There is a current trend in space-based remote sensing toward long duration missions that produce hyper-spectral imaging data. One instrument that is uniquely suited to hyperspectral imaging is the infrared Michelson spectrometer. Michelson spectrometers use a translating mirror stage to vary the optical path length of one leg of the interferometer. Infrared applications often require cooling of this stage to achieve optimum performance. This cryogenic mirror stage is a critical spectrometer component that must be designed and constructed to achieve high reliability and performance during long duration missions. This paper concentrates on three specific areas of optimization. First, an accelerated lifetime test was performed on the mirror stage, with particular attention to the flexural pivots in the joints of the structure. There was no change seen over 22 million translation cycles. Second, a vibration model was created to predict the stage's response to launch and operational accelerations. The model's results closely matched measured values obtained during shake tests of the mirror stage. Third, a cryogenic mirror design was improved to decrease its weight and increase its stability over a wide temperature range. The improved design offers excellent performance for cryogenic operation.
Electroless nickel is often plated on the surface of aluminum mirrors to improve the ability to polish the mirror surface. Electroless nickel plating can cause a bimetallic effect, creating distortion of a mirror surface if it is heated or cooled. Published data listing the thermal coefficient of expansion and Young's modulus as a function of temperature for electroless nickel is not readily available. This study examined using bimetallic bars to measure the Young's modulus and thermal coefficient of expansion of electroless nickel as it is cooled from room temperature to 100 K. A test chamber was developed which can accurately measure the rotations of a bimetallic bar as it is cooled. Elementary beam theory equations for a bimetallic beam were developed. These equations indicate that by testing beams with a variety of beam thicknesses, one should be able to determine modulus and thermal coefficient of expansion data for electroless nickel. The results show that the method fails to find accurate values. Very small measurement errors cause large changes in the modulus values. By using typical values for Young's modulus and the measured beam rotations, values for thermal coefficient of expansion for electroless nickel were obtained.
Space based optical instruments are evolving toward large apertures and requiring high sensitivity at longer wavelengths. Instruments that collect light at wavelengths longer than about 15 microns often use Potassium Bromide (KBr) as part of the optical system. Since KBr has rather poor mechanical properties, many engineers have been hesitant to design instruments with KBr optics larger than a few centimeters. This problem is made more difficult by the fact that sensors in these longer wavelengths are often operated at cryogenic temperatures to minimize self- emission. The overall objective of this effort was to examine methods of mounting KBr optics to improve their vibrational, optical, and thermal characteristics. A legacy KBr mount is examined and revised to increase its robustness and scalability. Using finite element models and dynamic testing, the limits of the current design was explored. An alternative design using a bonded support was investigated. A new thermally engineered composite material (TECMat) was developed that appears to match the thermal expansion of KBr over a wide temperature range. TECMat's general properties and possible methods of implementing it in optical mount are described.
This paper describes the design of a 10-channel infrared (1.27 to 16.9 micrometers ) radiometer instrument known as SABER (sounding of the atmosphere using broadband emission radiometry) that will measure earth-limb emissions from the TIMED (thermosphere- ionosphere-mesosphere energetics and dynamics) satellite. The instrument telescope, designed to reject stray light from the earth and the atmosphere, is an on-axis Cassegrain design with a clam shell reimager and a one-axis scan mirror. The telescope is cooled below 210 K by a dedicated radiator. The focal plane assembly (consisting of a filter array, a detector array, a Lyot stop, and a window) is cooled to 75 K by a miniature cryogenic refrigerator. The conductive heat load on the refrigerator is minimized by a Kevlar support system that thermally isolates the focal plane assembly from the telescope. Kevlar is also used to thermally isolate the telescope from the spacecraft. Instrument responsivity drifts due to changes in telescope and focal plane temperatures as well as other causes are neutralized by an in-flight calibration system. The detector array consists of discrete HgCdTe, InSb, and InGaAs detectors. Two InGaAs detectors are a new long wavelength type, made by EG&G, that have a long wavelength cutoff of 2.33 micrometers at 77 K.