CH beam splitters replacement

In order to relax the depolarization problem due to the CH pick-up beam splitter and make the CH immediately available, yesterday we replaced the pick-up beam splitters of the CH telescope.

The available beam splitters are from ULO Optics, thickness is the same as the replaced II-VI beam splitters, the reflectivity is around 8% (instead
of 15% of II-VI).

The different wedge of the new BS caused misalignement of the beam wrt the AXICON on both benches, that we re-aligned. We also took the chance to do a better centering of the CH wrt to the ring on both benches.

In order to decouple the CH and ring powers, we moved the wave-plates
into their original positions, after the CH pick-up BS.
We re-calibrated the ring powers on both benches.

Pni(W)=5.12*Pr_TCS_NI_Mean-0.243
Pwi(W)=56*Pr_TCS_WI_Mean+0.684

We also calibrated the CH power as a function of the steps. Since the motors do not have end-stops, it is MANDATORY not to exceed
the reported values.

With the present set-up the maximum available power for the NI CH is 40mW and for the WI is 65mW. With 40mW of CH the image of the PC has a low SNR. We show in fig 1 and 2 an example of a previous measurement on the West IM (Logbook entry 21706) taken with 40mW and 80mW respectively, averaged over 10 sec.

Fig 3 and 4 show the rings with CH on for North and West benches respectively.

We found a problem with the North CH stepping
motor, solved by Cavalieri and Nenci (see Logbook
21948).

Next week, the two II-VI beam splitters will be characterized and if necessary they will be put back on the benches.