Journal ArticleQuantifying the Limits of TESS Stellar Rotation Measurements with the K2-TESS OverlapBoyle, Andrew W (ORCID:0000000160372971); Mann, Andrew W (ORCID:0000000336541602); Bush, Jonathan (ORCID:0000000294469250)The Transiting Exoplanet Survey Satellite (TESS) has provided stellar rotation periods across much of the sky through high-precision light curves, but the reliability and completeness of these measurements require careful evaluation. We assess the accuracy of TESS-derived rotation periods by leveraging a cross-matched sample of ∼23,000 stars observed by both TESS and the K2 mission, treating K2 periods as a benchmark. Using causal pixel models to extract light curves and a Lomb–Scargle (LS) periodogram to identify rotation signals, we quantify the empirical uncertainties, reliability, and completeness of TESS rotation period measurements. We find that uncertainties on TESS-derived rotation periods are typically below 3% for stars with periods <10 days. Rotation periods are generally reliable out to 10 days, with ≳80% of measurements matching the K2 benchmark. Completeness and reliability drop dramatically for periods beyond ≃12 days due to the 27 day sector limitation. Stricter cuts on TESS magnitude and LS power improve reliability; the highest LS power tested (>0.2) ensures >90% reliability below 10 days but removes over half of potential detections. Stitching consecutive-sector light curves reduces period uncertainties but does not improve overall reliability or completeness due to persistent systematics. Our findings and code provide a framework for interpreting TESS-derived rotation periods and inform the selection of quality cuts to optimize studies of stellar rotation, young associations, and gyrochronology.Astrophysical Journal2025-05-2710671749The Astrophysical Journal98522330004-637Xhttps://doi.org/10.3847/1538-4357/adcecc2143763National Science Foundation