A reassessment of the Pantheon+ and DES 5YR calibration uncertainties: Dovekie

Type Ia supernovae (SNe Ia) are crucial tools for measuring the accelerating expansion of the Universe, comprising thousands of SNe across multiple telescopes. Accurate measurements of cosmological parameters with SNe Ia require a robust understanding and cross-calibration of the telescopes and filters. A previous cross-calibration effort, `Fragilistic', provided 25 photometric systems, but offered no public code or ability to add new surveys. We provide a publicly available open-source cross-calibration solution with an eye to the future. By jointly using the Pan-STARRs (PS1) and all-sky telescopes, we characterised the measured filters from 11 photometric systems---including CfA, PS1, Foundation, DES, CSP, SDSS, and SNLS---using published observations of field stars. For the first time, we derived uncertainties on effective filter transmissions and modified filters to match the data. With the addition of direct observations of DA white dwarfs, we simultaneously cross-calibrated our zero points across photometric systems and trained a new SALT model to propagate to cosmology. Gaia With improved uncertainties from DA WDs, we find improvements to the calibration systematic uncertainty of times 1.5 for the Pantheon+ sample, with a new systematic photometric uncertainty of σ_w( ) = 0.013 for FlatwCDM, and modest improvements to that of the DES5YR analysis. We find good agreement with previous calibration and show that even these small calibration changes can be amplified by up to a factor of six in the inferred SN,Ia distances; this is driven by calibration sensitivity in the colour--luminosity relations and SALT training. Initial results indicate that these changes cause dμ/dz = 0.041 ± 0.009 across our redshift range. These may have a potentially larger impact in w0/wa space and on inferences regarding evolving dark energy. phot