Solar spectral irradiance measurements: visible to near-infrared regions

Abstract

A spaceborne spectral irradiance monitor (SIM) is being developed at the Laboratory for Atmospheric and Space Physics, University of Colorado, to measure solar spectral irradiance and its variation for the NASA Earth Observing System (EOS) SOLar STellar Irradiance Comparison Experiment (SOLSTICE). The SIM consists of two independent and identical prism spectrometers. Each channel is equipped with an electrical-substitution radiometer and additional photodiodes to provide spectral coverage from 0.3 µm to 2.0 µm with a resolving power greater than 30. One of the spectrometers is used daily, while the other is used on a lower duty cycle to account for instrument degradation caused by solar exposure. The prism is rotated on a flex pivot and driven with a voice-coil motor. Closed-loop drive control and wavelength calibration are achieved by using a concave mirror on the prism table to focus a second solar beam onto a linear charge-coupled device (CCD) located at the focal plane of the prism. Radiation-induced changes in prism transmission are measured in flight by using one of the spectrometers to transfer monochromatic light to the other spectrometer. Visible and infrared photodiodes on a flip-arm measure the ratio of the beam into and out of the prism, thus measuring the prism transmission as a function of wavelength.