A motional Stark effect MSE diagnostic is now installed and operating routinely on the MAST spherical tokamak, with 35 radial channels, spatial resolution of 2.5 cm, and time resolution of 1 ms at angular noise levels of 0.5°. Conventional albeit very narrow interference filters isolate or polarized emission. Avalanche photodiode detectors with digital phase-sensitive detection measure the harmonics of a pair of photoelastic modulators operating at 20 and 23 kHz, and thus the polarization state. The component is observed to be significantly stronger than , in reasonably good agreement with atomic physics calculations, and as a result, almost all channels are now operated on . Trials with a wide filter that admits the entire Stark pattern relying on the net polarization of the emission have demonstrated performance almost as good as the conventional channels. MSE-constrained equilibrium reconstructions can readily be produced between pulses.