Last night Jeff and Corey had difficulties with large pitch motion of the test masses.
This is described in a couple of alogs from Corey, and can easily be seen on the summary pages
- In the attached screenshot you can see that typical of our CSOFT/spot position/dPdtheta instability, the circulating powers are all oscillating, and the optical levers for the X arm are moving together. However, something is wrong with the ITMY oplev, so we cannot see the motion of ITMY.
- The sum of ITMY looks like there are interference fringes. These went away as soon as the interferometer unlocked, so they are probably either caused by 1064nm or interference between optical lever beams caused by the interferometer pitch motion. Jeff K and I looked at a trend of the sum and in full lock at 30W input power about 2% of the power on the oplev is light from the interferometer (similar to ITMX). Jeff B and I turned off the OpLev laser, and we can see that the fluctuations we are seeing in this sum today are mostly from the optical lever laser, but they are smaller than what we saw last night.
- Krishna, Jim, Patrick and I also tried to see if there is a problem with the ITMY oplev by driving L2 at 0.45 Hz in pitch, we were able to reproduce amplitude similar to what ITMX did last night without seeing anything strange on the OpLev.
Suggestion for the operators if you have this problem again:
1) If you see noise in the ITMY oplev laser sum again, try moving CPY around to see if you can get rid of it. (The ITMY laser is off for now, but you might still see noise on the SUM).
2)If you see that the circulating power is fluctuating at 0.4 Hz, try turning on the PSL 3rd loop.
- From the site map, choose PSL> ISS 2nd loop
- In the lower left of the 2nd loop screen, there is a checkered/shaded box that is labeled 3rd loop, click on this to open the third loop filter bank.
- Set the gain of PSL-ISS_THIRDLOOP_SERVO to 0, make sure the INPUT and output are enabled
- Ramp the gain to 0.5 with a 3 second ramp. This causes a big glitch and can sometimes unlock the IFO when done with the ISS 2nd loop DC coupled. (It usually doesn't unlock, but there is a risk).
The fringes seen on ITMY must be due to fringes of the OpLev laser with an OpLev ghost beam coming off CPY.
In the first attached screenshot I am driving CPY in L with 600 counts at 0.1 Hz in R0 DAMP L. You can see that the fringing got worse then better as I moved CPY to improve the camera image (36936), then I moved the CP to maximize the fringes. I went to the floor and unplugged the OpLev fiber, and the fringing went away, so this is not from IFO beams.
