Daytime Photometry of Starlink Satellites with the Huntsman Telescope Pathfinder

Abstract 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° to midday. Starlink satellites are found to be particularly bright at $$3.6\pm 0.05$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>3.6</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.05</mml:mn> </mml:mrow> </mml:math> mag, $$\sigma = 0.6\pm 0.05$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>σ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.6</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.05</mml:mn> </mml:mrow> </mml:math> mag in Sloan r′, or $$\sim 11\times$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>11</mml:mn> <mml:mo>×</mml:mo> </mml:mrow> </mml:math> 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.