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We achieve this by constructing a fully parameterized b-spline model of the sky using the SKY frame. We then perform a 2D FFT cross correlation of the science frame and sky model to determine flexure offsets at the 0.01 pixel level. These subpixel offsets are applied to the sky frame to register it to the object. We have found that this produces excellent registration of the model. However, even then we see noticeable residuals in the cores of OH lines, because even a 1% variation can be stronger than object signals. To mitigate this, we use a list of known OH lines, and calculate the variation in OH intensity between the SKY and SCIENCE frames, within 1 resolution element (50 km/s) of a catalog OH line. This is done for each of ~300 lines and the corrections are added to the sky model. We have debated whether this is best practice but ultimately settled on adjusting the OH lines to mitigate residuals. Users who wish to turn of this feature should contact R. Simcoe for information on how to do this.