Toward a Data-driven Model of the Sky from Low Earth Orbit as Observed by the Hubble Space Telescope

Abstract

The sky observed by space telescopes in Low Earth Orbit (LEO) can be dominated by stray light from multiple sources including Earth, Sun, and Moon. This stray light presents a significant challenge to missions that aim to make a secure measurement of the extragalactic background light (EBL). In this work, we quantify the impact of stray light on sky observations made by the Hubble Space Telescope (HST) Advanced Camera for Surveys. By selecting on orbital parameters, we successfully isolate images with sky that contain minimal and high levels of earthshine. In addition, we find weather observations from CERES satellites correlate with the observed HST sky surface brightness indicating the value of incorporating such data to characterize the sky. Finally, we present a machine-learning model of the sky trained on the data used in this work to predict the total observed sky surface brightness. We demonstrate that our initial model is able to predict the total sky brightness under a range of conditions to within 3.9% of the true measured sky. Moreover, we find that the model matches the stray-light-free observations better than current physical zodiacal light models.