SKYSURF-4: panchromatic Hubble Space Telescope all-sky surface-brightness measurement methods and results

The diffuse, unresolved sky provides most of the photons that the Hubble Space Telescope (HST) receives, yet remains poorly understood. The HST Archival Legacy program SKYSURF aims to measure the 0.2–1.6 μm sky surface brightness (sky-SB) from over 140,000 HST images. We describe a sky-SB measurement algorithm designed for SKYSURF that is able to recover the input sky-SB from simulated images to within 1% uncertainty. We present our sky-SB measurements estimated using this algorithm on the entire SKYSURF database. Comparing our sky-SB spectral energy distribution (SED) to measurements from the literature shows general agreements. Our SKYSURF SED also reveals a possible dependence on the Sun angle, indicating either nonisotropic scattering of solar photons off interplanetary dust or an additional component to zodiacal light. Finally, we update the diffuse light limits in the near-IR based on the methods from Carleton et al., with values of 0.009 MJy sr⁻¹ (22 nW m⁻² sr⁻¹) at 1.25 μm, 0.015 MJy sr⁻¹ (32 nW m⁻² sr⁻¹) at 1.4 μm, and 0.013 MJy sr⁻¹ (25 nW m⁻² sr⁻¹) at 1.6 μm. These estimates provide the most stringent all-sky constraints to date in this wavelength range. SKYSURF sky-SB measurements are made public on the official SKYSURF website and will be used to constrain diffuse light in future papers.