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8.
MICROSCOPY
8.13. Instrument Stability
Such a variety of things can destroy the quality of an electron micrograph that it is foolish to take a chance with the microscope itself. One
should check its stability every day before use. This can best be done
while one checks the objective pole-piece compensation (discussed in
the previous section).
High magnification is used with an optical microscope viewer. It is
simplest to superimpose specimen dirt on a piece of dust or a scratch mark
on the viewing screen. Then any small movement can be detected. It may
be that a drift will be observed that is due to specimen instability rather
than to trouble with the microscope. This is particularly apt to be the case
when pure plastic supporting films are used without carbon stabilization.
In a micrograph images will be fuzzy in one axis [Fig. 34(1)]. One can
check this by examining rough contours of the specimen screen itself.
These should not show drift for more than a few seconds after the stage
has been moved, which should be sufficient time for thermal equilibration.
Most of the time, when there is image instability, the culprit is a contaminating film or a particle of dust in one of the critical places in the
microscope column, and these should be considered and checked first.
But it may be circuit instability! Most microscopes, unfortunately, do
not provide ways of adequately testing circuit instability. The gauges
that are provided are generally relatively sluggish voltmeters and ammeters. These do well enough to indicate the general level of circuit
operation, but tell nothing of momentary circuit breaks, superimposed
ac-ripple [Fig. 34(m)] or, in general, what radio technicians call "noise."
In the absence of adequate test equipment for quickly spot checking
circuit instability, the microscope operator cannot do much when image
instability appears except first clean up the column. In at least nine out
of ten instances this will eliminate the trouble. Since the objective aperture is so apt to be the cause of difficulty, it is usually well to examine
image stability with this removed if one is suspicious of circuit instability.
It is an advantage to have a microscope more or less permanently
equipped with built-in "power monitoring" equipment. Rapid voltage
fluctuations, ac-ripple, and the like can be seen best on an oscilloscope
screen. This writer knows of several instruments that have been custom
equipped with an oscilloscope, suitably tapped in through switches to
the important circuits of the microscopes. By flicking a switch, the be-
8.
MICROSCOPY
8.13. Instrument Stability
Such a variety of things can destroy the quality of an electron micrograph that it is foolish to take a chance with the microscope itself. One
should check its stability every day before use. This can best be done
while one checks the objective pole-piece compensation (discussed in
the previous section).
High magnification is used with an optical microscope viewer. It is
simplest to superimpose specimen dirt on a piece of dust or a scratch mark
on the viewing screen. Then any small movement can be detected. It may
be that a drift will be observed that is due to specimen instability rather
than to trouble with the microscope. This is particularly apt to be the case
when pure plastic supporting films are used without carbon stabilization.
In a micrograph images will be fuzzy in one axis [Fig. 34(1)]. One can
check this by examining rough contours of the specimen screen itself.
These should not show drift for more than a few seconds after the stage
has been moved, which should be sufficient time for thermal equilibration.
Most of the time, when there is image instability, the culprit is a contaminating film or a particle of dust in one of the critical places in the
microscope column, and these should be considered and checked first.
But it may be circuit instability! Most microscopes, unfortunately, do
not provide ways of adequately testing circuit instability. The gauges
that are provided are generally relatively sluggish voltmeters and ammeters. These do well enough to indicate the general level of circuit
operation, but tell nothing of momentary circuit breaks, superimposed
ac-ripple [Fig. 34(m)] or, in general, what radio technicians call "noise."
In the absence of adequate test equipment for quickly spot checking
circuit instability, the microscope operator cannot do much when image
instability appears except first clean up the column. In at least nine out
of ten instances this will eliminate the trouble. Since the objective aperture is so apt to be the cause of difficulty, it is usually well to examine
image stability with this removed if one is suspicious of circuit instability.
It is an advantage to have a microscope more or less permanently
equipped with built-in "power monitoring" equipment. Rapid voltage
fluctuations, ac-ripple, and the like can be seen best on an oscilloscope
screen. This writer knows of several instruments that have been custom
equipped with an oscilloscope, suitably tapped in through switches to
the important circuits of the microscopes. By flicking a switch, the be-