Demonstration of how XRT is used to constrain the source boundary threshold as seen by STIX. a) XRT Be-thick image showing the source boundary (blue contour; Eq. (2)) and the epipolar planes that bound it (green). b) Corresponding 6–10 keV STIX image with three source contours (inner dashed red line: 50%; solid red line: 37%, outer dashed red line: 30%) and the same epipolar planes as in panel a (green) projected to the STIX viewing angle. Only one contour (solid red) is tangent to both bounding epipolar planes and so represents the source boundary as seen by STIX.