Sub-pixel shifting of WFPC image is a promising strategy to recover resolution lost by undersampling. The costs are negligible since multiple exposures of the same field are used in most cases anyway to allow removal of CR hits.
The software discussed in this paper is available from the author.
Acknowledgments
I thank Bob Fosbury for suggesting this project, David Koo for providing the WFPC images to test the algorithm, Richard Hook for his co-adding code and advice, and Leon Lucy for valuable discussions.