A decrease of OgCAVITES is often associated to a decrease of sensitivity. Starting from thurday 21 in the evening and in the following hours, such a situation was quite clear. In fig 1 we can see the Range_BNS going down together with OgCAVITIES, while other possible sources of problems where more stable (OgPR, microseism). The difference visible in the sensitivity at the beginning and at the end of that period was relevant below 100 Hz (fig. 2), not the typical situation of a 'flat noise' growing up. This is something already observed during O3a, and reported in the logbook at least twice (but I'm not able to find the entries). I tried to fit the difference, and I found something going as 1/f^1.3 (fig 3). Looking at band-rms of hrec in some clean region below 100 Hz, one can see that there was a gradual worsening for each of them (fig 4).
Some statistics over O3b data have been done, taking into account only locks in stable conditions (at least 1 hours and half after the beginning, excluding also the glitches).
Fig 5: Range_BNS vs OgCAVITIES. Low OgCAVITIES often corresponds to low Range
FIg 6: OgCAVITIES vs NI temperature. The correlation is quite good. As already known, around 19.57 C there is a good working point, corresponding to low finesse of North Arm.
Fig 7: OgCAVITIES vs WI temperature. Good correlation even there. The good working point 19.88 C is confirmed (corresponding to high finesse of West Arm).
Fig 8: OgCAVITIES vs difference of Arm Finesse (roughly computed, to be checked).