This is the second in a series of three papers in which we present a measurement of the mean flux of the optical extragalactic background light and the cosmological implications of that result (see Bernstein, Freedman, & Madore 2002a & 2002c). The extragalactic background light (EBL) is the spatially averaged surface brightness of all extragalactic sources, resolved and unresolved. As such, the absolute flux of the EBL is a powerful and fundamental cosmological constant which can significantly constrain galaxy formation and evolution scenarios. Like all diffuse backgrounds, however, the optical EBL is very difficult to isolate from foreground sources, which are two orders of magnitude brighter. At high Galactic and ecliptic latitudes (> 30°), the sky flux observed from the ground is dominated by terrestrial airglow and zodiacal light (ZL), each with a surface brightness of ~ 23 AB mag arcsec-2. The Hubble Space Telescope (HST), which orbits at an altitude of 600 km, avoids atmospheric emission, but the total sky flux is still dominated by ZL. An accurate measurement of the ZL is therefore crucial to a successful detection of the diffuse EBL.
Our measurement of the EBL involves simultaneous HST and ground-based observations. From HST we measure the total flux of the night sky, including ZL. Using spectrophotometry over the range 3860-5150Å (1.25Å per pixel) taken with the Boller & Chivens Spectrograph on the duPont 2.5m Telescope at Las Campanas Observatory in Chile, we measure the absolute flux of the ZL contributing to the HST observations, which we can then subtract. In Bernstein, Freedman, & Madore (2002a, henceforth Paper I), we present the full details of the coordinated program to measure the EBL. In this paper, we present the ground-based measurement of the absolute flux of the ZL. As calibration of these data is crucial to the scientific goals, the data acquisition, reduction, and flux calibration are discussed here in detail.
Background regarding the nature of the ZL is given in Section 2. The observations, data reduction, and flux calibration are discussed in Section3. In Section 4, we briefly described the complications which arise due to atmospheric scattering, which redirects off-axis flux into and on-axis flux out of the line of sight. Detailed calculations of the atmospheric scattering relevant to precisely our observing situation (defined by the observatory location and positions of the Sun, Galaxy, and target relative to eachother and the horizon) are relegated to the Appendix, and summarized in Section 4. In Section 5, we describe the technique used to measure the zodiacal light flux in reduced spectra. The results are summarized in Section 6.