Medium- and small-scale grid distortions
The medium- and small-scale distortions represent the final sub-mas corrections that were accumulated and applied to the astrometric intermediate data. They are properties of the modulating grid as convolved with the average projection characteristics of the telescopes. The displays show the mean distortions over the two fields of view, as well as the differences between the two fields. Data in the along-scan direction are affected by integration, which is shortened close to the edge of the grid.
The modulating grid was manufactured by means of a grid of 168 by 48 individual prints, and this structure of the prints can be seen in the small-scale distortion corrections as a function of the across-scan coordinate. The figure below shows a detail of this correction. The saw-tooth like appearance implies that the grid-elements are slightly tilted, creating localized offsets at a level of up to 0.2 mas.