Summary
Large-etalon Fabry-Perot Spectrometers (FPS) measure upper atmosphere winds and temperatures by observing spectral profiles of atmospheric emission lines with sufficient resolution to determine the Doppler broadening, from which temperature is deduced, and any overall Doppler shift of the line caused by a prevailing wind. Inevitably changes in the instrument’s environment cause instrument drift that affects the accuracy of wind measurements. A standard approach uses a frequency-stabilised laser to monitor instrument drift; an alternative is described here. The La Trobe FPS located at Davis, Antarctica, has a fixed-space etalon in a hermetically sealed, temperature-controlled chamber. Temperature, pressure and relative humidity in the chamber, in the laboratory and outside the building are recorded. It is found, surprisingly, that the drift can be calculated accurately by considering just the variation of refractive index of the gas in the etalon chamber deduced from pressure and temperature measurements obtained by sensors located in the chamber. The FPS can be more stable than the laser because of occasional mode jumps and small drifts that occur even in a frequency-stabilised laser. Consequently these problems can be avoided by using the refractive index technique to correct for instrument drift. This also eliminates the need to observe the laser line for calibration purposes, resulting in increased time resolution of sky observations. Furthermore, pressure and temperature sensors have longer operating lifetimes than lasers so the overall reliability of the FPS at a remote site is improved, which is particularly important for Davis which can only be visited once a year.