SKYSURF: Constraints on Zodiacal Light and Extragalactic Background Light through Panchromatic HST All-sky Surface-brightness Measurements. I. Survey Overview and Methods

We give an overview and describe the rationale, methods, and testing of the Hubble Space Telescope (HST) Archival Legacy project SKYSURF. SKYSURF uses HST's unique capability as an absolute photometer to measure the similar to 0.2-1.7 mu m sky-surface brightness (sky-SB) from 249,861 WFPC2, ACS, and WFC3 exposures in similar to 1400 independent HST fields. SKYSURF's panchromatic data set is designed to constrain the discrete and diffuse UV to near-IR sky components: Zodiacal Light (ZL), Kuiper Belt Objects (KBOs), Diffuse Galactic Light (DGL), and the discrete plus diffuse Extragalactic Background Light (EBL). We outline SKYSURF's methods to: (1) measure sky-SB levels between detected objects; (2) measure the discrete EBL, most of which comes from AB similar or equal to 17-22 mag galaxies; and (3) estimate how much truly diffuse light may exist. Simulations of HST WFC3/IR images with known sky values and gradients, realistic cosmic ray (CR) distributions, and star plus galaxy counts were processed with nine different algorithms to measure the Lowest Estimated Sky-SB (LES) in each image between the discrete objects. The best algorithms recover the LES values within 0.2% when there are no image gradients, and within 0.2%-0.4% when there are 5%-10% gradients. We provide a proof of concept of our methods from the WFC3/IR F125W images, where any residual diffuse light that HST sees in excess of zodiacal model predictions does not depend on the total object flux that each image contains. This enables us to present our first SKYSURF results on diffuse light in Carleton et al.

Automated summary

This article provides an overview of the Hubble Space Telescope (HST) Archival Legacy project SKYSURF, describing its rationale, methods, and testing. SKYSURF utilizes HST's unique photometer capability to measure sky-surface brightness in various wavelengths, aiming to constrain different components of the sky. The article outlines the methods used in SKYSURF to measure sky brightness levels, estimate the discrete extragalactic background light, and assess the existence of truly diffuse light. Simulations and algorithms are employed to accurately measure the sky brightness values.