There's a comb visible in recent DARM data at multiples of 0.996795 Hz (last digit uncertain). This is suspiciously similar to a comb seen before O2 (then measured at 0.996798 Hz), which was due to the HWS. Just as before, the comb appears strongly in corner station magnetometer channels.
I've attached an image showing teeth of the comb below 100 Hz (Oct 24, ~50 minutes starting at 9:30 UTC). It is also visible in other lock stretches within the past few weeks. When this comb was seen before, it coupled only intermittently to DARM; its presence seems steadier now.
Relevant past alogs:
- September 2016 post on comb updates (see first heading, and second comment)
- HWS cameras at X and Y were placed on a separate power supply October 25, 2016. This got rid of the comb in magnetometer channels, and it wasn't seen in DARM again until now, as far as I can tell.
- Potentially related recent thread about 57 Hz comb with an HWS connection
Hey Ansel,
The Hartmann camera sync frequency is set to 1Hz for both cameras at the vertex.
Hi Daniel, Right, some of us were wondering in an email thread if that frequency could be set to something different for a period of time, ideally with DARM in low noise, but that's not essential for things to be seen in magnetometers. thanks, Keith
Keith and Ansel,
I adjusted the ITMY and ITMX HWS sync frequency to 55 Hz from gps 1224538388 to 1224539253. Please don't hesitate to let me know if you need anything else related.
Yes, this couples very strongly into DARM. A line at 54.8ish Hz appears with the change (spectrum attached).
Hi Daniel - yes, that definitely had an effect. I checked both DARM, and also the magnetometer channel where the comb was showing up most strongly. The comb disappears in both channels during the time segment you indicated, and reappears afterward.
Indeed this is a known problem with the HWS cameras (FRS 4559). And the problem was solved by placing the cameras on alternate power supplies in 2016. I suspect that following the post-O2 disconnecting and reconnecting of the HWS table and cables, the cameras got plugged into the chassis power supply again. We should do the following for all 8 HWS:
- Confirm that the HWS cameras are plugged into the chassis. If so,
- Get the plugged into a separate power supply, and
- Lock out the chassis power supply to the cameras to prevent plugging them back in there in future, and
- After consultation with Rana, look at getting them plugged more permanently into a dirty power supply (like the one for the vacuum system) and check they're properly isolated from the clean power
- Also - enable EPICS control & read-back of the camera frame rate
This should be done for the ETMs as well (where the 57Hz is an issue). The original isolation fix does not appear to be robust enough.
- Mitigation of 1Hz at LLO ITM HWS: https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=31339
- ITM HWS ground isolation at LLO: https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=30350
- LHO appearance of 57Hz ETM HWS lines: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=44495
- Reappearance of lines from LLO ETM HWS: https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=41248
Block diagrams (DCC is down so I can't get hyperlinks right now)
- Corner station: E1100892
- End stations: E1100891