Jenne and I reviewed the ISS status this morning. Jenne noticed that the RIN of the 2nd loop out-of-loop PD array shows increased coherence between 7~30Hz. I continued to investigate the comparison of the ISS state between now (6/8 19:00 UTC) and May 1st (5/1 1:00 UTC).
The 2nd loop out-of-loop floor level shows the stability of 1e-8 [1/rtHz] for both cases. However, the stability below 30Hz is worse than the one on May 1st. (Attachment 1. The blue curves are the current, and the red curves are the ones on May 1st.)
Attachment 2 shows the ISS QPD pointing. The yaw shows the shift from 0 to +0.3 while the pitch stayed ~-0.7. (Is that Okay?)
The 2nd loop PDs (inner/outer) exhibited 5~7% decrease, while the input power was also decreased 5%. So there looks some real decrease of the incident power on the PD exist. (Note tthat the QPD sum decreased -6.5%).
Can we change the features in the power spectrum by realigning the ISS PDs?
Rick sent me a couple of reference alogs regarding the ISS QPD centering:
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=30915
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=29583
It seems that the last ISS QPD centering was done on Oct 27, 2016. (Attachment 1)
I've checked the ISS QPD pointing since this last centering (Attachment 2). The pointing gradually drifted out from the center in the pitch diection during the first two months. There was no yaw drift. After the vent, there was no big change in pitch, but was in yaw.
I took the in-loop/out-of-loop spectra of the invac ISS array outputs right after the last centering (Attachment 3). The spectra were basically the same as the ones seen before the vent, and thus were better than the current level.
Because there was no intentional offset on the QPD pointing, we should try to center the beam on the ISS QPD (in the next possible occasion) to see if this helps to reduce the residual intensity noise at the low freq band.
