Danny, TJ, Georgia, TVo
Big ups to Sheila for running our measurement after ISC was finished with their work for the night.
Yesterday Georgia and Danny put in an iris on ITMX to crop out a prompt reflected beam from an in-chamber lens so that we can try to compare the spherical power on both ITMX and ITMY HWS when injecting 6 Watts of power into the ring heaters (3 top, 3 bottom):
Note that there is still a bit of clipping on ITMX on the top right corner. Using the results of a COMSOL model here, where it quotes
| Model Prediction |  |
| HWS ITMX Measured |  |
| HWS ITMY Measured |  |
From yesterday:
Danny installed an iris on the ITMY HWS path directly before the HWS camera.
This iris blocks a problematic stray beam that appears to be reflected off a surface between the viewport and the SR3 baffle.
Attaching screenshots of the camera image (with hartmann plate removed) before (attachment 1) and after (attachment 2) iris install. We were a bit concerned about the new fringes and any noise they might introduce. Note the two screenshots were taken with different lighting conditions (table door open/closed) so the intensity difference is not a concern.
For reference, here is how the ITMX HWS return beam looked back in 2014 when everything was first installed.
2014 versus 2018
..
And here's a view with the 2014 and 2018 beams overlapped. Roughly 50% of the HWS probe beam is clipped.
I think I've tracked down the source of the problem with the HWSX probe beam clipping. The issue stems from the fact that the new HWSX STEER M1 optical mount required the base to be moved. This was known and we aimed to keep the optic face in the same location. However, in placing the new optical mount, the wrong face has been kept in the same location - resulting in a displaced front surface.
We aligned the in-vacuum optics assuming the front surface had not moved(aLOG 39053). I'll need to investigate further to trace out the beams but this is almost certainly the cause of the clipping we're seeing.
The attached images show an overlay of two photos of the HWSX STEER M1 optic in 2014 (aLOG 12615) and 2018 (aLOG 39071)
FRS issue (https://services.ligo-la.caltech.edu/FRS/show_bug.cgi?id=11691)
After looking closely at the in-chamber photos, I've tried to estimate what the optical axis is doing. It should move in the -Y direction by ~7-10mm in the Hartmann Scraper Baffle.
If that's the case, then the beam size (at one beam radius) going through the aperture will look something like the following. The red beam is getting close to the edge of the aperture.
TJ took some photos of the in-vacuum optics with his phone and we can see relatively well along the optical axis of the HWS (although not with enough resolution to see the scraper baffle).
We'd like to try to do this with the chamber illuminated and a good SLR camera that is placed in the optical axis and focussed at the same distance as the scraper baffle. We should see a series of concentric circles and ellipses that are the apertures of all the optics and baffles. If, as I suspect, the HWS scraper baffle is now off center relative to the beam, it should be visible as such in this image.