To see if the OM2/beckhoff coupling is a direct electronics coupling or not, we've done A-B-A test while the fast shutter was closed (no meaningful light on the DCPD).
State A (should be quiet): 2023 Oct/10 15:18:30 UTC - 16:48:00 UTC. The same as the last observing mode. No electrical connection from any pin of the Beckhoff cable to the OM2 heater driver chassis. Heater drive voltage is supplied by the portable voltage reference.
State B (might be noisy): 16:50:00 UTC - 18:21:00 UTC. The cable is directly connected to the OM2 heater driver chassis.
State A (should be quiet): 18:23:00- 19:19:30 UTC or so.
DetChar, please directly look at H1:OMC-DCPD_SUM_OUT_DQ to find combs.
It seems that even if the shutter is closed, once in a while very small amount of light reaches DCPDs (green and red arrows in the first attachment). One of them (red arrow) lasted long and we don't know what was going on there. One of the short glitches was caused by BS momentarilly kicked (cyan arrow) and scattered light in HAM6 somehow reached DCPDs, but I couldn't find other glitches that exactly coincided with optics motion or IMC locked/unlocked.
To give you a sense of how bad (or not) these glitches are, 2nd attachment shows the DCPD spectrum of a quiet time in the first State A period (green), strange glitchy period indicated by the red arrow in the first attachment (blue), a quiet time in State B (red) and during the observing time (black, not corrected for the loop).
FYI, right now we're back to State A (should be quiet). Next Tuesday I'll inject something to thermistors in chamber. BTW 785 was moved in front of the HAM6 rack though it's powered off and not connected to anything.
I checked H1:OMC-DCPD_SUM_OUT_DQ and don't see the comb in any of the three listed intervals (neither state A nor B). Tested with a couple of SFT lengths (900s and 1800s) in each case.
Since it seems that the coupling is NOT a direct electronics coupling from Beckhoff -> OM2 -> DCPD, we fully connected the Beckhoff cable to the OM2 heater driver chassis and locked the OMC to the shoulder with an X single bounce beam (~20mA DCPD_SUM, not 40mA like in the usual nominal low noise state). That way, if the Beckhoff is somehow coupling to OMC PZT that might cause visible combs in the DCPD.
We didn't see the comb in this configuration. See the 1st attachment, red is the shoulder lock and green is when 1.66Hz comb was visible with the full IFO (the same time reported by Ansel in alog 73000), showing just two largest peaks of 1.66Hz harmonics visible in the green trace. (It seems that the 277.41Hz and 279.07 Hz peak are 167th and 168th harmonics of 1.66Hz.) Anyway, because of the higher noise floor, even if the combs are there we couldn't have seen these peaks. We've had a different comb spacing since then (alog 73028) but anyway I don't see anything at around 280Hz. FYI I used 2048 FFTs for both, red is a single FFT and the green is an average of 6. This is w/o any normalization (like RIN).
In the top panel of 2nd attachment, red is the RIN of OMC-DCPD_SUM_OUT_DQ of the shoulder lock, blue and dark green are RIN of 2nd loop in- and out-of-loop sensor array. Magenta, cyan and blue green are the same set of signals when H1 was in observing last night. Bottom panel shows coherence between DCPD_SUM during the shoulder lock and ISS sensors as well as IMC_F, which just means that there's no coherence except for high kHz.
If you look at Georgia's length noise spectrum from 2019 (alog 47286), you'll see that it's not totally dissimilar to our 2nd plot top panel even though Georgia's measurement used dither lock data. Daniel points out that a low-Q peak at around 1000Hz is a mechanical resonance of OMC structure causing the real length noise.
Configurations: H1:IMC-PWR_IN~25.2W. ISS 2nd loop is on. Single bounce X beam. DCPD_SUM peaked at about 38mW when the length offset was scanned, and the lock point was set to the middle (i.e. 19mA). DC pointing loops using AS WFS DC (DC3 and DC4) were on. OMC QPD loops were not ON (it was enabled at first but was disabled by the guardian at some point before we started the measurement). We were in this state from Oct/17/2023 18:12:00 - 19:17:20 UTC.
BTW Beckhoff cable is still fully connected to the OM2 heater driver chassis. This is the first observation data with such configuration after Fil worked on the grounding of Beckhoff chassis (alog 73233).
Detchar, please find the comb in the obs mode data starting Oct/17/2023 22:33:40 UTC.
The comb indeed re-appeared after 22:33 UTC on 10/17. I've attached one of the Fscan daily spectrograms (1st figure); you can see it appear in the upper right corner, around 280 Hz as usual at the start of the lock stretch.
Two other notes:
- The comb is now back to its original spacing of 1.6611 Hz.
- There are new strong lines visible at +/- 1.235 Hz from the comb teeth. If this structure has appeared before, I'm not aware of it. I've attached an image showing the lines (2nd figure, average of DMT-ANALYSIS_READY data between 22:00 10/17 and 02:00 10/18) and a comparison with older data from Sept 20th (3rd figure).
Just to see if anything changes, I used the switchable breakout board at the back of the OM2 heater driver chassis to break the thermistor connections but kept the heater driver input coming from the Beckhoff. The only two pins that are conducting are pins 6 and 19.
That happened at around Oct/18/2023 20:18:00 to 20:19-something UTC when others were doing the commissioning measurements.
Detchar, please look at the data once the commissioning activities are over for today.
Because there was an elevated noise floor in the data from Oct/17/2023 18:12:00 mentioned in Keita's previous comment, there was some doubt as to whether the comb would have been visible even if it were present. To check this, we did a direct comparison with a slightly later time when the comb was definitely present & visible. The first figure shows an hour of OMC-DCPD_SUM_OUT_DQ data starting at UTC 00:00 on 10/18 (comparison time with visible comb). Blue and yellow points indicate the comb and its +/-1.235 Hz sidebands. The second figure shows the time period of interest starting 18:12 on 10/17, with identical averaging/plotting parameters (1800s SFTs with 50% overlap, no normalization applied so that amplitudes can be compared) and identical frequencies marked. If it were present with equivalent strength, it looks like the comb ought to have been visible in the time period of interest despite the elevated noise floor. So this supports the conclusion that the comb *not* present in the 10/17 18:12 data.
Following up, here's about 4 hours of DELTAL_EXTERNAL after Oct 18 22:00. So this is after Keita left only the heater driver input connected to the Beckhoff on Oct/18/2023 20:18:00. The comb is gone in this configuration.
