Keita, Sheila, Jenne, Evan, Hang
The mystery about AS 90 WFS may be (partially) due to the drift of dark offset.
We checked a time of today when IMC-MC2_TRANS_SUM was 0, and found that the output of a segment of AS 90 WFS was as large as ~100 cts when it was dark. As a comparison, the total sum of 4 segments at DC_readout was ~ 1000 cts. After corrected for it, the AS 90 WFS signal looked more reasonable.
We then checked the dark offset of the same WFS roughly days ago, and found that it drifted by ~ 100 ct (see, e.g., two seg1s in the attached plots). This large drift seemed make it not a very reliable sensor as the drift of dark offset was at least comparable to the signal we looked for. If we wanted to use it, we might have to check the offset frequently.
I looked at the dark offset drift of the AS 90 signals over several hours Monday night, when the PSL was shuttered. The dark offset for AS 90 signals jump around by a few tens of counts, even though the amount of light into the vacuum is not changing. Over the same period, the AS 36 and AS45 signals are stable to within one count. When we are at the Increase Power step of lock acquisition, the signals change from ~100 counts to several hundred counts when the power is changed from 2W to 8W, so these offset jumps are 10% or more of the signal size. Seems no good.
I only looked at the AS_A signals for this time stretch, but I have no reason to suspect that AS_B will be any different. Note that the last ~hour and a half of this time stretch the cleaning crew was working on getting HAM6 ready for the vent, so it's not surprising that we see offset changes, since we know that happens any time anyone is near the HAM6 racks.
