The Planetary Imaging Concept Testbed Using a Recoverable Experiment Coronagraph (PICTURE C) is a high-altitude balloon-borne observatory that uses a vector vortex coronagraph to image debris disks and exo-zodiacal dust around nearby stars. The engineering flight of PICTURE-C launched in September of 2019, successfully demonstrated several key technologies needed for the upcoming science flight, currently scheduled for September of 2022. The flight environment presents several challenges in thermal and mechanical effects. Low ambient pressure and temperature, approximately 4 Torr and 240 K respectively, combined with significant and varying solar irradiance, lead to time-dependent and anisotropic thermal deformation of the optics and their supporting structure. A constantly swaying observatory in a 1g environment, and the mechanical strains of a pointing system keeping the instrument on target lead to both a sag and flexing of the support structure. In order to characterize how these effects limit the ultimate performance of the mission, we present a finite-element model of the flight instrument, implemented in COMSOL, which takes into account the interactions with the environment. We present the comparison of this thermal model with the temperature data available from flight 1.
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