Today I observed that the B4 beam has a very different shape whether the North Arm is locked or Unlocked.
Figure 1 show the images of the beam B4 on PC, SPRB cameras and also B2 beam.
Figure 2 show the same images with the arm unlocked.
The beam looks much better when it is reflected from the North Arm locked, especially true for the two cameras on SPRB and the carrier image.
The "avocado" shapes seems to appears when the North arm is unlocked.
Figure 3 show the beam radius fit given by GxC and recorded in the rds frame.
To be noticed that I have added 4 signals called SPRB_B4_Cam1/2_zoom_FitWaistX/Y. These signals are the fit obtained in the central area of the cameras, the goal is to avoid getting artefact in the projection due to secondary beams.
The values given by the SPRB_B4_Cam1/2_zoom_fitWaistY are smaller than the one given by SPRB_B4_Cam1/2_fitWaistY because it exclude the secondary beam or most of it.
I ran my script that gives the beam profile of B4 for NArm locked (fig. 4) and NArm unlocked (fig. 5)
I don't want to comment too much because I would like to redo this test putting the secondary beam out the cameras because I am not sure it has no impact of the data.
Anyway the astigmatism of B4 seems reduced and the beam shape much better with the North Arm locked.
So I would say that some part of the astigmatism is not coming from the SPRB telescope (including viewport).
