Optimal proper motion measurements with the Wide Field and Planetary   Camera
  An optimal maximum-likelihood technique for computing point-source image centroids from many, slightly offset, CCD frames is presented. The method is especially useful for measuring stellar proper motions from data taken with the Wide Field and Planetary Camera aboard the HST, and also provides a means to identify very compact non-stellar sources. We work though the example problem of obtaining image centroids of objects in the Hubble Deep Field. 
Rodrigo Ibata and Geraint Lewis