288
8.
MICROSCOPY
rhe sweep is decreasing or increasing in amplitude. If the former, the
correct side was chosen, if the latter, it is necessary to reverse the direction of the gun and condenser movements. Alignment is achieved when
the illumination expands symmetrically on either side of cross-over.
Once the condenser lens has been aligned it should not need much
attention. However, if small, externally centerable condenser apertures
are being used, these will have to be put in after the basic alignment
is complete, and will require alignment themselves. This is done once
again by varying the condenser control through cross-over. The illumination will sweep until alignment is achieved when it will expand symmetrically. The aperture is moved in an arbitrary direction until the
sweep pattern starts to change or illumination dims. In the latter case,
one has been going in the wrong direction, and the movement must be
reversed. When the sweep pattern begins to change, the aperture is then
moved at right angles to the original direction, and the same criteria
are used. Alignment is achieved by successive approximation, alternating
the two axes of corrective movement.
8.11. Filament Replacement and Alignment
Filament replacement is the commonest maintenance task. Three different adjustments must be performed with some precision. (1) The tip
of the filament must be mechanically centered with reasonable accuracy.
If centering is done very carefully each time, there will be little need
for optical alignment subsequently. Precision here pays dividends later.
A centering jig which the manufacturer may supply or which can be made
in a machine shop is quite worthwhile.
(2) The distance of the filament tip from the anode is critical in a
biased gun. If it is too close, excessive beam currents will be drawn which
are apt to overload seriously the circuits of the microscope, and destroy
regulation. If the distance is too far, beam intensity will be weak. A
depth gauge or jig which the manufacturer may supply or which can be
made easily is almost essential.
(3) After the gun is reassembled, when the microscope is to be put
back in use again, some alignment probably will be necessary, but no
more than translational adjustments of the gun. If the previous
filament
was centered carefully, and the new filament is also well centered, no
source or condenser alignment should be required.
Every time a gun is disassembled to change a filament, the operator
8.
MICROSCOPY
rhe sweep is decreasing or increasing in amplitude. If the former, the
correct side was chosen, if the latter, it is necessary to reverse the direction of the gun and condenser movements. Alignment is achieved when
the illumination expands symmetrically on either side of cross-over.
Once the condenser lens has been aligned it should not need much
attention. However, if small, externally centerable condenser apertures
are being used, these will have to be put in after the basic alignment
is complete, and will require alignment themselves. This is done once
again by varying the condenser control through cross-over. The illumination will sweep until alignment is achieved when it will expand symmetrically. The aperture is moved in an arbitrary direction until the
sweep pattern starts to change or illumination dims. In the latter case,
one has been going in the wrong direction, and the movement must be
reversed. When the sweep pattern begins to change, the aperture is then
moved at right angles to the original direction, and the same criteria
are used. Alignment is achieved by successive approximation, alternating
the two axes of corrective movement.
8.11. Filament Replacement and Alignment
Filament replacement is the commonest maintenance task. Three different adjustments must be performed with some precision. (1) The tip
of the filament must be mechanically centered with reasonable accuracy.
If centering is done very carefully each time, there will be little need
for optical alignment subsequently. Precision here pays dividends later.
A centering jig which the manufacturer may supply or which can be made
in a machine shop is quite worthwhile.
(2) The distance of the filament tip from the anode is critical in a
biased gun. If it is too close, excessive beam currents will be drawn which
are apt to overload seriously the circuits of the microscope, and destroy
regulation. If the distance is too far, beam intensity will be weak. A
depth gauge or jig which the manufacturer may supply or which can be
made easily is almost essential.
(3) After the gun is reassembled, when the microscope is to be put
back in use again, some alignment probably will be necessary, but no
more than translational adjustments of the gun. If the previous
filament
was centered carefully, and the new filament is also well centered, no
source or condenser alignment should be required.
Every time a gun is disassembled to change a filament, the operator