Schematic drawing of the transformation T from the (α, β) cartesian space to the (ρ, θ) polar space. In this transform, the ring corresponding to the vignetting region becomes a rectangle (here only half the corona region is shown). The drawing is not to scale, since OM corresponds to the Sun radius (about 960 arcsec) and the width (MM’) is the vignetted region (about 100 arcsec).