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 Pachón, 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.