Today Sheila realigned the beam on IM4 trans QPD (81735). This beam has been clipped at least since the O4 break due to various reasons (in this particular alog we tried to fix it, but then undid the fix, I'm adding here so we have a general time reference for when we first noticed the problem: 76291). As a result, the PRG calibration for O4b has been fishy, since it is normalized by the IM4 trans QPD power. Today, with the beam properly centered, I checked the PRG calibration to make sure it's still good.
Craig did this after ITMY was replaced: 58327. I followed his same steps with help from Ryan S and Sheila. First, we ran initial alignment on the y arm, which uses the green camera references. We waited until that alignment was converged and then stepped through the input_align_yarm states in the ALIGN_IFO guardian. I recorded the IM4 trans value as well as the y arm transmission value.
When we run a normal initial alignment, we lock input align with the x arm, so I used that opportunity to similarly record the x arm transmission value and IM4 trans. As a note: the green input alignment references are set when we converge the full IFO ASC, therefore I believe the single arm alignment after we run the green initial alignment is probably very close to the full IFO alignment.
Then, we paused in DARM_TO_DC_READOUT in the ISC LOCK guardian; this is just after all the full IFO ASC converges at 2W. I grabbed the x and y arm transmission values and IM4 trans.
Channels: H1:IMC-IM4_TRANS_NSUM_OUT16, H1:LSC-TR_X_NORM_OUT16, H1:LSC_TR_Y_NORM_OUT16
Single arm lock values:
Y-arm trans: 0.91, IM4 trans: 1.95
X-arm trans: 1.01, IM4 trans: 1.98
Full lock values:
Y-arm trans: 1684
X-arm trans: 1708
IM4 trans: 1.98
Calculation:
PRM transmission = 0.031
PRG = Tp * Y-arm trans (full ifo) / Y-arm trans (yarm only) * Input power (yarm only) / Input power (full ifo) (copied directly from Craig's alog)
PRG from Y arm = 56.5
PRG from X arm = 54.0
PRG reported from H1:LSC-PR_GAIN_OUT16 at the 2W lock time: 54
Therefore, I think our PRG calibration is correct. We can begin to cultivate a good O4b power up data set for modeling.