P17	March 1998

¹ Institut d'Astrophysique et de Géophysique - Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium
² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

^* Directeur de Recherches FNRS, Belgium

Abstract

     Based upon the subtraction of TinyTim PSFs from PC1 point-like objects, a method has been developed to determine the optimal values for the telescope jitter and the Z4 relative focus during calibration or science observations. Using these jitter and focus values, an optimal TinyTim PSF, computed over a resampled grid, is then iteratively fitted to the object, yielding an improvement in the PSF centering, more accurate photometric results and a better detection of underlying structures.