Jennie W, Sheila

Today I got a chance to redo some of my output chain measurements (alog #82555) that gave us a confusing result when we were heating up and cooling down the OM2 heater. The confusiong was that our optical gain got better for cold OM2 compared to hot, but the loss through HAM6 predicted by stepping the DARM offset and comparing it to the power at the anitsymmetric port predicted that the loss was worse with cold OM2 and gave us unreasonably low (~65 %) throughput estimate for the fundamental TM00 mode through HAM6. We realised that the OM3 and OMC alignment are being changed by the ASC during this time so that could be affecting the comparison.

Steps:

Turn off OMC ASC at 18:43:00 UTC.

Run auto_darm_offset_step.py from /ligo/gitcommon/labutils/darm_offset_step at 18:45:01 UTC.

Results: P_AS = 62.996 mW + 1.162 * P_DCPD

This makes the throughput estimate for the TM00 mode through HAM6 to the DCPDs to be 86.0 % of the power at the AS port. Which seems more reasonable than the 65% we got last time. We need to do this measurements again when we have improved kappa C somehow to check it gives us the same loss estimate between two times.

We were then going to purposely change kappa C as a comparison to the estimate the DARM offset step gives us, byt changing the QPD offsets based on this measurement 82383(I got the sign wrong last time I did this).

I turned the OMC ASC back on before doing this, however when I changed the H1:ASC-OMC_A_PIT_OFFSET to 0.45 this saturated the OM3 suspension so this was set back to nominal.

I think I got signs wrong yet again so we will try what we now think are the correct ones another commissioning period but maybe do it slowly so as not to cause saturations.