  The ultracold neutron (UCN) source at the Paul Scherrer Institute serves
mainly experiments in fundamental physics. High UCN intensities are the key for
progress and success in such experiments. A detailed understanding of all
source parameters is required for future improvements. Here we present the UCN
source components, elements of the neutron optics, the characterization of
important related parameters like emptying times, storage times or transmission
probabilities of UCN which are ultimately defining the UCN intensity delivered
at the beamports. We also introduce a detailed simulation model of the PSI UCN
source, used to analyze the measurements and to extract surface parameters.
