A signiﬁcant fraction of all dark energy measurement systematics with LSST will come from the complex physical eﬀects of the atmosphere, telescope, and camera that distort the light from astrophysical objects. This distortion is imprinted in the images, and therefore the most direct and robust way of mitigating and understanding these systematics is through high-ﬁdelity image simulations. We may use codes for simple image simulations through a parameterized point-spread-function (e.g., GalSim), but for detailed simulations in the analysis tasks, we will require signiﬁcantly more detailed simulations.

To transform the catalogs of astrophysical objects into images, the DESC will use a high ﬁdelity simulator developed in cooperation with the LSST Project called the photon simulator (phoSim). The photon simulator does this using a photon Monte Carlo approach. A Monte Carlo approach produces simulated images eﬃciently and encodes an arbitrary complexity of atmosphere and instrument physics in terms of photon manipulations. The basic approach of the simulator is described on this page.

**Convenor: John Peterson (Purdue) [peters11 at purdue dot edu]**

- Phosim Code Relase (John Peterson, Purdue)
- Simulations Recipe Book (Debbie Bard, Jim Chiang, Phil Marshall, SLAC)