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Spectral Resolution of One Million Achieved in the Visible (1Jun95) (from NSO, NOAO Newsletter No. 42, June 1995) The 632 groove/mm visible light grating is yielding practical resolving powers in excess of 10^6 at the 13.5m McMath-Pierce spectrometer. Operated double-pass, certain optical aberrations cancel, giving a theoretical resolving power of 2.6 X 10^6 at the sodium D-lines in fifth order. While we have not been able to determine the exact resolving power empirically, a comparison with FTS spectra, using I_2 absorption tube line widths, indicates that it is better than 10^6. At the suggestion of Roger Angel (Steward) this high resolution has allowed us to measure the column amount of mesopheric Na D_2. Seen against the core of solar disk Na D_2, the hyperfine structure of this telluric component is nicely revealed (see figure). Thought to be meteoric in origin, it has long been known that this thin, 90 km high layer is variable. Jian Ge (Steward) plans continuing observations to learn how this sodium layer fluctuates at our latitudes. The result is neccessary to specify laser requirements for Adaptive Optics Guide Star systems. [Figure not included] Sample scan at 5 airmasses showing the D_2 profile with the tiny atmospheric absorption part in the line core. By taking the ratio of low sun to high sun profiles, and expanding the wavelength scale to 0.1 nm and the intensity by a factor of 30, the hyperfine structure is displayed (dashed lines). William Livingston, Lloyd Wallace
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