We have measured the mean surface brightness of the zodiacal light along a single line of sight towards an extragalactic target using ground-based spectrophotometry with a 300 arcsec2 field of view. The observations were made on on 27 and 29 November 1995, simultaneous with HST observations of the same field. The goal of this coordinated program is a measurement of the optical extragalactic background light. Because the zodiacal light at optical wavelengths results from sunlight scattered by interplanetary dust concentrated in the ecliptic plane, the flux towards an extragalactic field varies seasonally. Variations in the interplanetary dust cloud with time and the solar cycle may also affect the flux of zodiacal light. The measurement of zodiacal light present here is therefore uniquely relevant to the date and target field.
Our results incorporate explicit calculation of net effect of atmospheric scattering on terrestrial measurements of the zodiacal light, and show that these effects are small (< 10%) for zodiacal light measurements far from the Sun. We find the mean flux to be 109.4 × 10-9 ergs s-1 cm-2 sr-1 Å-1 at 4650Å (see Figure 13), and the color to be 5( ± 1)% redder than the solar spectrum per 1000Å. The statistical uncertainty in the mean flux is 0.6% (1), and the systematic uncertainty is 1.1% (1). We discuss additional systematic effects which might influence this measurement beyond those which are quantified here. Our results are in good agreement with previous measurements of the ZL at similar orientations with respect to the ecliptic plane and scattering geometry (see Leinert et al. 1998 for a recent review). This is the only optical measurement to date which isolates the ZL from other uniform backgrounds, including diffuse Galactic light and extragalactic background light. The color of the ZL as a function of the line of sight through the interplanetary dust cloud is further addressed in Paper I.
We thank an anonymous referee for comments which have significantly improved the analysis. We would also like to thank Carnegie Observatories and specifically L. Searle, A. Oemler, and I. Thompson for generous allocations of observing time at Las Campanas Observatory during the course of this work. The observations would not have been successful without the generous efforts of O. Duhalde, E. Cerda, I. Thompson and especially night assistant H. Olivares. We would also like to thank R. Kurucz for kindly providing an electronic version of the solar spectrum and for helpful discussions. We have benefited greatly from discussions with J. Dalcanton, S. Shectman, and A. Williams. J. Dalcanton and I. Thompson provided the data for measurements of the solid angle of the instrument. RAB would like to thank R. Blandford, A. Readhead, and W. Sargent for financial support in the early stages of this work. This work was supported by NASA through grants NAG LTSA 5-3254 and GO-05968.01-94A to WLF and through Hubble Fellowship grant HF-01088.01-97A awarded by STScI to RAB.