In order to coadd the images, they must be previously aligned.The jitter data reduction procedure uses cross-correlation techniquesin the image domain to detect the offsets with subpixel precision, usingthe telescope offsets provided in the template as first estimates.Best matching areas (points of interest) are detected by a fast brightobject detection algorithm. The number of pixel operations is optimizedfor the telescope precision in offseting (up to +/-5 pixels relativepointing error for SOFI).The frames are then resampled one by one using a4x4 Lanczos kernel tabulated on 1000 values per pixel. Photometry errorsdue to resampling noise should remain negligible and rather homogeneousover the wholeimage. Coaddition is then performed, adding a temporal filter to removespurious pixel values before averaging along the z-axis.
The whole process of sky estimation, subtraction, offset estimation,image registration and frame stacking takes about 30 seconds CPU timeper frame on an UltraSparc 1.