Philip Castro, Applied Optimization Inc.; Tamara Payne, Applied Optimization Inc.; Joseph Moody, Brigham Young University; Stephen Gregory, Applied Optimization Inc.; Phan Dao, AFRL; Roberto Acosta, NASIC/GSMS
Keywords: Photometry, Spectral, Transformations, Calibrations, Characterization, SSA
Abstract:
Space Situational Awareness (SSA) observations are sometimes performed with a filter. The traditional filters used are that 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 Sloan filters have wider bandpasses than Johnson-Cousins filters, are essentially non-overlapping while the Johnson-Cousins filters overlap, and are on the AB magnitude system while Johnson-Cousins filters are on the Vega magnitude system. The most recent large astronomical surveys in the optical have used the Sloan filters: the Sloan Digital Sky Survey and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). 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 improvements in 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 Johnson-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 from one photometric system to another.
A number of such transformations exist within the astronomical community for stellar objects, such as main sequence stars. However, the spectral energy distributions (SEDs) of main sequence stars are not the same as satellites. While the SEDs of main sequence stars are unchanging, 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 these satellites and juxtaposed them with the stellar transformations established by the astronomical community. The results of this analysis will be presented. Finally, we conclude on the future prospects of the historical data collected in Johnson-Cousins.
Date of Conference: September 11-14, 2018
Track: Poster