There is maybe a problem with the ghost beams coming from the BS and hitting the Iris in front of the sample.
I am afraid that this will produce some additional scattering light which superpositions the backscatter-signal and disturb the measurements especially for wide angles.
Torii-san and I tried to set an obliqued ND filter in front of the Iris to remove the ghost beam hitting the surface of the Iris.
The first results seem to support my fears.
Estimated influence on photocurrent: 0.1 - 0.2 nA