Sub-pixel Interpolation of PSF image for analysis of FRD

Residuals between spectrograph PSF images at different FRD are dominated by a dipole structure that indicates a shift between centroids of the images. The absolute position of the simulated PSFs is held constant between these images in the current version of the 2D-DRP code. That is, centroid shift is real and due to the changing FRD. However, in the spectrograph calculations are made relative to the centroid of the image. Thus, for a consistent analysis of the FRD effect on the spectrograph images, the centroid must be the same between images.