Naturally, the apparent {x,y} position of the table deviates somewhat from a
perfectly uniform, rectilinear coordinate system. When in use as a coordinate
measuring machine, the Leitz control system applies software corrections to
the apparent positions in order to correct for this effect. Because of the
high stability of the machine in its temperature-controlled environment, it is
expected that these corrections will remain systematic and fixed for long
timescales (> 1 year), and hence that software correction provides a viable
means of obtaining an accurate measuring system. Measurement of NPI gauge
blocks confirmed that, with corrections in place, the large-scale absolute
accuracy of the table (in the absence of the optical system and measuring
statically as a coordinate measuring machine) was indeed accurate to
micron.
During a scan, some of these systematic errors (the larger component) are removed by applying our own real-time position corrections: others are removed during the image-processing stage.