Jenne, Kiwamu,

We have spent some time today trying to fix the long-standing issue with MICH_DARK (which started on 20th of December as far as I know, alog 24349) because we needed to fix it for a calibration measurement scheduled in this week.

We finally fixed it and now MICH_DARK locks.

The reason why it has been not locking seems to be due to too high sensing noise in ASAIR_A which kept saturating the beam splitter's DACs (assuming that the digital filters and DACs of the beam splitter have been unchanged). I have checked the whitening filters and gain of ASAIR_A, but they have been unchanged at least in the past 4 months or so. I don't have quantitative comparison of the noise floor back then and now. So at this point we have no idea why the sensing noise increased.

Anyways, we have inserted two extra low-passes in order to shave off sensing noise above the unity gain frequency (which is typically somewhere between 4 and 10 Hz). These low-passes fixed the issue. I have not changed any foton filters; I have recycled unused filters. The first filter resides in LSC-MICH FM5, and the second in BS_ISCINF FM6.  In addition, I adjusted the gains of the MICH loop so that it smoothly grabs a fringe. Now the initial acquisition gain is -333 and the final one is -500. We have edited the following guardian codes, both of which are checked into svn: ALIGN_IFO.py, lscparams.py and ISC_DRMI.py. The guardian was tested 5 or 6 times with the new settings and we have not seen a failure yet.