Facility-specific assessment of indoor airflow stability for repeatable interferometric testing of large optics

Indoor airflow is a major source of uncertainty in interferometric testing of large optics, yet quantitative criteria for identifying conditions that support repeatable measurements remain limited. We simultaneously measured the INTENSE-based effective Fried parameter (r₀) and interferometric root-mean-square wavefront error (RMS WFE) of a 1 m-class mirror under four airflow conditions involving forced airflow, local mixing, and no fan operation. For each condition, the primary analysis used ten 1-min measurements, with five additional 1-min measurements used for reproducibility assessment. Session-mean effective r₀ alone did not consistently indicate repeatability: the condition with the largest mean effective r₀ differed between sessions, whereas the combined forced-airflow and local-mixing condition exhibited the smallest observed repeat-measurement fluctuations in both effective r₀ and RMS WFE. Normalized effective-r₀ fluctuation was used as a scale-adjusted descriptor for intersession comparison, whereas absolute fluctuation showed a stronger exploratory association with RMS WFE fluctuation. Analyses of low-order Zernike modes and temporal power spectral density provided complementary modal and spectral information. Within the present facility and sampled time scales, these results suggest that the fluctuation of effective r₀ may complement its mean value and the corresponding RMS WFE statistics when evaluating measurement repeatability.