a) Time series of single-frame rms wavefront error σ_φ as measured by i) the central PSF Strehl ratio under the Maréchal approximation (σ_S, green) and ii) calibrated cMWS measurements (σ_WFS, blue). Slow variations in seeing quality seen in σ_S are visibly traced by the sensor. b) Correlation plot between the two independent estimates of rms wavefront error, with colour indicating point density. The solid black line denotes σ_S/σ_WFS = 0.45 as is expected theoretically from Noll (1976), which corresponds well to the core regions of the correlation (1.45 < σ_S < 1.65). The outer regions have a significantly shallower gradient (red), distorted by various systematic error sources.