I've been doing some investigation into the ETMY HWS measurements. The HWS shows negative lensing when the IFO is locked. However, this doesn't seem very physical. I had 4 working hypotheses as to what might be going on:
- We're not imaging the ETM HR surface correctly onto the HWS
- The TMS telescope is heating up from absorption on the secondary mirror
- There is somehow cross-coupling from optic alignment changes
- It's real distortion from an absorption ring about 100mm in diameter.
Without going into a lot of detail on these, I'm pretty convinced that the issue is Number 3: Cross-coupling from absorption. The main reasons for this is that he HWS spherical power and OPLEV PIT/YAW signals are highly correlated. Also, a linear change of -200 micro diopters in defocus over 7 minutes followed by a sudden stop is not remotely physical for a thermal effect in glass.
Why there is such a strong coupling for ETMY HWS and whether or not it can be removed (optically or digitally) is still TBD.
Correlation of HWS SPHERICAL power PIT/YAW signals:
HWS wavefront from above time series: comparing t = 11.3 minutes to t=22.7 minutes
For LHO ETMY HWS, there is a direct correlation between the spherical power and the total intensity on all pixels on the camera.
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H1:TCS-ETMY_HWS_PROBE_TOTAL_PIXEL_VALUE
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H1:TCS-ETMY_HWS_PROBE_SPHERICAL_POWER
The attached plots start from t=1256708990
The next step is to determine a baseline for how much we expect this to be, i.e how much this exists for other ETM HWS (do we see excess intensity on the HWS when powering up those arms in the IFO).
Non-power up coupling:
My analysis of an earlier spherical power jump (at t0=1256521193 and unassociated with a power-up) suggested that intensity and spherical power might be linked here.
