United States Department of Agriculture reference ultraviolet spectroradiometer: current performance and operational experience at Table Mountain, Colorado

Abstract

At present the United States Department of Agriculture (USDA) Reference Spectroradiometric Network consists of three sites: Table Mountain, Colorado, Lamont, Oklahoma (the ARM program SGP site), and Beltsville, Maryland. At each site we deploy and continuously operate a 1-m cascaded additive-double Czerny-Turner scanning monochromator with a bialkali photomultiplier and photon-counting detection. Lambertian fore-optic errors are less than 1% over the range of zenith angles from 0 to 80°. The instruments use photon counting and make measurements at 290 nm not affected by stray light under typical conditions. The basic performance specifications of the instrument were demonstrated by a prototype at the 1997 North-American UV Spectroradiometer Intercomparison. Data shown here demonstrate that these are met in routine operation. The fundamental instrument performance specifications are: Optical resolution: 0.1 nm FWHM, triangular slit-function. Wavelength reproducibility: ′0.0025-nm 2<T with 296-nm Hg retrace-scan corrections applied, ′0.007 nm 2σ over typical diurnal variability, without correction. Wavelength accuracy: Limited by calibration systematic errors. Believed to be 0.005-nm worst case. Stray light: < 10 - 7 at 4 FWHM, 10 - 1 0 at 20 nm, slit-scattering function versus 325 nm HeCd. Angular response: less than 1% error from cosine over the range of zenith angles from 0 to 80°. Signal linearity: The instrument uses a photomultiplier with 2-ns rise-time and photon counting detection. The dual-threshold discriminator has a 700-Mhz synchronous signal counting limit. The maximum counting rates seen at the longest wavelengths are less than 10 MHz; less than 1/5 of the frequency where nonlinearity can be detected, as tested for the 1997 Intercomparison. 2000 was the first full year of operation of our instrument at the NOAA Table Mountain site (140.177 °N 105.276 °W, 1900 m asl) for which the operational and calibration frequencies justify making the data accessible to outside users for scientific application. We show performance in routine operation and issues of calibration over the period April 2000 to 31 December 2001.