Related entries:
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=29416
https://services.ligo-la.caltech.edu/FRS/show_bug.cgi?id=6446
Summary:
For a very long time we've been limping along with some of the TMSX BOSEMs (namely F1, LF and RT) somehow seemingly less sensitive than they used to be (see the above alog and FRS).
By merely pushing BOSEMs closer to the magnets using adjustment nuts and roughly setting them to half the open value, the response of these BOSEMs were restored.
We also changed RT BOSEM (SN 164) to the one Betsy gave us (SN 083) because we could, but it was unnecessary in a retrospect. We won't change it back because it's a pain.
Details:
We measured the OSEMINF_INMON for suspected ones fully open by pulling BOSEMs away from the mass using adjustment nuts. (In the case of RT we removed the BOSEM plate from the cage and didn't see a large change.)
We found that all of them were already very close to the open values, and that open values were smaller than they used to be judging from the offsets that were set a long time ago. The former means that BOSEM bodies are much farther away from the magnets than they used to be. Apparently TMSX sagged and rolled.
|
OSEMINF_INMON before (counts) |
Open count | 2*|Offset| before | |
| F1 | 19.1k | 21.1k | 25.44k |
| LF | 18.1k | 18.7k | 23.16k |
| RT | 21.4k | 22.1k (26.95k after the swap) | 26.172k |
We swapped RT (SN 164) with the known good one (SN 083). No particular reason for the choice of RT rather than LF, it's just that Corey was working on RT at that time and thus was convenient to test.
We set the offsets to half the corresponding open count, and adjusted the BOSEM depth so that they come close to the offset.
The attachment shows the coil to coil transfer coefficient (OSEMINF/COILOUTF_EXC) at about 0.02 Hz before/after the change for F1, LF and RT (it also shows F2 and F3 but there's no "after" for these). You want to compare pink open circle with red solid disk, or blue open triangle with blue solid triangle.
As you can see the sensitivity increased by a factor of 1.9 for F1, 2.75 for LF and 3.3 for RT.
| Measurement before | Measurement after | sensitivity increase (after/before) | |
| F1 | 8.1e-4 | 1.53e-3 | 1.9 |
| LF | 2.28e-4 | 6.27e-4 | 2.8 |
| RT | 2.54e-4 | 8.34e-4 | 3.3 (=1.2*2.8) |
As for RT, a factor of 1.2 came from the increase in the LED power (26.95k/22.1k=1.22), so the change caused by the depth of OSEM is 3.3/1.2=2.8, almost the same as LF.
I removed a factor of 3 that was added to the PIT damping gain at some point, now all damping gains are 1 (except SD).
I'm also somewhat worried about TMSY LF and RT just because the OSEMINF_INMON is much larger than the offset (second attachment). However, similar measurement for TMSY (third attachment, left half is TMSX, right half is TMSY), it doesn't look as if F3 and LF are terrible.
It seems as if TMS sank and LED lost power at the same time, so it would have been difficult to judge what was going on just by looking at the data.
J. Kissel
I've completed the update to the open light current compensation in the OSEMINF filters by installing the normalization gains (H1:SUS-TMSX_M1_OSEMINF_${DOF}_GAIN) for the above updated OSEMs (F1, LF and RT), and then accepted the new offsets and gains in the SDF system. See attached screenshots, but for the sake of future searchability:
OSEM Open Light Current OFFSET GAIN
(ADC counts) (OLC / -2) (30000/OLC)
F1 21030 -10515 1.4265
LF 18700 -9350 1.6043
RT 26950 -13475 1.1132
