Time-resolved I-band photometry of calibration spheres and NaK droplets

We have developed a program to obtain photometry of Earth-orbiting satellites using the U.S. Air Force advanced electrooptical system 3.6 m telescope's Visible Imager instrument, which acquires time-series charge-coupled device images at typical rates of 0.1 to 2.1 Hz. Observations of 12 spherical satellites provide measurements of I-band albedos (i.e., reflectances) and evaluation of surface nonuniformities. Best-fit specular + diffuse albedo models enable brightness predictions and a means to grade the quality of the spheres as photometric calibration reference objects. CalSphere-4A appears to be the highest quality calibrator of the 12, with the best-fit model reproducing 90% of the measurements to within +/- 0.03 stellar magnitudes. This four-decade-old sphere reflects in a predominantly diffuse fashion and has a total albedo of approximate to 67% that is somewhat lower than expected for fresh white paint, effects likely due to space weathering. The aluminum Lincoln Calibration Sphere-1 shows predominantly specular reflection (albedos approximate to 59%) but with +/- 0.3 magnitude deviations between data and model that recur every 30-90 s, likely due to one or more surface irregularities. Three other predominantly specular aluminum spheres display less frequent but larger amplitude deviations. Observations indicate that sodium/potassium droplets leaked from Soviet radar ocean reconnaissance satellite reactors are highly reflective specular spheres that can also serve as optical calibrators.