Not that this is a surprise or anything, but I was noticing that the light on the OMC trans camera changes noticeably as the IFO thermalizes.
I've temporarily increased the exposure of the camera to 7000 (usually 569). You can't see much at 2W, but you start to see it at 20W and 35W - again not so surprising. What is perhaps more interesting is the way the light there changes after we've been at 50W for a while. The titles of the attachments include how long we were at each power. All images were taken during the same lock, so the 2 min at 50W image is after 5 min at 20W, and then 5 min at 35W, so it's not straight 2W->50W on this acquisition.
Since the original images are .tiffs, I've included a screenshot of all 6 images. Top row, left to right: 2W, 5min@20W, 5min@35W. Bottom row, left to right: 2min@50W, 6min@50W, 30+min@50W.
From the picture @2min 50W, I could see that this mode is probably one of the 9th-order modes.
I made the attached HOM map, based on the transverse mode spacing (TMS) of this OMC, measured during the final test.
It is likely that this 9th order mode is +9MHz sideband.
In order to try to push this mode away from the resonance, you can tune the DC voltage of the OMC PZT.
There must be two or three carrier TEM00 resonances in the OMC PZT range (0~100V) as one FSR corresponds to ~40V of the PZT voltage.
Choose the lowest voltage one. The DC voltage of the PZT changes the curvature of an OMC mirror and then the TMS by about 10ppm/V.
(Increasing the PZT voltage will not give us a good solution. It causes 10th -45MHz (cyan) mode comes into the resonance.)
You may already be at the lowest resonance, or the lowering the operating PZT voltage may not be enough to push the resonance away.
If this resonant HOM is still problematic, we need to reduce the leakage 9MHz sidebands from the interferometer.
[Sheila, Jenne]
We now lock the OMC on the carrier with the lowest PZT voltage possible, which is about 20V. We used to be locking with about 60V. It's not yet clear if this is enough of a change - we need to do some intensity noise coupling measurements with both lock points.
Here are some camera images though. Recall that in the last alog (29395) we spent some time at medium PSL powers before going up to 50W, so the total IFO thermal state isn't the same when we first arrive at 50W. The top row of images is from one lock, and the second row is from a different lock, although the times since arriving at 50W are accurate in the image titles. Top left is 2W DC Readout, then just after getting to 50W, then 2 min and 5 min at 50W. In a different lock the second row shows 35 and 45 minutes after arriving at 50W.
Posting Jenne's pictures for comment above
