Comparison of PSF similarity between reference-science and science-science pairs. The residual fractional rms of difference images are plotted as a function of relative PA/rotational offset. The black dots represent science-science subtractions, the blue dots represent science-reference subtractions, and the red dots represent science-science differences with acceptable self-subtraction. For the science-reference points, the relevant quantity is the time difference, which has an almost linear relationship to the PA difference.