<title>Optical filters for the wind imaging interferometer</title>

Abstract

Optical interference filters were required for the Wind Imaging Interferometer (WINDII), a Canadian instrument to fly on NASA's Upper Atmosphere Research Satellite (UARS). The WINDII instrument is a Michelson interferometer which measures wind speeds and temperatures in the upper atmosphere by analyzing airglow emissions. The original perception of the optical filters as a straightforward, low-risk element of the WINDII system, and the evolution of this view into a realization of the challenge and complexity of the filter requirements, are presented. The challenges discussed include the tight manufacturing tolerances required to achieve a wide field of view simultaneously with a narrow passband, and the problem of drifting of the filter passband over time. Changes in the scientific approach in order to relax the tolerances, tilting of the filters relative to the instruments's optic axis, and manufacturing changes in order to minimize the passband drift, are described. Other innovations in the filter design are also discussed, including a filter which incorporates a correction for chromatic aberration, and a 'split' filter which has two separate passbands.