For the upcoming ISS array swap, we plan to bypass the IMC, which is known to be a pain, but we need a stable beam for the array alignment.
Once the corner volume is vent, we use the QPD on the old array and IMC-IM4_TRANS as the initial reference for bypassing the IMC. Once IMC is bypassed, we will center REFL WFS BEFORE removing the old array and record the RM1/RM2 PIT and YAW. This way, even if we somehow suspect e.g. the pointing of the beam going to the IM4 moved after removing the old array, we can still restore the pointing by looking at the REFL WFSs in addition to IM4.
We measured the beam positions on these QPDs today even though we'll repeat this later.
IFO configuration:
- IMC was locked with 2W.
- REFL beam diverter was closed so no light comes out. (POP bdv was also closed just because.)
- The cover on the REFL septum window was removed.
- PRM, all IMs and RMs are aligned. (Watchdog of RMs were reset.) ITMs are misaligned.
Arrow ("->") means before and after the REFL centering servo (DC1 and DC2) was turned ON:
| PSL-ISS_SECONDLOOP_QPD | IMC-IM4_TRANS | ASC-REFL_A_DC | ASC-REFL_B_DC | SUS-RM1-M1_DAMP INMON | SUS-RM1-M1_DAMP INMON | |
| PIT | -0.814 | 0.366 | -0.884 -> 0 | -0.995 -> 0 | 293 -> 168 | -363 -> 68 |
| YAW | 0.655 | -0.146 | 0.590 -> 0 | 0.220 -> 0 | -176 -> -214 | 277 -> -70 |
Septum cover is left OFF but the PSL light pipe was closed after this. REFL centering was turned off but I didn't bother to offload the ASC output to RM sliders.
REFL WFS nubmers as of now are not super meaningful as we'll still have to lock HAM2 down, which potentially change the relative alignment between HAM1 and HAM2. (But it's good that the beam is still hitting REFL WFSs after HAM1 was locked down even though Jim noted that the ISI position of HAM1 is not good. )
I'll open the light pipe tomorrow and quickly repeat the measurement after Jim locks down HAM2 HEPI.
Jim locked HAM2 HEPI today. I opened the PSL light pipe and locked IMC, and the beam was already reasonable on REFL WFSs without centering servo.
I'm convinced at this point that Jim does a good job that the angle change won't be large enough to lose the beam in HAM1 even after Jim locks down HAM2 in air. We will very likely find the beam on REFL WFSs after bypassing the IMC using ISS array QPD and IM4 trans.
As before, "->" means before and after the refl centering was turned ON.
| ASC-REFL_A_DC | ASC-REFL_B_DC | SUS-RM1_M1_DAMP INMON | SUS-RM2_M1_DAMP INMON | |
| PIT | 0 -> 0 | -0.39 -> 0 | 190 -> 174 | 108 -> 14 |
| YAW | 0 -> 0 | 0.07 -> 0 | -210 -> -214 | -69 -> -59 |
After this,
- Light pipe was closed.
- Septum cover is left OFF.
- IMC was taken offline.
- Centering servo was turned off (again no offloading to the RM sliders).