Contrast uncertainty (median over sky positions) as a function of fringe-tracker phase error, for different values of target magnitudes. Dotted line is for no intensity fluctuations, solid line for (realistic) 2% RMS intensity fluctuations and dashed line for poor 10% intensity fluctuations. For high target fluxes, fringe tracker phase error dominates, and for low target fluxes, thermal background dominates. Residual target shot noise never dominates at an optics temperature of 290 K.
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