Detection analysis of HD 135344B observed in the narrowband NB_1.64. Left panels: Q_IPS and U_IPS images divided by the white lines into the four quadrants of the expected butterfly pattern. Right panel: annulus-wise CCFs, with the S/N shown against the annulus radius in pixels. The results for the Q_IPS and U_IPS images are plotted in blue and red, respectively. The shaded region specifies the coefficients used in normalising and converting the CCF into a S/N function. The 5σ detection limit is indicated with a horizontal line.