Photometric calibration in u-band using blue halo stars

We develop a method to calibrate u-band photometry based on the observed color of blue galactic halo stars. The galactic halo stars belong to an old stellar population of the Milky Way and have relatively low metallicity. The "blue tip" of the halo population -- the main sequence turn-off (MSTO) stars -- is known to have a relatively uniform intrinsic edge u-g color with only slow spatial variation. In SDSS data, the observed variation is correlated with galactic latitude, which we attribute to contamination by higher-metallicity disk stars and fit with an empirical curve. This curve can then be used to calibrate u-band imaging if g-band imaging of matching depth is available. Our approach can be applied to single-field observations at $|b| > 30^\circ$, and removes the need for standard star observations or overlap with calibrated u-band imaging. We include in our method the calibration of g-band data with ATLAS-Refcat2. We test our approach on stars in KiDS DR 4, ATLAS DR 4, and DECam imaging from the NOIRLab Source Catalog (NSC DR2), and compare our calibration with SDSS. For this process, we use synthetic magnitudes to derive the color equations between these datasets, in order to improve zero-point accuracy. We find an improvement for all datasets, reaching a zero-point precision of 0.016 mag for KiDS (compared to the original 0.033 mag), 0.020 mag for ATLAS (originally 0.027 mag), and 0.016 mag for DECam (originally 0.041 mag). Thus, this method alone reaches the goal of 0.02 mag photometric precision in u-band for the Rubin Observatory's Legacy Survey of Space and Time (LSST).