Solar–Stellar Irradiance Comparison Experiment II (Solstice II): Examination of the Solar–Stellar Comparison Technique

The Solar–Stellar Irradiance Comparison Experiment (SOLSTICE) measures the solar spectral irradiance from 115 to 320 nm with a resolution of 0.1 nm. The Sun and stars are both observed with the same optics and detector, changing only the apertures and integration times. Pre-launch calibration at SURF allows us to measure both with an absolute accuracy of 5%. The in-flight sensitivity degradation is measured relative to a set of stable, early-type stars. The ensemble of stars form a calibration reference standard that is stable to better than 1% over timescales of centuries. The stellar irradiances are repeatedly observed on a grid of wavelengths and our goal is to measure changes in the absolute sensitivity of the instrument at the 0.5% per year level. This paper describes the details of the observing technique and discusses the level of success in achieving design goals.