TLDR: CO2X looks misaligned on the HWS and phase camera images. Whether we want to realign it is up for discussion as CO2X is kept on during lock. This was because it helped RF and carrier builds up at the time.

Over the weekend with DRMI we were doing some phase camera tests. We decided to have a look at what effect the CO2s would have on the sidebands and see if what the phase camera is outputting makes sense. We had the phase camera running to take pictures of all the sidebands every 20s. We then dropped the power of CO2Y to 0W left if for 20 mins, increased it to 2W, waited, then returned it to nominal. We repeated the same with CO2X. We had DRMI ASC runnning during this process.

Looking at the raw phase camera images isn't particularly enlightening, unless something is completely wrong. One aspect that needs to be worked on is how we process and display the phase camera data in a usable manner. As we have both the amplitude and phase of each sideband we figured the best idea to try first is do a higher-order Hermite gaussian decomposition and analyse the change in mode content of each field. Ideally we would do this decomposition in the PRC mode propagated to the phase cameras position, however we don't actually know what that is in practice. Instead we just have to pick some beam basis that is something not too dissimilar to the beams we are seeing. I did a rough least-square fit for a gaussian HG00 to the data and got beam parameter values of:

X: w0=0.001362, z=6.688
Y: w0=0.001478, z=8.384

so the beam basis we are using here is slightly astigmatic. This doesn't necessarily mean the PRC mode is astigmatic, just that the beam at the phase camera is. We then run a decomposition of the 9 and 45 fields, plotted is the 1st and 2nd order mode changes to start with. Shown is that the 2nd order mode content of the beam varies as we change the CO2 power, as we'd expect. CO2Y has an ideal response, all we see is 2nd order changes, pretty much equal in amount between X and Y, so CO2Y looks well aligned.

However, CO2X looks more problematic. It induced both a 1st and 2nd order mode content change. It moves the fields diagonally and also creates a more astigmatic field. An image of the difference in shown here at the peak of the CO2X effect. The shape change (at the phase camera position) is mostly in the radius of curvature of the field and the misalignment mostly translation.

Averaging the upper and lower sideband I compare the 9 and 45 mode changes in figure. We can resolve a different shape between the 9 and 45 sidebands. The sidebands experience a different misalignment when CO2X is changed, the X displacement (HG10) change is different between the two. We can also see the lensing experienced in X is also different between the sidebands.

For reference I looked back at the HWSX images during this change. Not much can be seen from the weekend images. Comparing the CO2X switch on a week or so ago (with ring heaters off), we get this image. Comparing this to the cooldown of the IFO at the start of the commissioning break, we can see they not well centered. Looking at a power up at the end of September a dead pixel is causing an issue, but we can kind of see a deformation centered around 500, 400. Either way these don't look well aligned.