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.

o

NO ROTATION: this is the simplest case. One image is taken to be the "reference" and the second image will be shifted to match it. The tasks to use are `offsets' and `shiftfind'. `offsets' builds 4 cross-correlation "images", one for each WFPC group, and `shiftfind' measures the relative shift in each group by locating the correlation peak. For non-WFPC data, use task `crossdriz' instead of `offsets', with all rotation angles set to zero.

o

WITH ROTATION: the procedure for measuring the rotation between two images is to rotate the image against the reference by a set of angles and to compute the cross-correlation between each rotated image and the reference. A plot of the cross-correlation peak height as a function of rotation angle shows a peak at the "correct" (best fit) rotation angle. The task that executes these steps is `rotfind'. It relies on the `drizzle' task to perform the actual rotations, and on the `nfit1d' task to fit the cross-correlation maximum.

`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.