FASER: a tool for vectorial point spread function simulation with applications in stimulated emission depletion microscopy.

We introduce FASER, a software package designed to simulate the excitation point spread functions (PSFs) of microscopes. It is written in Python as a plugin for the open-source platform Napari. Using a full-vectorial computational approach to simulate the electromagnetic fields within the focal region makes precise predictions and allows detailed analyses of excitation PSFs. FASER is intended as a pedagogical tool enabling users to explore the impacts of various geometrical and optical parameters of practical importance on the performance of the microscope. It supports the modeling of complex beam profiles, including donut and bottle-shaped beams, which are commonly used in advanced microscopy techniques such as stimulated emission depletion (STED) microscopy. Through specific simulations and accessible illustrations, we showcase FASER's capabilities in capturing characteristic features of STED microscopy, making it a practical resource for researchers and students in optical microscopy to explore and optimize high-resolution imaging techniques.