PCA-disc image where each vertical profile has been normalized by the maximum spine brightness to enhance the vertical position of the spine. The two green circles have a radius of 0.4″ and 2″ respectively. The PA of the main disc (29°) as measured by Lagrange et al. (2012a) is indicated by a solid red line, whereas the best fit PA of 30.8° (NE) and 211.0° (SW) as measured from the L′ images are shown with dashed lines. The waviness of the spine is discussed in Sect. 5.2.