A COOLED OPTICS HIGH RESOLUTION SPECTROMETER FOR THE INFRARED
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Date
1976
Journal Title
Journal ISSN
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Publisher
Ohio State University
Abstract
A high resolution 3 m focal length grating spectrometer utilizing liquid nitrogen cooled order-sorting optics and a novel grating table drive is being assembled for use in the 2 $\mu_{m}$ to $27 \mu_{m}$ spectral region. The spectrometer uses 25 cm echelle gratings in a Littrow type design with an off-axis paraboloid as the collimating element. In a four-pass mode, the theoretical resolution of the spectrometer is $0.006~cm^{-1}$. The grating table is driven by a servo controlled D.C. torque motor with angular position signals obtained from a set of inductosyn plates. The angular position of the grating is measured with an accuracy of 0.5 are seconds which corresponds to a frequency calibration precision of $0.74 \times 10^{-4} cm^{-1}$ to $9.3 \times 10^{-4} cm^{-1}$ over the wavenumber interval from $400 cm^{-1}$ to $5000 cm^{-1}$. The overlapping orders from the high resolution spectrometer are separated by a 0.8 m Ebert spectrometer servo controlled to continuously track the proper order from the high resolution system. This second spectrometer is cooled to $77^\circ K$ to act as a narrow band cold filter greatly reducing the thermal background radiation falling on the detector. This is expected to give up to two orders of magnitude improvement in signal-to-noise compared to uncooled spectrometers.
Description
Author Institution: Earth Observation Systems Division, NASA/Goddard Space Flight Center; Space Science Division, Jet Propulsion Laboratory