Context :

Aims : We measured the Sloan g ’ magnitudes of the Starlink ’ s STARLINK-1130 ( Darksat ) and 1113 LEO communication satellites to determine the effectiveness of the Darksat darkening treatment at 475.4 nm .

Methods : Two observations of the Starlink ’ s Darksat low Earth orbit ( LEO ) communication satellite were conducted on 2020/02/08 and 2020/03/06 using Sloan r ’ and g ’ filters , respectively .
A second satellite , STARLINK-1113 , was observed on 2020/03/06 using a Sloan g ’ filter .
The initial observation on 2020/02/08 was a test observation conducted when Darksat was still in the process of manoeuvring to its nominal orbit and orientation .
Based on the successful test observation , the first main observation took place on 2020/03/06 , along with an observation of the second Starlink satellite .

Results : The calibration , image processing , and analysis of the Darksat Sloan g ’ image gives an estimated Sloan g ’ magnitude of \mathbf { 7.46 \pm 0.04 } at a range of 976.50 km .
For STARLINK-1113 , an estimated Sloan g ’ magnitude of \mathbf { 6.59 \pm 0.05 } at a range of 941.62 km was found .
When scaled to a range of 550 km and corrected for the solar and observer phase angles , a reduction by a factor of two is seen in the reflected solar flux between Darksat and STARLINK-1113 .

Conclusions : The data and results presented in this work demonstrate that the special darkening coating used by Starlink for Darksat has darkened the Sloan g ’ magnitude by 0.77 \pm 0.05 mag when the range is equal to a nominal orbital height ( 550 km ) .
This result will serve members of the astronomical community who are actively modelling the satellite mega-constellations to ascertain their actual impact on both amateur and professional astronomical observations .
Both concurrent and subsequent observations are planned to cover the full optical and NIR spectrum using an ensemble of instruments , telescopes , and observatories .