========== Background ========== Summary ------- SImSiT (Satellite Image Simulation Toolkit) is a Python package for generating synthetic telescope images containing stars and satellites. It is built on top of GalSim (:cite:t:`Rowe2015`) and SSAPy (:cite:t:`SSAPy2023`; :cite:t:`Yeager2023`) and is intended to support the development and testing of algorithms for satellite imaging, detection, calibration, and related analysis tasks. SImSiT supports multiple observing configurations, including sidereal tracking (where stars remain approximately point-like and fast-moving satellites appear streaked) and target tracking (where the tracked satellite appears nearly point-like and background stars may appear streaked). By enabling realistic simulation of telescope, detector, and tracking effects, SImSiT helps users study satellite observations under a range of observing conditions. SImSiT has also been used in studies of closely separated satellites in ground-based astronomical images (:cite:t:`Pruett2023`). Statement of need ----------------- The astronomy community has developed many image simulation tools, including mission- or telescope-specific packages such as `Romanisim `_, which generates high-fidelity images for the Nancy Grace Roman Space Telescope (:cite:t:`Spergel2015`), as well as end-to-end simulation frameworks for instruments such as the Vera C. Rubin Observatory LSSTCam (:cite:t:`Peng2013`). More general packages also exist for astronomical scene generation and optical simulation, including GalSim (:cite:t:`Rowe2015`), SkyMaker (:cite:t:`Bertin2009`), and ray-tracing frameworks such as PhoSim (:cite:t:`Peterson2015`). Separately, software packages have been developed for modeling satellite observational characteristics (:cite:t:`Fankhauser2023`). SImSiT is intended to bridge these domains by combining realistic astronomical image simulation with satellite orbit-based motion and observation geometry in a single package. With the growth of large astronomical imaging surveys such as the Vera C. Rubin Observatory LSST (:cite:t:`Ivezi2019`) and the Zwicky Transient Facility (:cite:t:`Bellm2019`), along with the increasing number of visible satellites (:cite:t:`Mroz2022`), realistic simulation of satellite-containing images has become increasingly important for both astronomy and space-domain applications. Method ------ SImSiT combines GalSim-based image rendering with SSAPy-based orbital and observational geometry calculations. GalSim enables users to configure observing parameters such as detector properties, optical distortion, vignetting, and point-spread functions. These models can be used either to approximate existing instruments or to explore prospective observing configurations. SSAPy provides orbit propagation and related geometry calculations, allowing users to simulate satellites in a range of orbit classes and observing scenarios. SImSiT can generate star fields from Gaia-based catalog inputs and produce simulated image products together with associated reference metadata such as WCS information and source tables.