Stage
movement Stage
movementThe specimen stage movement is controlled by a track ball (normally the right-hand one, but the assignment can be changed by the user) and/or by Multifunction knob. The movement has two modes of operation:
In the track ball mode
(the default) the stage will move whenever the track ball has
been moved, in the direction and with a displacement related to
the direction and displacement of the track ball. As soon as the
track ball stops moving, the stage will stop moving as well. For
normal operation at moderate to high magnifications this is the
preferred mode. However, for searching at low magnifications the
track ball mode requires that the user keeps moving the track
ball continuously.
In the 'joy stick' mode
the stage will move in the direction indicated by the movement of
the track ball and with a speed related to the displacement of
the track ball, and the stage will keep moving in the indicated
direction without requiring further input through the track ball.
The direction and speed of movement can be influenced by further
control of the track ball. To stop the stage movement
in the 'joy stick' mode, press one of the track ball
buttons.
Switching between the two modes is achieved by pressing the two
track ball buttons simultaneously.
Speed control
The speed of movement of the stage is related to three parameters :
Note 1: One exception is at the lowermost speed setting. In this case the CompuStage will make its smallest steps, independent of magnification. At low magnifications this may mean that the stage doesn't seem to move at all. If the latter is the case, click once on the right-hand track-ball button to switch the speed one step up.
Note 2: At very low magnifications the speed control may appear to function no longer (speed up doesn't increase the speed of the CompuStage). This means that the CompuStage has reached its maximum speed and can go no faster.
Specimen stage movement by Multifunction knob
The specimen stage movement can also be assigned to the Multifunction knobs. In this case the Multifunction knobs duplicate the track ball directions (that is, Multifunction X = track ball X). The knobs are thus not connected directly to a stage axis (technically this is not possible within the software architecture). For most microscopes this means that the X axis (the direction of the a tilt axis) is approximately connected to the Multifunction Y knob.
Stage axesThe CompuStage is a goniometer with five axes. Three of these are orthogonal translation movements X, Y and Z. The other two are mutually perpendicular rotation movements, a and b. The a tilt is parallel to the X axis and b tilt parallel to the Y axis.
The orientation in space of these axes can only be described at a tilt at 0° (since the other axes are mounted on top of the a tilt, they will change their orientation with it). At a tilt 0° the X and Y axes are horizontal and the Z axis vertical. The X axis runs along the rod of the specimen holder. If you look down on the microscope and define the front of the microscope as south, the X axis thus runs NW-SE, with NW in the + direction and the Y axis NE-SW with SW as the + direction. Up is the + direction of the Z axis. The X axis describes a truly linear motion, the Y and Z are in fact circular motions but with such a wide radius that the motion remains close to linear.
The Y and Z motions cause the specimen holder rod to pivot around the conical face where it narrows down, just beyond the O-ring.
Because of this pivoting motion, the end of the holder on the outside of the CompuStage moves in the opposite direction!
Below is a schematic 3-D view of the specimen holder tip and the orientation of the stage axes.
Note: The notation differs from that on the earlier CompuStage of the CM microscopes where the sign of the X and Y axes is reversed.
Eucentric
heightThe CompuStage is constructed in such a way that it is possible to tilt around the a tilt axis without having large apparent movements of the point of interest on the specimen. This is called eucentric tilting and is achieved by bringing the point of interest to the same height as the a tilt axis itself : the eucentric height.
Method 1 Eucentric Focus
Preset
Once the microscope has been aligned (as it normally
should be), the eucentric focus preset (obtained by pressing the Eucentric Focus button) sets
the objective-lens setting to the correct value for a specimen
that would be exactly at the eucentric height. If the specimen is
not at the eucentric height, it will appear out of focus. If you
now bring the specimen into focus by changing the Z height, not the focus, it will go to the
eucentric height. It may help to switch on the wobbler, since the apparent
displacement between the two wobbler images makes it easy to see
whether the height is changed in the right direction (the
displacement between the images becomes smaller).
Method 2 The Alpha
Wobbler
Since the image displacement on tilting is minimized at
the eucentric height, the eucentric height can be set by
minimizing the displacement while the stage is tilting. For this
purpose the CompuStage has the Alpha Wobbler function. When this
function is activated, the CompuStage is tilted continuously
between two preset tilts (typically -15 and +15°). Change the Z
height to make the displacement smaller. When the displacement is
minimized the specimen is at the eucentric height.
MaxiTilt
systemBecause tilting takes place around the eucentric point, the motion of the specimen holder may require a lot of space (see picture below).
In the restricted space available between the pole pieces of some objective lens types, there may not be enough room to tilt very far. But the pictures above demonstrate that the available tilt range is very much dependent on the position of the stage (mostly the Y and Z axes). The MaxiTilt system of the CompuStage provides a flexible way of keeping the tilt within a safe range, while at the same time maximizing the tilt available. The MaxiTilt senses what the maximum tilt is for the current stage position. If this range is exceeded (a situation called a pole touch), the CompuStage will move a little bit back on the axis that was changed last. If there is no axis identifiable, the B or A tilt will move back. The microscope will display an information message that a pole touch has been detected.
sEntry
systemThe CompuStage is equipped with a SafeEntry (or sEntry) system that prevents holders from being inserted into microscopes where the objective-lens pole-piece configuration is not compatible with the particular holder (like thick holders into objective lenses with a gap that is too narrow for the holder to fit). The sEntry system consists of a key (a pin varying in shape and diameter) on the holder defining the holder dimensions and a lock on the CompuStage defining the pole-piece configuration. The length of the sEntry key is such that if an incompatible holder is inserted, the key (blocked by the lock) prevents the holder from going between the pole pieces.
Although older specimen holders designed of the manual goniometer will fit in the CompuStage (provided their O-ring is exchanged for a - thicker - CompuStage O-ring), these holders are not equipped with a sEntry key and may therefore be unsafe with objective pole-piece configurations with narrow gaps such as the U-TWIN.
Page last modified on 11/13/00