I've attached the oplev trends for PIT ITMX and ETMX over the last 12 hours, (7urad, pp) these can be compared to the trends in T1300563
(page 11 and 16). You can see that we had about 1/5 the fluctuations in pitch durring HIFOY that we do now. After Hugh's work on friday the longitudnal motion is reduced (now we see about 2-3 fringes per second) to levels where wwe should have enough range to lock stably. As was documented in the alogs 9384 and 9381, the unexpected low finesse of the ETM means that higher order modes ( caused by misalingment or mode mismatching) cause offsets in our locking signals. As a result we are more sensitive to pitch fluctuations that we should have been, and our lock is not really stable yet. So if SUS/SEI can work on reducing the pitch fluctuations, we should give them whatever time they need with the optics. Also, we should simulate if our locking signals would be OK with the level of pitch fluctuations seen durring HIFO Y.
Last, I am going into the LVEA to look for parts in the east bay and work on ISCT1.
Out of LVEA
SHG output green power is 0.14mW, ALS prisim is in place.
Seems like the short term fluctuation of ETMX PIT was 3 to 4 urad pp after it quieted down in the plot.
The ETM rotation is amplified by a factor of R_ETM/(R_ETM+R_ITM-L)=12.6 to give the rotation of the cavity axis where R_ETM=2241.5m, R_ITM=1936.5m, L=4000m. (Replace R_ETM with R_ITM in the numerator to produce the amplification factor for ITM, which gives a factor of 10.9.)
The same ETM rotation is going to shift the cavity waist vertically by angle*R_ETM*dITM/(R_ETM+R_ITM-L)=angle*1260m where dITM is the distance from the cavity waist to the center of the curvature of the ITM (about 100m). (Replace dITM with dETM, about 80m, for ITM rotation.)
| ETM angle (urad p-p) | cavity angle (urad p-p) divided by 20urad divergence angle | cavity waist vertical/lateral shift (mm p-p) divided by 8.5mm waist radius | |
| PIT | 3 to 4 when quiet |
(38 to 50) / 20 = 1.9 to 2.5 |
(3.8 to 5.0)/8.5 = 0.5 to 0.6 |
| YAW | 1 | 13 / 20 = 0.65 | 1.3/8.5 = 0.15 |
In OAT and HIFO-Y not only was the mirror motion itself smaller, but the geometry was also better because the denominator in the amplification factor, R_ETM+R_ITM-L, was (2303+2312-4000)=615m instead of 177m, i.e. the cavity axis rotation per mirror rotation was a factor of 3.5 smaller than it is now.