268
8.
MICROSCOPY
soft iron screws between the lens elements in such a fashion that intrinsic
asymmetries in the magnetic flux of the pole-piece were exactly counterbalanced (first RCA model equipped this way as a standard fitting was
the EMU-2D). With sufficient care, practically all traces of intrinsic
astigmatism could be removed by this method. To this day, the investigator whose specimens do not require the use of an aperture can use his
compensated pole-piece for months or even years without problems of
adjustment, but those of us who must use an aperture are confronted
daily with the possibility of astigmatism, and the necessity of correcting
for it as it appears.
Of course, one can replace apertures with the expectation (often in
vain) of eliminating astigmatism. The very nature of the alignment
procedure endangers the aperture, so that by the time a new one is
aligned it may well be contaminated and be a source of new astigmatism.
Thus, until 1956, we were working precariously at best and often used the
microscope knowing that some astigmatism was present, but felt that we
would not necessarily improve things by attempting corrective measures.
Then the Canal Industrial Corporation added to their externally centerable aperture holder a group of six radially arranged, nonmagnetic pins,
opposing pairs of which could be given a variably controlled and stable
electrostatic charge. This could be adjusted so that its asymmetric effect
was set to counterbalance the astigmatism of the lens system. Thus, the
lens could be compensated while the instrument was in operation simply
by turning two knobs, one changing the azimuth of superimposed charge,
and the other controlling its magnitude. From the practical point of view
this has been a very important development for now one can correct
astigmatism as it appears and with little effort. There is now no excuse
for compromising with quality. Modern microscopes include this feature
or its magnetic equivalent.
Although the objective lens is the very heart of any microscope and
deserves primary attention, the optical performance of modern electron
microscopes has been improved in yet other ways. Although a properly
placed, suitably small objective aperture does much to remove scattered
electrons, some get through even this tiny hole and, indeed, with the
main beam itself. It became apparent that contrast could be improved
further by placing additional apertures in the projector pole-piece. These
apertures fortunately need not be very small to be nonetheless effective.
Simple mechanical alignment is usually enough, and contamination is
not a serious problem with them. In general, modern microscopes come
8.
MICROSCOPY
soft iron screws between the lens elements in such a fashion that intrinsic
asymmetries in the magnetic flux of the pole-piece were exactly counterbalanced (first RCA model equipped this way as a standard fitting was
the EMU-2D). With sufficient care, practically all traces of intrinsic
astigmatism could be removed by this method. To this day, the investigator whose specimens do not require the use of an aperture can use his
compensated pole-piece for months or even years without problems of
adjustment, but those of us who must use an aperture are confronted
daily with the possibility of astigmatism, and the necessity of correcting
for it as it appears.
Of course, one can replace apertures with the expectation (often in
vain) of eliminating astigmatism. The very nature of the alignment
procedure endangers the aperture, so that by the time a new one is
aligned it may well be contaminated and be a source of new astigmatism.
Thus, until 1956, we were working precariously at best and often used the
microscope knowing that some astigmatism was present, but felt that we
would not necessarily improve things by attempting corrective measures.
Then the Canal Industrial Corporation added to their externally centerable aperture holder a group of six radially arranged, nonmagnetic pins,
opposing pairs of which could be given a variably controlled and stable
electrostatic charge. This could be adjusted so that its asymmetric effect
was set to counterbalance the astigmatism of the lens system. Thus, the
lens could be compensated while the instrument was in operation simply
by turning two knobs, one changing the azimuth of superimposed charge,
and the other controlling its magnitude. From the practical point of view
this has been a very important development for now one can correct
astigmatism as it appears and with little effort. There is now no excuse
for compromising with quality. Modern microscopes include this feature
or its magnetic equivalent.
Although the objective lens is the very heart of any microscope and
deserves primary attention, the optical performance of modern electron
microscopes has been improved in yet other ways. Although a properly
placed, suitably small objective aperture does much to remove scattered
electrons, some get through even this tiny hole and, indeed, with the
main beam itself. It became apparent that contrast could be improved
further by placing additional apertures in the projector pole-piece. These
apertures fortunately need not be very small to be nonetheless effective.
Simple mechanical alignment is usually enough, and contamination is
not a serious problem with them. In general, modern microscopes come