1. Introduction

PSFEx (PSF Extractor) is a computer program that extracts precise models of the Point Spread Functions (PSFs) from images processed by SExtractor and measures the quality of images. The generated PSF models can be used for model-fitting photometry or morphological analyses. The main features of PSFEx are:

• Modelling of any arbitrary non-parametric or parametric, bandwidth-limited, PSF.
• Reconstruction of PSF from undersampled images using super-resolution on the pixel basis, the Gauss-Laguerre basis [4] or a user-provided vector basis.
• Modelling of PSF variations as a polynomial function of position in image, any SExtractor measurement, or any numerical FITS parameter.
• Tracking of hidden PSF dependencies using Principal Component Analysis [5].
• Computation of PSF homogenisation kernels (to convert variable instrumental PSFs to constant round Moffat [2] profiles).
• Automatic selection of point sources.
• Compatibility with SExtractor FITS or Multi-Extension FITS catalogue format in input,
• VOTable-compliant XML output of meta-data.
• XSLT filter sheet provided for convenient access to metadata from a regular web browser.