I reset the ITMY HWS magnification to 7.5x (it's set to 17.5x by default in the code).

aidan.brooks@opsws4:~$ caput H1:TCS-ITMY_HWS_MAGNIFICATION 7.5

Old : H1:TCS-ITMY_HWS_MAGNIFICATION  17.5

New : H1:TCS-ITMY_HWS_MAGNIFICATION  7.5

Alos, I had a quick look at the change in the spherical power measured by the ITMY HWS after the IFO lost lock this morning.

1. The measured change in defocus was, very roughly, 0.8 micro-diopters. (over about 40 minutes taken from immediately after 1141749205)
2. Around 95% of the lens forms/decays in this time. 
3. The magnification was set to 17.5 instead of 7.5, so we need to multiply the spherical power by (17.5/7.5)^2 = 5.44x
4. The steady-state, double-passed, lens measured by the HWS is then 8E-7/0.95*5.44 = 4.6E-6 diopters
5. The single-pass, steady-state, lens is then 2.3E-6 diopters
6. The single-pass, steady-state, lens gain (diopters per Watt absorbed) is 4.87E-4 D/W for self absorption
7. Therefore I calculate about 4.7mW absorbed power in ITMY
8. The arm power was around 11.9kW during this time
9. Hence, the absorption is (very roughly) 4E-7

This is a very rough calculation - and assumes that the HWS-Y beam is correctly aligned (we need to combine our evidence to really confirm this).

Nevertheless, I've set the ITMY absorption to 4E-7 in the simulation.

aidan.brooks@opsws4:~$ caput H1:TCS-SIM_ITMY_SURF_ABSORPTION 4E-7

Old : H1:TCS-SIM_ITMY_SURF_ABSORPTION 4e-09

New : H1:TCS-SIM_ITMY_SURF_ABSORPTION 4e-07

For comparison, ITMX absorption estimated to be 5.7E-7.