8.7.
OBJECTIVE ALIGNMENT
283
section, voltage alignment is of considerable importance when one is
striving for great resolution. Thus, practically all microscopists today
advocate voltage alignment rather than current alignment. The price
one pays is a slight inconvenience, for the image can be expected to move
as one focuses, since one is not apt to be looking at the center of this
rotation.
Voltage alignment is achieved by deliberately varying the accelerating
voltage and finding the center of image rotation. The objective lens is
then moved upon the projector lens until this axis is centered in the
viewing screen. In some of the older microscopes it is necessary to turn
off the high voltage switch and observe the expanding image as the high
voltage condensers discharge to see the axis of rotation. More modern
instruments provide a special switch which makes a moderate, controlled
alteration in the accelerating voltage which is a great convenience.
If the intermediate lens is not coupled to the projector lens and therefore must be aligned separately, it must not be functional when voltage
alignment is done. In one way or another it must be disconnected. In
the RCA EMU-3 and EML microscopes, one position of the magnification control cuts out this lens (taps 7 and 8, respectively). Voltage alignment is done at these positions. In the RCA EMU-2 microscopes the
coupling of the intermediate and projector lenses means that only a
compromise voltage alignment is possible. It is important to do this
with the microscope set at as high a magnification as one expects to
use commonly.
Voltage alignment should not be done with a good objective aperture
in place. The aperture will often become asymmetrically contaminated
by the beam sweeping across its rim when the accelerating voltage is
changed.
Once objective alignment has been achieved, it will not change except
as the electrical circuits change. But there will be gradual changes as
reference batteries deteriorate, and there may be sudden changes when
tubes or other defective parts are replaced. Thus, when any work has been
done on the fundamental regulating circuits, the high voltage circuits,
or the objective lens circuit, the objective alignment should be checked.
It is also wise and convenient to check the alignment every time an objective aperture is removed for cleaning or replacement. Taking out
the objective pole-piece and replacing it will make small changes in the
optical axis, but if the pole-piece is returned as nearly as possible to its
original position these should be so minor that they can be ignored and,
OBJECTIVE ALIGNMENT
283
section, voltage alignment is of considerable importance when one is
striving for great resolution. Thus, practically all microscopists today
advocate voltage alignment rather than current alignment. The price
one pays is a slight inconvenience, for the image can be expected to move
as one focuses, since one is not apt to be looking at the center of this
rotation.
Voltage alignment is achieved by deliberately varying the accelerating
voltage and finding the center of image rotation. The objective lens is
then moved upon the projector lens until this axis is centered in the
viewing screen. In some of the older microscopes it is necessary to turn
off the high voltage switch and observe the expanding image as the high
voltage condensers discharge to see the axis of rotation. More modern
instruments provide a special switch which makes a moderate, controlled
alteration in the accelerating voltage which is a great convenience.
If the intermediate lens is not coupled to the projector lens and therefore must be aligned separately, it must not be functional when voltage
alignment is done. In one way or another it must be disconnected. In
the RCA EMU-3 and EML microscopes, one position of the magnification control cuts out this lens (taps 7 and 8, respectively). Voltage alignment is done at these positions. In the RCA EMU-2 microscopes the
coupling of the intermediate and projector lenses means that only a
compromise voltage alignment is possible. It is important to do this
with the microscope set at as high a magnification as one expects to
use commonly.
Voltage alignment should not be done with a good objective aperture
in place. The aperture will often become asymmetrically contaminated
by the beam sweeping across its rim when the accelerating voltage is
changed.
Once objective alignment has been achieved, it will not change except
as the electrical circuits change. But there will be gradual changes as
reference batteries deteriorate, and there may be sudden changes when
tubes or other defective parts are replaced. Thus, when any work has been
done on the fundamental regulating circuits, the high voltage circuits,
or the objective lens circuit, the objective alignment should be checked.
It is also wise and convenient to check the alignment every time an objective aperture is removed for cleaning or replacement. Taking out
the objective pole-piece and replacing it will make small changes in the
optical axis, but if the pole-piece is returned as nearly as possible to its
original position these should be so minor that they can be ignored and,