1. INTRODUCTION

2. PRINCIPAL COMPONENT ANALYSIS OF POINT SPREAD FUNCTIONS
2.1. Optimal Basis Functions for Modeling a PSF Variation
2.2. Mathematical Formalism

3. PCA APPLICATION TO ACS DATA

4. DISCUSSION
4.1. Effects of Drizzling Methods on PSF Characteristics
4.2. Focus Dependency
4.3. Comparison with a shapelet approach
4.4. Comparison with TinyTim
4.5. How to Select the Right PSF Template

5. CONCLUSIONS