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Four observations of Starlink's LEO communication satellites, Darksat andSTARLINK-1113, were conducted on two nights with two telescopes. The Chakana0.6\,m telescope at the Ckoirama observatory (Chile) observed both satelliteson 5\,Mar\,2020 (UTC) and 7\,Mar\,2020 (UTC) using a Sloan r' and Sloan i'filter, respectively. The ESO VISTA 4.1\,m telescope with the VIRCAM instrumentobserved both satellites on 5\,Mar\,2020 (UTC) and 7\,Mar\,2020 (UTC) in theNIR J-band and Ks-band, respectively. The calibration, image processing, andanalysis of the Darksat images give r\,$\approx$\,5.6\,mag,i\,$\approx$\,5.0\,mag, J\,$\approx$\,4.2\,mag, and Ks\,$\approx$\,4.0\,magwhen scaled to a range of 550\,km (airmass $=1$) and corrected for the solarincidence and observer phase angles. In comparison, the STARLINK-1113 imagesgive r\,$\approx$\,4.9\,mag, i\,$\approx$\,4.4\,mag, J\,$\approx$\,3.8\,mag,and Ks\,$\approx$\,3.6\,mag when corrected for range, solar incidence, andobserver phase angles. The data and results presented in this work show thatthe special darkening coating used by Starlink for Darksat has darkened theSloan r' magnitude by 50\,\%, Sloan i' magnitude by 42\,\%, NIR J magnitude by32\,\%, and NIR Ks magnitude by 28\,\%. The results show that both satellitesincrease in reflective brightness with increasing wavelength and that theeffectiveness of the darkening treatment is reduced at longer wavelengths. Thisshows that the mitigation strategies being developed by Starlink and other LEOsatellite operators need to take into account other wavelengths, not just theoptical. This work highlights the continued importance of obtainingmulti-wavelength observations of many different LEO satellites in order tocharacterise their reflective properties and to aid the community in developingimpact simulations and developing mitigation tools.

Optical-to-NIR magnitude measurements of the Starlink LEO Darksat satellite and effectiveness of the darkening treatment