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Issue #5
Affects EDR, DR1

Zero-point calibration

Issue raised May 02 2016, 02:20; last modified Jun 05 2017, 12:10

Photometric zero-points for the EDR and DR1 are derived per image through comparison to the AAVSO Photometric All-Sky Survey (APASS). The DR1 dataset has improved the treatment of zero-points, however, several imperfections are still present, which may cause zero-point residuals with 3-4% rms in g- and r-bands, over 5% in z- and v-band, and up to 10% in u-band. The u-band suffers from a red leak that significantly changes the u-band flux of red stars with airmass. An M0 star shows an apparent u-v colour that is 0.4 mag bluer at airmass 2 than it is at airmass 1.

Zero-point calibration will be improved in future data releases, eventually becoming fully internally derived, with HST spectrophotometric standards as the fundamental references.