This paper describes an approach to mounting Potassium Bromide (KBr) optical elements that are expected to survive launch vibrations and a cryogenic environment. These KBr optics constitute the beamsplitter and compensator for a high-resolution, infrared Fourier transform spectrometer (FTS). This spectrometer is part of the Tropospheric Emissions Spectrometer (TES) instrument which will operate in the 3.2 to15.4 μm spectral range. TES is part of NASA's Earth Observing System (EOS) initiative to better understand our Earth's environment. TES is designed to obtain data on tropospheric ozone and other gas molecules that lead to ozone formation. These data will be used to create a three-dimensional model describing the global distribution of these gases to better understand global warming and ozone depletion. TES uses a Connes interferometer where the clear aperture (CA) responsible for splitting the science beam is distinct and separated by 108 mm from the CA which recombines the split beams. KBr has a low elastic limit and a high coefficient of thermal expansion, is highly soluble in water and is susceptible to degradation from humidity. These characteristics make it a rather difficult optical material to mount and protect from environments typically resisted by glass optics. The design described here uses a diameter to thickness aspect ratio of 6:1 (based on a 190 mm diameter) resulting in a rather massive element. Due to instrument mass and volume constraints in the interferometer, a pseudo-rectangular shape for the optical elements was devised and a graphite/cyanate ester support structure was designed to minimize the mass of the entire beamsplitter assembly. Vibration isolation of the optical elements was provided by RTV silicone pads, which were also designed to meet thermal stress concerns for the 180K operating environment. Both structural and thermal analyses were performed to verify the initial design. Further vibration and thermal testing of development units is expected to uncover any unforeseen problems and to verify compliance in areas of concern. This paper addresses RTV silicone material properties required to properly support the KBr optics and predicted KBr stresses and RTV preloads and deflections derived from an analytical model of the design configuration. Results from thermal and vibration testing of development units will also be presented (if available) and compared to preliminary thermal and structural models.
展开▼
