John,
A follow up to our brief discussion on stage precision (and perhaps how it relates to doing stage coordinate to image position calibrations):
I quote from the Cameca SX100 product description 17th Jan 2008 version "A large travel (50mmx80mm) automated specimen stage featuring: -Micro stepping motors (minimum 0.1um)" and "Closed loop optically encoded specimen positioning (+/_0.5um)"
On my instrument if I go to a relatively high mag (say 30um FOV) I can move the stage with the fine-adjust rollers (such that I see a shift in the electron image) without the stage coordinates updating (as observed in the roller window). Ergo, I conclude that this is consistent with positioning precision, as defined by the motors, better than 1um (probably 0.1um) but accuracy is only guaranteed at +/-0.5um because of the encoder limitations.
From my memory of my old SX50 that I was constantly fixing, the quadrature encoders have 4 bit-states per division on the encoder grating. I don't know what the grating spacing to um conversion is, but if we assume 1um, it is likely that all 4 states need to change for an update of the position (in um) The changing of the bit states is how Cameca immediately knows if there is a stage movement problem - if the the encoder bits do not change in the correct order, an error has occurred.
But here is where the plot thickens: If I use peaksight to define a stage scan SE image with 0.2um spacing and uncheck the 'continuous' motion button, The image is acquired. Now I can't tell if x is moving in steps, and it may be continuous, but Y cannot be moving continuously. The Y stage position duly updates 1um every 5 lines in X, and the image, although a little distorted in Y, looks similar to the beamscan equivalent (see attached). Hence, it looks like Peaksight can move the motors in steps less than 1um. I speculate wildly that perhaps during ROM based mapping an absolute position is not being used? this would mean that the stage could be moved by one step of the motor, without any positional feedback required? or, the encoder has better than 1um precision? Perhaps someone with more insight could clarify.
Anyway, not really of any consequence, except that this implies a stage map with 0.1um pixels is perfectly valid (for SE images at least), and very useful for stage/image calibration (assuming no distortions); if only the dwell could be less than 5ms!
Gareth