Stray light analysis of SAMOS: a DMD-based multiple object spectrograph and imager

SAMOS, the SOAR Adaptive-Module Optical Spectrograph, is a multi-object spectrograph and imager built to utilize the adaptive optics system of the SOAR telescope in Cerro Pachon, Chile. This medium-resolution spectrograph has been designed around a digital micromirror device (DMD), which behaves as a slit mask. The DMD enables adaptable slit sizes and dynamic slit positions without the fabrication of custom components.(1) Due to the diffractive and scatter effects of the micromirror array, the inclusion of a DMD requires special considerations when quantifying stray light. We utilize results from an electromagnetic finite-difference time-domain (FDTD) simulation, along with an opto-mechanical model of SAMOS, to conduct a stray light analysis. Ansys Lumerical is used for the FDTD simulation and Photon Engineering's FRED software is used to merge the models and run a bulk of the analysis.(2) Our results model specular ghost images and scattered light on the SAMOS focal plane arrays. This information is used to recognize problematic opto-mechanical surfaces and make system-level performance predictions. Our analysis identifies, characterizes, and allows for the mitigation of stray light using a streamlined set of macros written for FRED. This process is applicable to other astronomical instruments and can be used to improve the opto-mechanical design of a wide variety of systems.