arXiv subjects

Sarah E. Caddy

Publications and source records attributed to Sarah E. Caddy.

4 recordsLinked to original sources

arXiv2026

The First Observations of Moonlit Satellites

Optical Space Domain Awareness (SDA) operations have traditionally relied on Sunlit passes near the terminator, limiting continuous tracking of low-Earth orbit (LEO) satellites. In this work, we show that by accounting for all four major illumination sources: Sunlight, Earthshine, Moonlight, and Lunar-Earthshine, large LEO satellites like the ISS can be optically monitored for a full 24 hour period. We present what we believe to be the first quantitative observations of the International Space Station (ISS) and Chinese Space Station (CSS) illuminated only by Moonlight and Lunar-Earthshine under night-time conditions. Using a 0.6 m telescope, we obtain 147 detections of the ISS with a median brightness of V = $12.02 \pm 0.17$ mag, approximately $13.1 \pm 1.3$ magnitudes fainter than in daylight. For interpretation, we extend a satellite brightness model based on reflected Sun light (lumos-sat) to include Moonlight and Lunar-Earthshine, and validate this model both against the new observations and independent Ansys Systems Tool Kit simulations, achieving residuals of $0.03 \pm 0.80$ mag. The simulations confirm that Lunar-Earthshine dominates the illumination of nadir- facing components, boosting their radiance by at least 100 times relative to Starlight alone. Applying this validated model to existing and proposed large satellites, we show that Moonlight and Lunar-Earthshine will make these future spacecraft well within detection limits of even modest ($\sim 0.6$ m) optical SDA systems for most of the night, and for up to $\sim 11$ nights per lunar month at local midnight. When Moonlit observations are combined with daytime and twilight observations, this enables near-continuous ($\sim 24$ h) optical monitoring of LEO satellites from a single site, significantly increasing SDA tracking capabilities.

astro-ph.EP↗

arXiv2025

Daytime Photometry of Starlink Satellites with the Huntsman Telescope Pathfinder

The rapid increase in satellite launches in recent years, and the pressure of launches planned into the next decade, demands an improvement in the efficiency of space domain awareness facilities. Optical facilities form an important component of global space domain awareness capabilities, however traditional optical telescopes are restricted to observing satellites during a small twilight window. In this work we explore expanding this operational period to encompass the entire day to dramatically improve the observing opportunities at a single site. We explore daytime space domain awareness observations with the Huntsman Telescope Pathfinder, an instrument built using predominantly off the self components, and Canon telephoto lenses. We report successful detections and photometric light curves of 81 Starlink satellites from Sun altitudes ranging 20 degrees to midday. Starlink satellites are found to be particularly bright at $3.6 \pm 0.05$ mag, $σ= 0.6 \pm 0.05$ mag in Sloan r', or $\sim 11\times$ brighter than twilight conditions. We conclude this surprising observed brightness is due to the contribution of Earthshine beneath the orbiting satellites. We also compare our observations to existing satellite optical brightness models and find that satellite optical brightness during the day can only be well described by a model including an Earthshine component. We find that observed light curves are more complex than simple geometric models predict, but generally agree within an order of magnitude. Finally we suggest improvements to satellite optical brightness models by incorporating weather data to measure the actual Earthshine under a satellite.

astro-ph.IM↗

arXiv2022

Towards a data-driven model of the sky from low Earth orbit as observed by the Hubble Space Telescope

The sky observed by space telescopes in Low Earth Orbit (LEO) can be dominated by stray light from multiple sources including the 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 correlates with the observed HST sky surface brightness indicating the value of incorporating such data to characterise 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.

astro-ph.IM↗

arXiv2019

The Huntsman Telescope

The Huntsman Telescope, located at Siding Spring Observatory in Australia, is a system of ten telephoto Canon lenses designed for low surface brightness imaging in the Southern sky. Based upon the Dragonfly Telephoto Array, the refractive lens-based system provides an obstruction free optical path, which reduces the number of scattering surfaces and allows easier access to lower surface brightness levels. In this proceeding, we present an analysis of the impact of flat fielding uncertainty on the limiting low surface brightness levels. We show that a fairly standard set of flat-field data can be well-characterised to a $\sim0.1\%$ level. This corresponds to a 5-$σ$ lower limit of $\sim33$ magnitude per arcsecond$^2$, which means that flat fielding is not likely going to set Huntsman's low surface brightness limit. We also present early results of an exoplanet transient mode for Huntsman where all lenses work together to detect subtle variations in the luminosity of relatively bright $V=8-12$ magnitude stars. High-precision exoplanet imaging is ultimately limited by systematic uncertainties, so we anticipate multiple lenses will help to mitigate issues related to pixel-to-pixel and intra-pixel sensitivity variations. Our initial results show we can easily get $\sim0.4\%$ photometric precision with a single, defocused lens.

astro-ph.IM↗