With the purpose to shed some more light on the HWS measurement, we have analyzed the raw images before and after the appearance of the feature in the upper part of the wavefront.

Figure 1 shows the difference between the two raw images. As it can be seen, there is a significant modification of the illumination pattern on the HWS as soon as the ITF has unlocked. The change in the illumination pattern is responsible for the appearance of the feature.

This change in the illumination pattern can be explained with a displacement of the HWS probe beam (due to a misalignment of one of the optics or benches along the path) or with some spurious other beam (1064 nm or 532 nm) reaching the HWS plate. With the available data, it is not possible to discriminate between the two effects, even if a check of the LC signals (figure 2) seems to exclude the displacement of the probe beam.