Surface Interactions in Photon Monte Carlo Simulations

We implement a comprehensive simulation of photon surface interactions using a Monte Carlo approach. This is effective in simulating the interaction of light with telescope mirrors and lenses. We use a full electromagnetic solution to simulate the wavelength and angular dependence at surfaces. This includes bare interfaces, monolayer interfaces, protected layer coatings, and multilayer coatings. We handle special cases when multilayer data are incomplete or when there is photoconversion in the interface as with sensors. We implement interactions with surface microroughness and predict the corresponding angular distribution using a Monte Carlo implementation of the Harvey–Shack scatter theory for a microroughness power spectrum. Finally, we simulate surface interaction with contamination from dust or condensation using Mie scattering applied efficiently to individual contaminants. The combination of these implementations can efficiently simulate rough to polished surfaces of arbitrary materials that are fully cleaned or dusty. The observational consequences include complex wavelength- and spatial-dependent photometric errors, the dominant effect of the wings of point-spread functions (PSFs), dust rings, and wavelength- and angle-dependent throughput losses. We find agreement with the PSF wings of WIYN One Degree Imager observations of bright stars and the properties of dust rings, and demonstrate the ability to disentangle mirror microroughness from the turbulence PSF patterns. The comprehensive numerical implementation of surface interactions has wide applicability in nonastronomical applications as well.