--- This is a backpost from last week ---

The FSS and PLL are now pretty much locked all the time with the last reported settings (see PLL:1991). The PLL and FSS have all remained in locked state for periods of over 48 hours.

Craig and I had a first look at the scatter noise shelf with a white noise PZT buzzer to see where where the worst offending sources of parasitic interferometers would be.  The periscopes and around the PDs seem like the most likely points from our initial tests.

There was a lot of foam dust on the upward facing periscope mirrors (see pictures PSL:1980) and on the final MM mirrors before the ref cavs.  We gave these a clean as an initial test.  The north path 77.3 mm ROC mirror directly before the periscope has a permanent burn mark in the center: see picture below.  I tried a gentle wipe with methanol and then a little more of an aggressive scrub and it wont budge. Craig has cleaned another of the the same lenses and this is ready to switch in next time we need to do a serious realignment of the north cavity. We're leaving it for now as we want to get onto other issues like scatter in the transmission PLL beat path.

I took a reference PLL spectrum attached in the noise budget plot below. There is a new broad 2 kHz bump that we are trying to work out the source of. Previously tara had speculated that this was RFAM (PSL:1311) and Evan had speculated that it was FSS induced noise (PSL:1526). We re optimized the 14.75 MHz RF EOMs polariaziations in both paths reducing North to 80 dBm/sqrtHz and south to 90 dBm/sqrtHz. This didn't seem to be as good as some of my previous reported residual AM, its not clear why North was so difficult to reduce this time; previously North was optimizable down to the noise floor of the PD RF dark noise.  The 2 kHz hump is still there.  It might well be an artifact of the scatter.