Transformation between the Johnson-Cousins and Sloan Photometric Systems for SSA

Space Situational Awareness (SSA) observations are sometimes performed through a spectral filter. The traditional filters used are those of the Johnson-Cousins photometric system (B, V, R, and I). The SSA community has been observing with these filters for decades and therefore has historical data spanning this duration. More recently, the astronomical community is replacing the Johnson-Cousins system with the Sloan photometric system as the primary system for optical observations. The most recent large astronomical surveys in the optical regime have used the Sloan filters: the Sloan Digital Sky Survey and the Panoramic Survey Telescope and Rapid Response System (PanSTARRS). The Pan-STARRS 1 catalog sky coverage and its astrometric and photometric precision make it well suited for in-frame calibrations of satellite observations. Such in-frame calibrations would provide increased calibration cadence and the potential for improving accuracy by mitigating the effects of a changing atmosphere. Because a comparable catalog in the Johnson-Cousins photometric system that would allow in-frame calibrations does not exist, it makes sense for SSA observations to transition to the Sloan system. A consequence of transitioning from JohnsonCopyright © 2018 Advanced Maui Optical and Space Surveillance Technologies Conference (AMOS) – www.amostech. com Cousins to Sloan is the obsolescence of the historical Johnson-Cousins satellite photometry. To compare photometry between the Johnson-Cousins and Sloan systems, a transformation needs to be made to convert data from one photometric system to another. A number of such transformations exist within the astronomical community for stellar objects. However, the Spectral Energy Distributions (SEDs) of stars are not the same as those of satellites. Reflection for spacecraft can be modeled based on diffuse and specular reflection components, where the diffuse components’ reflected spectrum may have spectral characteristics of the material off which it reflects, thereby altering the SED from that of the Sun. While the SEDs of stars are largely static, the SEDs of satellites are not. Specifically, their SED may change with phase angle (e.g., solar panel contributions are phase angle dependent and typically make the SED bluer). To investigate the transformation between Johnson-Cousins and Sloan for satellites, we performed the following analysis. We observed four satellites sequentially in Johnson-Cousins filters (B, V, R, and I) and Sloan filters (g, r, i, and z), covering a large range of phase angle. We then empirically derived transformations between Johnson-Cousins and Sloan for each satellite’s observed data and for all of the observed satellite data as a whole, and juxtaposed these with an astronomical transformation. We found mixed results for the transformation relations. The r – V as a function of V – R relation provides a great fit for all of the observed satellite data with low root mean square (RMS) error and is exactly the same as the astronomical transformation. The r – z as a function of R – I relation provides a great fit for all of the observed satellite data, but has large RMS scatter and is distinct from the astronomical transformation. Thus, we do not recommend transforming historical satellite photometry observed in Johnson-Cousins to Sloan to compare to observations of satellites taken in the Sloan filters. Since the transformations are dependent on the SED of the satellite, and the satellites’ SEDs are variable, transformations generally yielded poor results for the two photometric systems we studied here, i.e. Johnson-Cousins and Sloan. Moreover, our supposition is that such attempts with any two photometric systems may yield similarly poor results.