This package contains tasks which can be used to combine dithered images using the "drizzle" algorithm. Some tasks are specialized for WFPC data only, others, including the`drizzle' task, can be used to process any image format supported by IRAF.
Two basic procedures are available, depending on the existence of only linear shifts between the images to be combined, or linear shifts plus a (small) rotation angle. Both procedures rely on cross-correlation to derive the relative shifts and rotation angles.
The main combining task is `drizzle', which has to be supplied with the linear shifts in X and Y and the rotation angle. It also supports rescaling of the pixel grid and corrections for geometric distortions, but only coefficients appropriate for WFPC-II data are currently available.
`avshift' uses the combined results from the 4 WFPC chips to asses the quality of the final results.
More information can be found in A. S. Fruchter, R. N. Hook, I. C. Busko, and M. Mutchler, 1997, "A Package for the Reduction of Dithered Undersampled Images", in "The 1997 HST Calibration Workshop", S. Casertano et al., ed. (Baltimore: Space Telescope Science Institute), in press. See http://www.stsci.edu/meetings/cal97.