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Abstract ATLAS (Asteroid Terrestrial Last Alert System) is a time-domain survey using four telescopes, covering the entire sky. It has observed 8378 spectroscopically confirmed Type Ia supernovae (SNe Ia), with thousands of cosmology-grade light curves (to be released as TITAN DR1). To prepare this massive, low-redshift dataset for cosmology, we evaluate and cross-calibrate ATLAS forced photometry using tertiary stars from the DES (Dark Energy Survey) Y6 release. The 5000 deg²DES footprint overlaps regions both in and out of the PS1 (Pan-STARRS DR1) footprint, allowing tests of the primary calibrator for the ATLAS Refcat2 catalog. Initial offsets are at the ∼40 mmag scale. To improve this, we determine Δ zero-point offsets for two cases: (1) pixel-to-pixel offsets within individual CCDs (reduced from ∼8 to ∼4 mmag rms) and (2) chip-to-chip offsets across the nine CCDs and filters (reduced from ∼17 to ∼3 mmag rms). We also identify the largest systematic uncertainty as a transmission-function color dependence, requiring shifts in the assumed ATLAS filters at the ∼30 mmag level if uncorrected. We validate our calibration using (a) CALSPEC standards, (b) an independent tertiary catalog, and (c) distance moduli of cross-matched SNe Ia, all showing improved consistency. Overall, we estimate combined calibration-related systematics at the ∼5–10 mmag level, supporting competitive cosmological constraints with the TITAN SN Ia dataset. 

Ground-based observatories will discover thousands of transients in the optical, but will not provide the NIR photometry and high-resolution imaging of a space-based observatory. WFIRST can fill this gap. With its SN Ia survey, WFIRST will also discover thousands of other transients in the NIR, revealing the physics for these high-energy events.