Sarah Caddy, Australian Astronomical Optics – Macquarie University; Lee Spitler, Australian Astronomical Optics – Macquarie University
Keywords: SDA, Daytime, Optical
Abstract:
Optical monitoring, classification and tracking of resident space objects has rapidly gained interest with the launch of thousands of satellites planned over the next decade. In order to improve our ability to monitor these space assets and ensure a safe and sustainable future in Space for generations to come, Space Domain Awareness (SDA) facilities must maximise productivity and coverage.
SDA observations at optical wavelengths using modest aperture telescopes and commercial-off-the-shelf components are a particularly attractive solution due to the low cost associated with building and maintaining these facilitates. However, observations at optical wavelengths have largely been limited to night time, terminator illuminated conditions – a period making up only ~15% of the 24 hour day. This has been in part due to the bright daytime sky, and the prediction of prohibitively faint target brightness which significantly reduces the productivity of observations.
The goal of our research is to demonstrate the potential of a small, modular commercial-off-the-self optical telescope facility to operate productively throughout the day following the discovery of a remarkable ~20 times increase in satellite brightness at optical wavelengths in observations taken during the day compared to terminator illuminated conditions.
We present our observations of Starlink satellites (V1.0, V1.5 and V2.0 mini) in addition to several large bright satellites in Low Earth Orbit (LEO) down to ~6th magnitude (V band) during the day between Sun altitudes of 0 to 60 degrees at Australian latitudes. We demonstrate the use of broadband photometry of bright reference stars throughout the day to achieve high photometric accuracy of observations of satellites between 1-10%, motivated by the purpose of monitoring and classifying objected in LEO. We present observed light curves for the passes of satellites observed, with median Starlink daytime optical brightness in r’ band between 3.9 mag for V1.0 up to 3.5 mag for V2.0 mini. In addition we share our best practices found for observing during the day, and our findings of the impact of environmental factors such as seeing, scintillation and local cloud cover on the productivity of observations. Finally, we compare our measurements of satellite brightness to existing models to demonstrate the significant contribution of Earthshine to observed satellite optical brightness during the day.
The methodology used in this work utilises the Huntsman Telescope Pathfinder or mini-Huntsman herein – a pathfinder instrument for the Huntsman Telescope located at Siding Spring Observatory, originally designed for low surface brightness imaging of extended and diffuse astrophysical sources. Mini-Huntsman is located an Macquarie University Observatory in Sydney Australia, and consists of a single Canon 400mm EF 2.8L telephoto lens with a 7 inch aperture mounted on a software bisque MEII equatorial mount. We use a ASI183MM Pro CMOS camera capable of down to microsecond exposures, a filter wheel containing sloan r’ and g’ broadband filters, and a Astromechanics focuser. In addition, we use commercial solutions for image capture ASICap, as well as mount control and satellite tracking using TheSkyX.
We find that with an accurate pointing model with RMS ~53 arcsec and up-to-date ephemeris, satellites can be reliably tracked during the day within a field of view of 1.89 x 1.26 degrees. After applying flat field calibrations, we calculate a photometric zero point for satellite observations in sloan r’ and g’ filters by taking 1000 exposures of several bright reference stars throughout the day through a range of air masses and comparing to catalog magnitudes. To ensure an accurate result and eliminate the probability of detecting false positives from frequently passing seed packets, bugs, and leaves, we manually perform source detection of satellite targets for all frames in each ~1-2 minute pass using a source detection interface developed in python for this work. Metadata for each frame is calculate from the satellite ephemeris for which we verify the relationship between the variation in observed satellite brightness verses solar elongation angle for Starlink satellites during the day, as compared to terminator illuminated conditions at night in the literature. To explore the surprising increase in satellite optical brightness during the day, we extend the python software package “Lumosat” for daytime observations, and verify that the model, when including light scattered off the surface of the Earth known as Earthshine in addition to direct sunlight, can explain the increased brightness of satellites observed during the day. Our findings demonstrate that small aperture telescopes compose of commercial-off-the-shelf components can be productive facilities for SDA for the observation and monitoring Starlink satellites and other bright LEO satellites.
We anticipate that our findings will make optical daytime observations for the purpose of SDA more accessible to smaller aperture telescope facilities including those built from commercial-off-the-self components. This work may serve as a starting point for rolling out multiple modular, low cost mini-Huntsman type facilities around the world to improve the coverage and productivity of SDA to ensure a safer and more sustainable usage of Space for generations to come.
Date of Conference: September 17-20, 2024
Track: SDA Systems & Instrumentation