1.    The Beam Defining Aperture.  There are two types of "final" aperture (i) is almost at the base of the final lens (through the side of the specimen chamber) and is known as a real aperture, (ii) the other aperture known as a virtual aperture will be found some way up the column from the final lens position.  The real aperture as its name suggests is the final unit within the column that affects the beam shape and its edge sharpness.  The virtual aperture casts a shadow down the column that has been found to have the same affect.  The real aperture is the older style of aperture system and usually does not require re alignment unless there has been a major change in column condition e.g. kV, Spot Size or aperture change.  The virtual aperture is found on most modern systems, it needs far more alignment correction as it is very much smaller.  This aperture system, whilst its alignment requirement is more regular, is well away from the "dirty" specimen area and therefore has the advantage of lasting a very long time before cleaning is required. 

 

When either aperture type is out of alignment the image will move when you focus.  Image movement when you correct astigmatism has nothing to do with the aperture alignment.  Alignment of either a real or a virtual aperture system is identical.  Use the wobbler to judge the direction of specimen movement.  Your aperture adjustment controls work as Y, up/down the screen

 

or X, left/right on the screen.  If the wobble direction is neither exactly up and down or left and right, simply take any one of the alignment controls and adjust it until the image moves either directly up and down or left and right.  Once you have made this correction go to the other alignment control and adjust it to minimise the new direction.  Only use one control at a time.  Each time the control forces the image to change from up/down or left/right to the other motion you change the control that you were using and continue with the correction.  A corrected aperture alignment will vary its image form depending upon the magnification in use and to a certain extent the model of microscope.  A corrected aperture alignment may produce an image that spins around the centre of the screen when the wobbler is on.  Alternatively the image will display a rather erratic movement which does not favour any particular direction.  Either motion will be found to be correct.

2.    Astigmatism.  This defect is the result of the demagnification of the condenser lenses in one direction being greater than the demagnification in another direction.  

When working it is possible that whilst moving through focus the image becomes stretched first in one direction and as you move through image focus the stretch is minimised before it is displayed at right angles to the first direction - this is astigmatism.  Never try to correct/compensate astigmatism (stigmate) when you are able to see the stretched image, always bring it to focus, the point when the image is neutral in form even if it is not sharp.  From this condition use each stigmator in turn to sharpen the image.  With gross astigmatism you may need to re focus and repeat the procedure.  I would advise moving to at least double your working magnification in order to correct the astigmatism, in which case if you do not make a perfect correction it should not spoil your image at the lower magnification. A quick check of your compensation is to move out of focus slightly, the image should go soft evenly in all directions if you have been successful.

 

Should the image move during the correction/compensation process the stigmators are out of balance.  Not all SEM have a correction to a stigmator imbalance that is within the reach of the operator, it is often an engineer task.  However do not be critical of the engineer if after a few months this alignment once again requires correction!  This problem is common on many of the older instruments.