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.

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).

Today I have produced some data with different exposure times for the end cameras, to be compared to the equivalent data acquired before the replacement of the WE mirror. The GPS stretchs are below and will be analyzed later on, for the moment we qualitatively evaluated the WE mirror to have a brighter spot wrt the NE, apparently due to more intense scattering from surface of the mirror.

First plot: ITF in LN3, powers circulating in the arms

Second plot: pictures of NE / WE mirrors - exposure time 200us for both.

GPS and exposure times below:

1429718937  26 Apr 2025 16:08:39 UTC  (26 Apr 2025 18:08:39 CEST)
NE/WE 30us for 60s

1429719018  26 Apr 2025 16:10:00 UTC  (26 Apr 2025 18:10:00 CEST)
NE/WE 50us for 60s

1429719098  26 Apr 2025 16:11:20 UTC  (26 Apr 2025 18:11:20 CEST)
NE/WE 55us for 60s

1429719179  26 Apr 2025 16:12:41 UTC  (26 Apr 2025 18:12:41 CEST)
NE/WE 100us for 60s

1429719268  26 Apr 2025 16:14:10 UTC  (26 Apr 2025 18:14:10 CEST)
NE/WE 150us for 60s

1429719353  26 Apr 2025 16:15:35 UTC  (26 Apr 2025 18:15:35 CEST)
NE/WE 200us for 60s

1429719455  26 Apr 2025 16:17:17 UTC  (26 Apr 2025 18:17:17 CEST)
NE/WE 250us for 60s

1429719541  26 Apr 2025 16:18:43 UTC  (26 Apr 2025 18:18:43 CEST)
NE/WE 450us for 60s

1429719646  26 Apr 2025 16:20:28 UTC  (26 Apr 2025 18:20:28 CEST)
NE/WE 5000us for 60s

1429719732  26 Apr 2025 16:21:54 UTC  (26 Apr 2025 18:21:54 CEST)
NE/WE 15000us for 60s

1429719832  26 Apr 2025 16:23:34 UTC  (26 Apr 2025 18:23:34 CEST)
NE/WE back to 1800us

Michal noticed that part of the mounting screws for the inner baffles (ath the bottom of the first image) got illuminated when the beam on the WE is not offset with respect to the center, so the question was if this could be due to stray light coming from the WI or it is something due to "local" source of stray light.

To answer to this question, the clearance of the payload baffles has been examined.

Pictures 2,3 are taken from the executive drawing of the payload baffles, arm side (pic 2: outer baffle, pic 3: inner baffle).

The inner radius of outer baffle is 238.5 mm, while the radius of the crcle where the centers of the mounting screws for the inner baffle are is rout_inn = 485 mm / 2 = 242.5 mm. But the holes have a radius of 4.5 mm, so the very inner part of these holes is on a circle of radius r = 238 mm.

This means that a small part of each hole (a circular segment of height 500 um) is indeed exposed to the light coming from the WI mirror.

Overall this does not seem to represent a problem for the stray light coupling (the involved area is very small) but does indicate the presence of additional small angle scattering after the replacement of the WE mirror.