Observations:
Before the installation of the monolithic suspensions, the phase camera images suffered from a second beam both on B4 and on B1p during the initial locking stages. After the installation these spurious beams vanished. Figure recombined.png shows the situation of PC B4 and PC B1p for the interferometer locked in recombined, before and after the installation of the monolithic suspensions. Additionaly the beam shape on the PC B4 during dark fringe has changed from round to astigmatic. No drastic change in astigmatism  are observed on B1p (see Fig. df.png). The alignment of the PC B4 telescope has been changed since the installation of the monolithic suspensions, while the telescope on B1p is not touched (see entry 41414).

Hypothesis:
The second beam on PC B4 during recombined (in the past) might have been caused by the BS just after the view port, that splits the beam between SFP and PC (the mount was broken and Matteo found during the realignment that it was clipping, at least after the replacement of the suspensions). As the beam leaving the view port is astigmatic, the ellipticity we observe now could be explained by a change in the distance between the two PC telescope lenses after the last alignment. The cause of the second beam we did not investigate. To check whether the PC telescope has changed, we remeasured the beam profile on 15.08.2018.

Beam profile:
Indeed the beam profile of B4 at the level of EPRB (outside the SPRB vacuum) has changed. Most likely cause is the PC telescope itself, as the beam on the SFP path looks the same as on the SPRB camera and both have not changed after the installation of the suspensions. Changing the distance between the two lenses by 9 mm explains the observed horizontal profile (see Fig. b4beamprofile.png for comparison). We decided not touch the telescope for now, as the interferometer was very unstable with a person on the BS tower (which is needed for the realignment due to the location of EPRB) and a change in telescope corresponds to a change in image point, but not in HOM content (which is the quantitiy we are currently most interested in).

Future action:
We will discuss a redesign of the telescope.