Abstract

High resolution atmospheric transmission spectra collected with a Fourier transform instrument sometimes exhibit small systematic shifts in the apparent positions of spectral features. Although small, these shifts result in line shape distortions such as "S-curving" when, in the data reduction, one spectrum must be divided by another. The origin of the shifting is in small residual angular deviations between the laser reference and measurement beams and the mechanical axis of the spectrometer. Thermal distortion of the instrument resulting from temperature gradients that change from spectrum to spectrum during a data acquisition operation are the source of the angular deviation. The authors typically measure spectra under field conditions by using trailer-mounted instruments. The level of temperature control necessary to eliminate shifting is not possible under these conditions. Strategies must be found to reduce or eliminate the effects during processing. The processing method that is explored here is to measure the relative shift between the individual spectra and then to interpolate one of them to the precise spatial frequency values present in the other prior to point-by-point division. Relatively simple interpolation schemes yield significant reduction of the distortion when applied to simulated data, and smaller but still good reduction when applied to measured spectra.

© 1992 Optical Society of America