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Experiments on Grating Performance at 12.3 Microns (1Mar94) (from NSO, NOAO Newsletter No. 37, 1 March 1994) Using the stellar dewar `DERF', which contains a bib single-channel detector cooled with liquid helium, and the IR grating of the 13.6-m spectrometer, we have managed recently to measure the Mg emission feature at 12,319.5 nm. The line was observed to have a strength of about 20% near the limb in agreement with FTS results. The resolving power of the grating at these wavelengths is about 0.2 nm, or comparable to the width of the Mg feature. However the performance in terms of S/N is far from optimum. Several difficulties are encountered. The filter passband is too great (about 3 fm), resulting in excess thermal noise from the spectrograph, most likely the grating. Also terrible channeling is found with all filters tried so far. The maximum acceptance angle at the detector amounts to only about 2 arcsec. Since the telescope diffraction limit at this wavelength is about the same, this might be considered a good match, but we have no way to improve throughput by lowering spatial resolution as would be normal practice. Alignment has been difficult. The sun is no longer the dominant signal. We see spectra as the grating is rotated even when the slit is not open! Emission from parts of the tank plus air molecular absorption within the tank exceed the solar component. In other words, the setup remains very experimental as we continue to learn about the problems of operation at these long wavelengths. Bill Livingston, Dick Joyce, Ken Hinkle, Doug Rabin
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