Imsim is a software package used to drive simulations of the LSST telescope and survey. Imsim is used both for small scale studies in the DESC Science Collaboration and LSST, and also for large scale data challenges being undertaken in the DESC. It produces simulated images from the 3.25 Giga-Pixel LSST camera which are suitable to be processed through the LSST Data Management pipeline. Imsim simulates the pointing of the telescope with information given by the OpSim program and takes as an input a catalog of astronomical sources from the CatSim package. How the light is distorted on the way to Earth including lensing and absorption by dust is also considered.
The images that are produced include the systematic effects of the atmosphere, optics and sensor physics on the observed point-spread-function. Imsim calls the GalSim software library for astronomical object rendering and is designed to be used inside of the LSST Data Management and LSST Simulation Group software environment. The LSST software libraries provide the LSST telescope and hardware specific information necessary to simulate the exposure such as pixel coordinates on the focal plane, telescope filter characteristics, the brightness of the sky etc. Using the LSST camera description, Imsim then prepares output files that simulate the image after electronic readout.
In many cases, Imsim supplies multiple models for the same effects. This means that varying levels of fidelity and speed can be selected for each feature depending on the particular need. Some of the available models for point spread functions and detector effects are simple approximations, while others include full physics treatments such as a fully physics based silicon model and ray-traced stochastic atmospheres. The resulting speed and throughput is dependent on the fidelity of the models selected. Special attention is payed to efficient generation when simulating extremely large numbers of objects. The driving code has the ability to dynamically adjust the computational approach for both very bright or extremely dim objects.
Imsim is an open source code currently under active development. It is developed by students and scientists at labs and universities across LSST and the Dark Energy Science Collaboration.
The Imsim codebase is located on GitHub at: https://github.com/LSSTDESC/Imsim
Communication with the Imsim development team should take place through the Imsim GitHub repository which is part of the DESC GitHub organization. Other questions can be directed to Chris Walter at Duke University.
