Virgo Logbook

Environmental Monitoring (Environmental Monitoring)

Tringali, Spinicelli, Chiummo, Fiori, Gargiulo - 16:06 Thursday 17 April 2025 (66579)

WE scattering source inspection

Given the scattered light issues at WE (64481), we performed some studies before the mirror replacement.

At first, before the opening of the tower, we acquired the IR images from the NI/WI and NE/WE mirror cameras in order to have a light scattering reference. The acquisition was carried out by varying the integration times during the several steps of the lock acquisition (CARM_NULL_1F, Locked_ARMS, and LN3) being careful to not saturate the camera. The gps times are reported in the attached files. Those acquisitions will be repeated after the recovery of the lock acquisition.

During the opening of the WE tower for the mirror replacement, we performed some visual inspections inside the tower, taking some pictures and gently tapping on some of the objects that could be considered potential sources of scattering:

basement and baffle of the TCS MIR 2 and 3
baffle installed inside the tower - HR mirror side (Figure 1 - green baffle)
baffle located within the cylindrical pipe (Figure 1 - orange baffle)

Overall, no significant issues were identified during the inspection, except some small vibration on the front tower baffle (see two attached videos) and for the absence of the tower baffle on the AR side of the mirror.

In Fig. 2 we have a picture of the front baffle and the closed valve, taken with a visible reflex camera. In Fig. 3 and 4 we have the same picture taken with an IR reflex camera and respectively with and without a 900-1350nm band-pass filter (thorlabs FGB67M). Lightening has been produced by a commercial flash (Sony HVL-F32M). Any particular hot spot has been detected.

Images attached to this report

Non-image files attached to this report

Comments to this report:

chiummo - 15:59 Thursday 24 April 2025 (66617)

The intervention to replace the WE mirror and the replacement of the uncoated payload baffles with the nominal AR-coated baffles has had an improvement on the stray light we can spot with the tower camera.

In the first picture, the situation just before the intervention, with the WE showing some additional brigth spots with respect to the NE.

In the second picture, taken after the recovery of the ITF up to CARM_NULL, the images from the two payload look much more similar to each other (with comparable circulating power situation, see third picture).

A more quantitative analysis could probably be done by adjusting the integration time of the cameras so to avoid saturation, to be possibly scheduled.

It can also be seen that, despite the disappearance of the bright spot on the bottom left of the image of the WE, the markers on the WE payload still look brighter than the equivalent on the NE (and also brighter than before!). It could be useful to swap the cameras to check whether it is related to some differences in the senstivity or it is a true effect of some larger scattering off the NI mirror wrt the WI for instance. To be also re-assessed after the tuning of the ring heaters is complete.

Images attached to this comment

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chiummo, mwas - 8:25 Friday 25 April 2025 (66624)

There is a mistake in the reference picture for the WE camera. As correctly pointed out by Michal, the reported picture of the WE pay cam refers to a time when the integration time of the camera was already decreased down to 450us (from the original 1800us). The correct picture is attached here, along with the circulating power comparison.

I took also a comparison of the illumination of the markers as seen from the camera (second picture: purple is the reference before WE intervention - Apr 3 15:30 UTC, blue is after the intervention). I tried to get a zero reference (third picture) which gives WE_MIR_DR_0 = 26500 a.u., and WE_MIR_UR_0 = 17800 a.u.. This yelds a variation: WE_MIR_DR_2 = 30500 - 26500 = 4000; WE_MIR_UR_2 = 20700 - 17800 = 2700 (after intervention) while WE_MIR_DR_1 =37300 - 26500 = 10800; WE_MIR_UR_1 = 21700 - 17800 = 3900 (before intervention). When accounting for the difference of circulating power in the west arm we have:

WE_DR2 = WE_MIR_DR_2/WE_pwr2 = 8.8; WE_UR2 = WE_MIR_UR_2/WE_pwr2 = 5.9;
WE_DR1 = WE_MIR_DR_1/WE_pwr1 = 22.2; WE_UR1 = WE_MIR_UR_1/WE_pwr1 = 8.0;

The conclusions are that the intervention on the WE payload (mirror + baffle replacement) have for sure removed the bright spot on the bottom left (possibly a spurious reflection of the viewport illuminated by a lot of stray light) and also some reduction on the illumination of the markers of the baffles (to be reassessed after the tuning of the RH is complete).

As a side note, the powers in the arms appear to be more unbalanced than before, but I guess that this also has to be reassessed after the work of recovery is complete.

(I attach here also the picture of the camera after the intervention for completeness, fourth picture).

Images attached to this comment