Improving PSF Processing Speeds

ASTERIX stores PSFs as 2D arrays of integrated probability. These arrays are pixel based. When applying the PSF the size of the PSF array determines the number of adjacent pixels which have to be visited. Because calculating a PSF for individual pixels, and then applying that PSF to each pixel in a dataset is computationally expensive, the PSF system includes strategies to speed up the process;

• Reduce the PSF size. The included energy fraction of the PSF, and hence pixel array size, can be reduced (as in pss).
• Precalculate the PSF. The spatial response can be pre-calculated and included in the dataset (see the Respfile Option below.)
• Calculate the PSF on a grid.The number of PSF calculations can be reduced by calculating PSFs, not at each pixel in the image, but over some coarser grid of positions. This method can be used either before applying the PSF (as in spresp) or on an `as needed' basis (see the Model Option below.)