8.3.
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271
know at first whether the aperture is too close to, or too far from the lens.
He then uses the coarsest focus control and goes up or down the scale.
The image of the aperture lip probably will tend to move off the field of
view and will have to be kept in view with its alignment screws. At some
level of lens current the rim of the aperture will completely disappear
from view and, as the controls are turned further, it will reappear again
but with reversed symmetry. Thus, there is a "cross-over" effect which
appears when the aperture passes the back focal plane. This will be the
clue as to whether the aperture is too close to the lens or too far away
from it. If the lens is operating as an unduly weak lens at low current
levels, the gap between the aperture and the lens is too long, and the
holder must be elevated. Conversely, if the lens current is above normal
so that the lens is too strong, the aperture is too near the lens, and the
gap must be lengthened. Successive adjustments will have to be made
until finally the aperture is close to the back focal plane when a specimen
is also in focus. Ideally, the aperture height should be adjusted so that it
is in the back focal plane when the instrument is in focus for a fairly
high magnification.
Fortunately, once the correct aperture position has been determined,
it will not need readjustment unless the specimen distance from the polepiece is altered. This means, however, if more than one specimen holder
is used, that all be positioned so that the specimen plane is identical. It
may preclude the use of stereoholders.
8.3. Objective Aperture Alignment
It has already been indicated that when apertures become asymmetrically contaminated, astigmatism appears which ruins good resolution.
Inevitably, apertures have to be replaced fairly often, which means that
microscopists must become adept at their alignment. The great hazard in
this is that contamination will occur as the aperture is being aligned if a
high intensity beam is allowed to bombard the rim of the aperture during
this process. It is necessary to work with a greatly spread beam, and
probably also with a beam current substantially below saturation.
If a between-the-lens aperture is being used, it is not possible to obtain
a focused image of its edge. However, at low magnification a circle of
illumination can be seen having a fuzzy limit. The center of this should
OBJECTIVE APERTURE ALIGNMENT
271
know at first whether the aperture is too close to, or too far from the lens.
He then uses the coarsest focus control and goes up or down the scale.
The image of the aperture lip probably will tend to move off the field of
view and will have to be kept in view with its alignment screws. At some
level of lens current the rim of the aperture will completely disappear
from view and, as the controls are turned further, it will reappear again
but with reversed symmetry. Thus, there is a "cross-over" effect which
appears when the aperture passes the back focal plane. This will be the
clue as to whether the aperture is too close to the lens or too far away
from it. If the lens is operating as an unduly weak lens at low current
levels, the gap between the aperture and the lens is too long, and the
holder must be elevated. Conversely, if the lens current is above normal
so that the lens is too strong, the aperture is too near the lens, and the
gap must be lengthened. Successive adjustments will have to be made
until finally the aperture is close to the back focal plane when a specimen
is also in focus. Ideally, the aperture height should be adjusted so that it
is in the back focal plane when the instrument is in focus for a fairly
high magnification.
Fortunately, once the correct aperture position has been determined,
it will not need readjustment unless the specimen distance from the polepiece is altered. This means, however, if more than one specimen holder
is used, that all be positioned so that the specimen plane is identical. It
may preclude the use of stereoholders.
8.3. Objective Aperture Alignment
It has already been indicated that when apertures become asymmetrically contaminated, astigmatism appears which ruins good resolution.
Inevitably, apertures have to be replaced fairly often, which means that
microscopists must become adept at their alignment. The great hazard in
this is that contamination will occur as the aperture is being aligned if a
high intensity beam is allowed to bombard the rim of the aperture during
this process. It is necessary to work with a greatly spread beam, and
probably also with a beam current substantially below saturation.
If a between-the-lens aperture is being used, it is not possible to obtain
a focused image of its edge. However, at low magnification a circle of
illumination can be seen having a fuzzy limit. The center of this should