Q_φ image illustrating the method to determine the PA and size of the outer disc. The image was 4 × 4 binned and smoothed via a Gaussian kernel with σ = 1 pixel. The brightest pixels of the regions inside the yellow circles indicate the location of the planetesimal belt which is schematically drawn as a yellow ellipse. For the clarity, the ellipse was shifted upwards (white dotted lines show the parallel shift of the brightest pixels). The position of star is marked by a yellow asterisk. The white line represents the disc major axis. The colour bar shows the counts per binned pixel.