Top: Zemax ray-tracing software views from above the focal gondola on the optical axis. These graphs show the optical rays (blue lines) for the declination of the star Deneb at several hour angles (− 40 min <H< + 15 min). In this projected view, each mirror (N, SW, SE) is at the end of a blue line. It falls in the Mertz field of view if it is located between the two red circles (in the blue area). The central obscuration is due to the central hole of the Mertz corrector (see Figs. 2 and 4 of Paper I): when a primary segment is aligned with this hole (i.e., the focal gondola is aligned with this segment and the star), the light is lost (the lost light can be blocked by a mask in the pupil plane, to avoid directly illuminating the camera). A graph is shown each time a primary mirror enters or exits the Mertz field of view. Note that the north mirror is represented, but during the observation of Deneb it was not aligned and was masked. Thus, we expected fringes on Deneb only when the SW and SE mirrors are in the field of view. Bottom: we give the name of the primary mirrors (N, SW, SE) that are in the field of view of the Mertz corrector during the tracking of Deneb. From 35 min to 20 min before the transit, only the SE mirror is in the field of view of the Mertz corrector, and no fringe acquisition is possible. We have recorded data on Deneb during 1 min 30 s starting 10 min before the transit (transit at H = 00 min i.e. 21:18:28 UT on September 05, 2013), and during 5 min, 9 min 30 s after the transit (see the observing log of Table A.1). During 18 min around the transit, the SE mirror of the small baseline is not in the field of view of the Mertz corrector (confirmed during the observation: Table A.1).