Derivation of CLAES Filter Shapes

Abstract

The Cryogenic Limb Array Etalon Spectrometer, CLAES, was launched on September 12, 1991 aboard the NASA Upper Atmosphere Research Satellite (UARS) and has been acquiring data on stratospheric composition since October 1, 1991. Overviews of the CLAES experiment and hardware are given by Roche et al (1) and Burriesci et al. (2). In the OSA Topical Meeting on Optical Remote Sensing of the Atmosphere in 1990 we presented three papers, (3,4,5) that described our progress up to that time in defining the system spectral transmission. In this paper, we describe refinements to CLAES characterization since then. These have come from further work with the pre-launch cold test data and from on-flight data. The spectral transmission characterization described here is currently being used in CLAES retrieval software. Figure (1) shows a schematic diagram of the spectrometer. In normal operation the CLAES instrument achieves a 0.2-0.65 cm-1 spectral resolution by passing the radiation through one of nine passband blocking filters (FWHM ~10 cm-1) mounted in a filter wheel, and one of four Fabry-Perot etalons which are mounted in a paddle wheel to provide for spectral scanning by tilting. The transmitted radiation then falls on a focal plane array consisting of a main array of 20 elements and an HCI array of 3 elements. The main detector array is used by eight spectral channels from 5.3 to 12.8 μm and takes atmospheric data in 20-2.5 km vertical increments. The 3-element HCL array is used only by the 3.5 μm channel where each detector spans approximately 16 km for S/N augmentation.