I performed in-vac health check for BS (fig1-25).
Though IMV1 gain becomes slightly smaller because it is close to the linear range, it would be acceptable, hopefully.
Figure 26 and 27 show the OSEM input signals during in-air and in-vac health check measurements, respectively.
IMV1 signals in fig27 is lower than those in fig26 and close to the edge of linear range.