We present the BLISMM (Blurring Interference Seeing Motion Monitor), a compact pupil-plane mask instrument measuring simultaneously the Fried parameter \(r_0\) , the per-frame coherence time \(\tau _0(n)\) , and the isoplanatic angle \(\theta _0\) from a single video sequence on a bright star. A three-aperture mask combines a DIMM channel for \(r_0\) [21], a Fizeau interferometric channel for \(\tau _0(n)\) , and an aperture scintillation channel for \(\theta _0\) [5]. The central contribution is a per-frame expression for \(\tau _0(n)\) derived directly from the contrast degradation of Fizeau fringes within individual exposures, without requiring knowledge of the atmospheric wind speed profile. Unlike the GDIMM [1], which estimates \(\tau _0\) from angle-of-arrival statistics leading to an effective wind speed, or the SHIMM [18], which derives \(\tau _0\) from the power spectrum of wavefront defocus over temporal sequences, the BLISMM provides an instantaneous \(\tau _0(n)\) at frame rate. On-sky validation at two French observatories (2014 and 2023) yields a linear correlation slope \(=1.00\) and \(R^2=0.98\) over 2033 frames between the fringe-based \(\tau _0(n)\) and an independent \(\tau _0\) estimate. Mean values from the 2023 campaign: \(r_0=55\) mm (seeing \(\approx 2.4''\) at 656 nm), \(\tau _0(n)=1.57\) ms, \(\theta _0=9.75''\) . The per-frame \(\tau _0(n)\) output enables real-time frame selection for lucky imaging without wind speed data.
