R. Short, J. Oberling

As we've been seeing the FSS RefCav TPD dropping over the past couple of weeks, this morning Jason and I did a proper tune-up of the RefCav alignment. We started by remotely aligning the beam into the RefCav from the control room using the two picomotor-controlled mirrors in the FSS path. With the ISS on and the IMC unlocked, the signal on the TPD started and ended at:

• Start: 0.69 V
• Finish: 0.76 V

Only seeing a slight improvement in the RefCav TPD signal with the picomotors, we then proceeded to head into the PSL enclosure to touch up the FSS path alignment. We started with a power budget on the FSS path (done with the ISS on and both PSL Guardians paused):

• FSS In: 300.0 mW
• AOM In: 295.5 mW
• AOM Out (Single Pass): 210.1 mW
  • Diffraction Efficiency: 71.1%
• AOM Out (Double Pass): 131.9 mW
  • Diffraction Efficiency: 62.8%
• EOM Out: 136.8 mW

The most obvious issue appeared to be the double pass through the AOM, as Jason had left the diffraction efficiency at around 73.8% after the last on-table FSS alignment, but we decided to start from the highest point upstream and make adjustments at each point. We adjusted WP05 to improve AOM IN, the AOM (mostly upwards in height) to improve single pass diffraction, and mirror M21 to improve double pass diffraction. Our results:

• AOM In: 300.0 W
• AOM Out (Single Pass): 223.7 mW
  • Diffraction Efficiency: 74.6%
• AOM Out (Double Pass): 158.1 mW
  • Diffraction Efficiency: 70.7%

Good improvements all around, and although neither diffraction efficiency ended as high as after the last alignment, with an unlimited maintenance window I suspect we would see more gains. We then checked the alignment through the EOM; seeing it was good with no clipping and measuring 158.4 mW out, we moved on to recovering the RefCav using the iris mounted in front of it. With the RefCav now locked, we touched up the alignment using the picomotor-controlled mirrors while watching the signal on the TPD:

• Start: 0.85 V
• Finish: 0.99 V

Excellent. Using a multimeter to watch the DC voltage, we then touched up the alignment onto the RefCav RFPD by adjusting mirror M25:

• Start: 1.210 V
• Finish: 1.316 V

To finish off our activities in the enclosure, we measured the RefCav's visibility:

• Unlocked: 1.316 V
• Locked: 0.220 V
• Visibility: 83.3%

After returning to the control room, we resumed the PSL Guardians, turned the ISS back on, and adjusted the ISS RefSignal to -1.97 V to keep the diffracted power around 2.5%. I have not accepted this value in SDF as it will likely need to be adjusted periodically this evening/tomorrow as the enclosure comes to thermal equilibrium. This closes WP 11537.