guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Microscope
b) The particle is moving along a circular off-centre path (Fig. 1-8,II), indicating that the
objective is not centred. The rotation axis of the image has to be shifted into the centre of the
reticule. This is achieved by turning the two centering screws located in the objective casing
or the nosepiece using the centring tools commonly provided with the microscope. Older
microscopes may have centring rings on the objectives (and hence no special centring tools
are required). The objectives are centred if the rotation centre of any circular particle path
coincides with the crosshairs intersection. An alternative way of achieving this is to rotate the
stage such that an observed particle is in the most distant off-centre position, 180° from its
position at the crosshairs intersection. By turning the centring screws, the particle is then
shifted half the distance towards the crosshairs intersection (Fig. 1-8,III).
To verify that the objective is precisely centred now, the particle is shifted into the crosshairs
intersection by carefully moving the thin section. If the particle remains in its position when
rotating the stage, the objective is centred (Fig. 1-8,IV). Otherwise, the centring procedure
has to be repeated.
All the objectives on the nosepiece have to be centred this way. Should an objective of high
magnification be already centred precisely, it is even easier to centre the lower-magnification
objectives. A small grain or object is positioned in the crosshairs intersection using the highmagnification
objective. Then the poorly centred objective is rotated into the optical path and
the particle is moved to the crosshairs intersection using the centering devices.
Caution! It is important to ensure that the nosepiece is positioned correctly on the tube and
the objective in use has clicked into place. Otherwise, centring can never be achieved (Ch.
1.7). Some microscopes (e.g., Olympus) have a stage that can be centred. The stage is then
centred on a single fixed objective and has to be aligned first, with that particular objective in
place, before the other objectives can be centred. Any attempt to centre the objectives with
the stage being off-centre will result in a serious misalignment of the light path.
What applies to all microscopes: To avoid de-centring of the objectives, they must never be
touched when changing magnification! Always grab the grooved rim of the nosepiece to
change between objectives.
Raith, Raase & Reinhardt – February 2012
Figure 1-8. Centring the objective
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