A method of stabilizing a Fabry-Perot interferometer with respect to a reference wavelength, while simultaneously stabilizing the parallelism of the flats, is described. The method consists of creating correcting signals proportional to the displacement of the fringe maxima of three auxiliary spectral sources with respect to a fixed reference point and feeding these signals back to the interferometer piezoelectric drive so that the correcting signals become time invariant, and the interferometer is stabilized. The stability of the finished instrument with respect to the reference wavelength and parallelism is near lambda/1000, at 546 nm, for a 6-h period; however, the actual parallelism of the etalon is limited by the surface defects of the flats. The method, as described, allows simultaneous stabilization and data taking, since the instrument is continuously updating itself.

Feedback stabilized fabry-perot interferometer.

Upper-atmospheric sodium abundances have been measured at Boca Raton, Florida (26.4°N, 80.1°W), from 1967 to 1971. These daytime spectroscopic absorption measurements were made with the Pepsios spectrometer using sunlight, and each line profile was computer-fitted to yield the abundance and analysis uncertainty. Results are consistent with existing twilight glow measurements and do not show the daytime enhancement reported for dayglow measurements. No diurnal variations are apparent. Short-term fluctuations are large and tend to obscure possible effects of hurricanes.

W. E. Lammer

Papers

Gravity-adjusted fabry-perot etalon.