Coronagraph inner working angle (${\theta }_{\mathrm{IWA}}$)

The coronagraph inner working angle describes how close to the blocked source a specified coronagraph can usefully transmit or detect a faint neighbor. It requires a criterion, such as a chosen fraction of the far off-axis coronagraph throughput, and is often expressed in units of $\lambda /D$. It is not automatically the coronagraphic focal plane mask radius or a guarantee of detection. Small separations make useful source transmission and accurate control of wavefront errors particularly demanding. ^[ref].