The location of the short on the OFCW is known to be between rings 181 and 182. The midpoint between those two rings is at z = 208.8 cm, as discernable in this schematic. Because we usually perform the distortion calculations within a box whose boundary is defined by the gating grid, and the gating grid is located at z = 208.707 cm, the ideal location to account for the short is just outside that box.

To deal with this, we perform the distortion calculation at a non-ideal location, offset by 1 mm to place it just inside the edge of the box. This offset position will have a small impact on the calculated distortions. To get an idea of the significance of the impact, I have plotted here the effects of shifting by an additional 1 mm (for a total offset of 2 mm from ideal). Shown are azimuthal (Rφ, top) and radial (R, bottom) distortion maps (for FullField) as a function of radius and z for the used 1 mm offset (left), and additional offset to 2 mm (middle) and their difference (right). All units on the plots are [cm].

The maximum Rφ and R distortion differences are on the order of 10^-7 cm and 10^-4 cm (1 micron) respectively. The conclusion is that this offset location effect is negligible.

-Gene

(click on the image for a larger, full resolution version)