The Blurring Interference Seeing Motion Monitor (BLISMM): a Compact Instrument for Simultaneous Measurement of R_0 , Per-Frame Τ _0(n) , and Θ _0 Without Wind Speed Measurement

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 τ _0(n) , and the isoplanatic angle θ _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 τ _0(n) , and an aperture scintillation channel for θ _0 [5]. The central contribution is a per-frame expression for τ _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 τ _0 from angle-of-arrival statistics leading to an effective wind speed, or the SHIMM [18], which derives τ _0 from the power spectrum of wavefront defocus over temporal sequences, the BLISMM provides an instantaneous τ _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 τ _0(n) and an independent τ _0 estimate. Mean values from the 2023 campaign: r_0=55 mm (seeing ≈ 2.4” at 656 nm), τ _0(n)=1.57 ms, θ _0=9.75” . The per-frame τ _0(n) output enables real-time frame selection for lucky imaging without wind speed data.