Briefly measured the green return beam on ISCTEY. The first attachment shows the beam profile about 5" or so downstream of the BS that splits WFS and PDH. This is very good, though you might be able to tell that some broad thing is on top of mostly Gaussian main beam.

There are several ghost beams that potentially interfere with the main beam, the closest one being AR reflection from the 1" lens between two MadCityLabs PZT mirrors. The ghost beam is about 3uW in the WFS path while the main beam is 730uW, so this is not that bad, but I'll misalign the lens slightly so that the beam will not come on top of the main beam. The second attachment shows the beam profile of this ghost beam measured at the same position as the first one.

There was also a strong ghost beam coming from the BS to split WFS and PDH path. Total power coming to this BS is about 5.7mW, 5mW goes to PDH and 730 goes to main WFS beam while the ghost beam is about 17uW. This one is shifted in YAW and is easy to block. The third attachment shows this beam measured at the same position as previous two, but the scan head shifted in YAW by about 3mm.

Also there are many many internal reflections coming out of Faraday that  remain there even when the beam path to the chamber is blocked. The total power of these is 115uW. The brightest one is off in PIT but it was easy to block this by using an iris.