Sudarshan, Gabriele
We significantly improved the beam centering on the ISS array: now we have good powers on all diodes, centered QPD and lower coupling of beam motion to dP/P
The procedure
- We wrote a python script that uses the picomotor 1 to maintain the beam centered on the QPD.
- We moved picomotor 8, mainly in the LEFT direction. After each motion, we ran the script to recenter the beam on the QPD. We then checked that we were improving the power levels.
After a lot of steps in the LEFT direction and few in the UP direction (for picomotor 8), we could get the beam centered on the QPD and good power levels on all diodes. Actually, we believe we are quite close to the maximum power for each diode.
The results
The following table compares the power levels (in counts) before our adjustment and at the end.
| Photodiode | Power before [cts.] | Power after [cts.] |
| CH24 - PD1 | 460 | 470 |
| CH25 - PD2 | 500 | 507 |
| CH26 - PD3 | 510 | 505 |
| CH27 - PD4 | 560 | 563 |
| CH28 - PD5 | 535 | 554 |
| CH29 - PD6 | 460 | 520 |
| CH30 - PD7 | 550 | 612 |
| CH31 - PD8 | 530 | 564 |
Then, we measured again the coupling of beam motion to dP/P, and we got much improved numbers:
| Photodiode | dP/P/dx PITCH [1/m] | dP/P/dx YAW [1/m] |
| CH24 - PD1 | < 30 | < 90 |
| CH25 - PD2 | 130 | 430 |
| CH26 - PD3 | < 30 | < 60 |
| CH27 - PD4 | 40 | 180 |
| CH28 - PD5 | 310 | 90 |
| CH29 - PD6 | 60 | 100 |
| CH30 - PD7 | < 45 | < 50 |
| CH31 - PD8 | < 50 | < 50 |
At this level it is difficult to find a better position looking only at the power levels. We might have to optmize the centering looking directly at the beam motion coupling.