Measured shifts in the locations of 60 ThAr spots from a flood-lamp exposure and from one of the science frames taken during the same night, illustrating a severe case of flexure. The shifts are indicated by arrows, calculated after the mean shift (ΔX = 0.21 and ΔY = 0.46 pixels) has been subtracted; the arrows have been magnified 1000 times for illustration purposes. The change in scale is clearly visible.