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