SPEXone polarimeter instrument thermal design

SPEXone is a 6u narrow angle, compact spectropolarimeter used to measure the characteristics of aerosols in the earth’s atmosphere. SPEXone is developed in a partnership between SRON Netherlands Institute for Space Research and Airbus Defence and Space Netherlands with support from the Netherlands Organization for Applied Scientific Research (TNO) as a Dutch contribution to the NASA PACE observatory launching in 2022. The SPEXone polarimeter consists of an aluminum housing with a radiator and contains one detector module, several mirrors and a grating. This paper presents the thermal design approach. The primary goal of the thermal design is to provide a stable thermal environment needed for mechanical and optical stability and performance. Several design challenges were faced. First, to create a stable thermal environment, temperature fluctuations and gradients need to be kept to a minimum. The temperature level of the detector will be controlled at 293K with an allowed narrow accuracy range of 0.15K (RMS). Also, the instrument thermo mechanical design should be insensitive for thermo elastic loads in order to ensure a stable optical alignment. Special attention has been put into the control of thermo mechanical effects by means of a Structural, Thermal and Optical Performance (STOP) analysis. Important part of the verification is to assess the spot size and spot position accuracy (allowed variation 11µm +/-5%). Second, the implementation of the design-to-cost principle led to the use of a COTS detector module. Some of the common design practices for narrow range active temperature controlled components could not be established. Special attention has been put in the design and verification of the detector temperature control. Analysis has shown that the design is fully compliant to all requirements. The instrument is approaching the final integration and test phase including a dedicated thermal balance test planned for the second half of 2019.