Knowledge Bank

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 $0.74\times 10-4cm-1$ to $9.3\times 10-4cm-1$ over the wavenumber interval from $400cm-1$ to $5000cm-1$. 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.