M87: using a set of bright globular clusters visible in all of the frames, we matched the AGN and isophotal center position with the coordinate system defined by the ACS frame. The result shows that while the AGN (diamonds) remains fairly stable, the isophotal center (crosses) has the larger scatter accounting for the discrepancies in the recovered offsets. The smaller error is associated with results from ACS, the larger with results from NICMOS. Results from only three images out of six are shown to make the image clear.