8.10.
CONDENSER
ALIGNMENT
287
and in some microscopes manufactured abroad, the equivalent of this is
accomplished electrostatically.
To achieve source alignment, the gun is tilted so that the beam moves
in a direction diametrically opposite to the asymmetric cloud. When the
image sweeps off the viewing screen, it is returned by gun translation.
In practice the source should be aligned at as low a magnification as is
feasible so that as the image of the source is deflected by tilting, its path
can be followed as long a way as possible. Also, one wants to see an
image of the entire source on the viewing screen. As successive approximations approach alignment, the principal cloud dwindles in size as its
counterpart increases, and becomes symmetrically disposed. If it seemed
originally that one had a single lateral cloud, this will break up into two
halos which will move to symmetrical positions at the ends of the long
axis.
Source alignment is not very critical with a biased gun, and if it is obtained originally with a well-centered filament, subsequent replacement
of the filament should not demand readjustment. Of course the tilt, once
established, must not be altered. Centering the illumination between
alignments must be done only by changing the gun translation.
8.10. Condenser Alignment
Condenser alignment is relatively simple. One is concerned with
centering the gun in relation to the condenser axis. One must begin
with a mechanically centered aperture, the larger the better to minimize
centering error. As the condenser control is moved back and forth
through cross-over, the beam will sweep across the viewing screen in an
unaligned instrument. The sweep actually represents an arc of a circle,
and one has to move the beam along a radius toward its center by
condenser translation, while keeping the beam in view by gun translation. In practice, the arc is apt to have such a slight curvature that it is
not apparent. It is usually simplest to choose a side arbitrarily and move
the cross-over spot at right angles to the sweep. One soon learns whether
FIG. 33. Images of "clouds" or "halos" produced when the image of the source is
focused at cross-over, and when the current is also below the bias position. The symmetrical cloud of (a) indicates good source alignment [(b) is natural size]. Asymmetrical
clouds (c)-(e) indicate misalignment which is corrected by successive approximation.
The gun is transversed in the direction of the main cloud, and then tilted in the
opposite direction, as indicated by the arrows.
CONDENSER
ALIGNMENT
287
and in some microscopes manufactured abroad, the equivalent of this is
accomplished electrostatically.
To achieve source alignment, the gun is tilted so that the beam moves
in a direction diametrically opposite to the asymmetric cloud. When the
image sweeps off the viewing screen, it is returned by gun translation.
In practice the source should be aligned at as low a magnification as is
feasible so that as the image of the source is deflected by tilting, its path
can be followed as long a way as possible. Also, one wants to see an
image of the entire source on the viewing screen. As successive approximations approach alignment, the principal cloud dwindles in size as its
counterpart increases, and becomes symmetrically disposed. If it seemed
originally that one had a single lateral cloud, this will break up into two
halos which will move to symmetrical positions at the ends of the long
axis.
Source alignment is not very critical with a biased gun, and if it is obtained originally with a well-centered filament, subsequent replacement
of the filament should not demand readjustment. Of course the tilt, once
established, must not be altered. Centering the illumination between
alignments must be done only by changing the gun translation.
8.10. Condenser Alignment
Condenser alignment is relatively simple. One is concerned with
centering the gun in relation to the condenser axis. One must begin
with a mechanically centered aperture, the larger the better to minimize
centering error. As the condenser control is moved back and forth
through cross-over, the beam will sweep across the viewing screen in an
unaligned instrument. The sweep actually represents an arc of a circle,
and one has to move the beam along a radius toward its center by
condenser translation, while keeping the beam in view by gun translation. In practice, the arc is apt to have such a slight curvature that it is
not apparent. It is usually simplest to choose a side arbitrarily and move
the cross-over spot at right angles to the sweep. One soon learns whether
FIG. 33. Images of "clouds" or "halos" produced when the image of the source is
focused at cross-over, and when the current is also below the bias position. The symmetrical cloud of (a) indicates good source alignment [(b) is natural size]. Asymmetrical
clouds (c)-(e) indicate misalignment which is corrected by successive approximation.
The gun is transversed in the direction of the main cloud, and then tilted in the
opposite direction, as indicated by the arrows.