Apparently I haven't aloged it last week, but oplevs recive the IR scatter from the arm. It's not a big deal, I'm writing this just for the record.
Attached shows one 7W lock when ETMY OPLEV was not working (no light). As soon as IR resonates in the arm, ETMY oplev segments jump up and the SUM goes to 560 counts (left and middle row). If the oplev laser was alive, this 560 counts would have been added on top of about 30k counts. Fortunately this is mostly common for all four segments, and the effect on the angle readout, when the oplev laser is alive, would have been about 0.03% of the full range, or about 0.03 urad.
The same thing happens to ETMX (right bottom) except that the oplev laser was alive and that the scatter increased the oplev SUM by only 360 counts.
For ITMs the SUM due to the arm scattering seems to be abount an order of magnitude smaller than ETMs, but the oplev power itself is also smaller (3000 to 4000 counts).
Along the same lines: quite a while ago during the DRMI locking, Jeff and I noticed that the BOSEMs of the HAM6 tip-tilts would pick up flashes from the DRMI lock acquisition that would generate spurious damping signals and shake the mirrors. This isn't a problem because we don't need those optics to be quiet before DRMI locks, but we thought it was interesting. I can't remember if the OMC SUS acquired some noise from the DRMI flashing.
After we noticed this, Jeff and I checked that it wasn't a problem for any of the corner-station mirrors that we don't actuate on to lock DRMI. We saw no evidence that DRMI flashing generated noise in the ITMs, PR2-3, or SR2-3. Of course it's impossible to tell if PR2, SR2, or BS OSEMs are affected by flashes since the LSC drive is banging on those optics when we're trying to acquire.