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Current cosmological analyses which use Type Ia supernova (SN Ia)observations combine SN samples to expand the redshift range beyond that of asingle sample and increase the overall sample size. The inhomogeneousphotometric calibration between different SN samples is one of the largestsystematic uncertainties of the cosmological parameter estimation. To placethese different samples on a single system, analyses currently use observationsof a small sample of very bright flux standards on the $HST$ system. We proposea complementary method, called `Supercal', in which we use measurements ofsecondary standards in each system, compare these to measurements of the samestars in the Pan-STARRS1 (PS1) system, and determine offsets for each systemrelative to PS1, placing all SN observations on a single, consistentphotometric system. PS1 has observed $3\pi$ of the sky and has a relativecalibration of better than 5 mmag (for $\sim15<griz<21$ mag), making it anideal reference system. We use this process to recalibrate optical observationstaken by the following SN samples: PS1, SNLS, SDSS, CSP, and CfA1-4. We measurediscrepancies on average of 10 mmag, but up to 35 mmag, in various opticalpassbands. We find that correcting for these differences changes recoveredvalues for the dark energy equation-of-state parameter, $w$, by on average$2.6\%$. This change is roughly half the size of current statisticalconstraints on $w$. The size of this effect strongly depends on the error inthe $B-V$ calibration of the low-$z$ surveys. The Supercal method will allowfuture analyses to tie past samples to the best calibrated sample.

Supercal: Cross-Calibration of Multiple Photometric Systems to Improve Cosmological Measurements with Type Ia Supernovae