New Estimates of the Sky Background for the HST Exposure Time Calculator

We present new spectra of the optical and near-IR sky background radiation visible to Hubble Space Telescope (HST) instruments . This background includes contribution from both the zodiacal light and the Earthshine light, as well as geocoronal lines. The spectrum of the zodical light is obtained by reddening and normalizing that of the Sun so that it fits measures of background radiation taken in a region near the north ecliptic pole at 0.45, 0.5, 0.606, 0.814, 1.25 and 2.2µm with HST/WFPC2 and COBE/DIRBE. The Earthshine spectrum is derived from the solar spectrum at optical and near-IR wavelengths and is based on direct IUE measures in the UV. These spectra provide better agreement with available observations than previous versions, and are useful for calculating the background expected during observations with HST. They have been included in the data base of the Exposure Time Calculators (ETC) of HST instruments. Since the intensity of the background varies depending on the specific situations encountered during observations, such as the angle between the line of sight and the plane of the ecliptic or with the Earth’s limb, we have implemented a suite of choices for the normalization of the spectra that should cover the range of conditions encountered in most observing programs.