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Photometric calibration of the Unistellar telescopes to the Johnson-Cousins System
Our purpose is to provide photometric data and tools to compute magnitudes from images obtained by Unistellar telescopes and enhance their usefulness to the broader astronomical community. To do so, we provide relationships transforming Unistellar complementary metal oxide semiconductor sensor Bayer filter bands (blue Be, green Ge, and red Re) to the Johnson-Cousins photometric system. To enable this calibration, stars with known spectra were observed with Unistellar telescopes by citizen scientists in the Unistellar Network. We obtained the zero-point magnitudes of the telescopes for each band by combining those spectra with the telescope sensor responses. Using the observations of 794 stars with known Johnson-Cousins magnitudes, we established the relations to transform the Unistellar magnitudes. The zero-point magnitudes and their associated errors for the three bands were derived. Polynomial expressions for the relation between the Unistellar colour Be−Re and the Johnson-Cousins magnitudes were obtained for stars with Unistellar colour −0.15<Be−Re<0.5. The uncertainties on these transformations increase linearly with the Re magnitude. These relations were applied to the observation of a type II supernova and an exoplanet transit, with the supernova example showing consistency with other published magnitudes but also demonstrating that narrow-line emission or absorption can bias the transformed magnitudes. Other limitations of the transformations were also identified, largely stemming from significant infrared contributions to all color channels and variability between individual telescope sensors. Nevertheless, the values and color relationships provided here can be used by professional and citizen astronomers alike to compute observed magnitudes and provide measurements in a standard system. This enhances the Unistellar Network’s ability to contribute accurate photometry to the astronomical community that can be easily and faithfully combined with measurements from other instruments.
