Spectral (top) and spatial (bottom) flexure measured from the location of an arcline (vertical shifts) and slitlet edges (horizontal shifts) respectively on raw detector frames, as a function of rotator angle. The 3 colours represent the 3 instrument segments (red/green/blue for segments 1/2/3). The lines trace sinusoidal fits to the data: while this is a good match for the spectral flexure, it is only an approximation to spatial flexure. Importantly, in both cases, the scale of the shift is ~±1 pixel, indicating that which pixels on the detector are illuminated depends on rotator angle.