Abstract

For over a decade, users of the FTS at the McMath Solar Telescope of the National Solar Observatory have had access to a simple but precise working wavenumber standard for absorption spectra in the form of a short sealed cell containing 10 or 20 Torr of N₂O. Because of the high pressure, the observed wavenumbers are not those of the unperturbed molecule, but a direct calibration of the 00⁰2 band at 4417 cm⁻¹in this cell against the accurately known P(7) transition in CH₄ yielded wavenumbers with a precision of the order of 0.000002 cm⁻¹ and an accuracy thought to be at least 0.00001 cm⁻¹ (Brault [1]). Some years later, this band was observed by Pollock, et al. [2] using laser heterodyne techniques, and when our cell values are corrected using the pressure shift which they observed, the absolute agreement is quite satisfactory: (NSO - Pollock, calculated) = 0.000035 ± .0000035 cm⁻¹; note that the absolute uncertainty in Pollock, et al. is quoted as .000080 cm⁻¹.

© 1992 Optical Society of America