Matteo, Simon

On Friday, we started to upgrade the absorption map to be capable of taking maps on the homogeneity of mirror-substrates in terms of their polarization.
The basic idea is to use a PBS nad two photodiodes (PSDs in our case) to distinguish between S and P polarization. Actually, we recognized that the incoming beam is 90% P and 10% S polarized so that we decided to put another PBS also in the incoming beam in addition to a half-wave plate which we use to check the differences in S and P polarization and the calibration of the system.

After some first checks, we found out that the PSD designated for sensing the S-polarization is very sensible to any scattered light which is automatically an issue due to the many ND filters we have to use for not saturating the AC channel (which is fed by the S-pol PSD). Therefore, we took long time to cover the path from the PBS to the PSD. We finally took a beam-pipe which does the job quite well (see attached pictures).

Over the weekend, we took two first maps. One with incoming-beam fully P-polarized and the other one having the half-wave plate rotated by 30 degrees. Qualitative pictures can already be calculated but we still need to calibrate the DC signal properly to calculate also a quantitative number.