Last week, sts correction gains were measured and calculated on HAM4/5/6 to improve the CPS sensor correction. We measured them today for the remaning chambers.
For the BSC chambers, we locked the platforms to the ground by increasing the blends to 750mHz (cf alog) and turned off the sensor correction. We measured the ratio between the T240 (stage 1) and the STS (ground) signal (should be 1 with the platform moving with the ground). The same measurement was also carried out between STS and L4C for hepi, since IPS sensor correction is used for the Z DOF.
All the measurements using the corner station STS B as input to the sensor correction (ITMX, BS, ITMY) show a factor of two mismatch with the hepi and platform sensors in the X DOF only. There is certainly a calibration error with the STS B seismometer. I checked the calibration filter in the input filter banks and it looks correct (10.17 nm/s /cts). We should investigate more. The other chambers and dofs need a relatively small correction.
The ISI gains (T240/STS for BSCs and GS13/STS for HAMs) are
| X | Y | Z | |
| HAM2 | 1.036 | 0.995 | 0.877 |
| HAM3 | 0.976 | 0.958 | 0.812 |
| ETMX (T240 for gnd sensor) | 1.114 | 1.106 | 1.105 |
| ETMY | 0.985 | 0.974 | 0.995 |
| ITMX | 1.964 | 0.976 | 0.993 |
| ITMY | 1.982 | 0.984 | 0.993 |
| BS | 1.99 | 0.986 | 0.994 |
The BSC - HEPI gains (L4C/STS) are
| X | Y | Z | |
| ETMX | 1.110 | 1.107 | 1.091 |
| ETMY | 0.998 | 0.991 | 0.969 |
| ITMX | 2.167 | 1.063 | 0.972 |
| ITMY | 2.011 | 1.006 | 0.963 |
| BS | 2.09 | 1.055 | 0.924 |
Attached are examples of the transfer functions for ITMY
To calculate the gains I wrote a script for the bscs in a similar fashion as Sebastien's : it is called BSC_gain_matching_calculation(IFO,Chamber,start_time,duration) located under /ligo/svncommon/SeiSVN/seismic/BSC-ISI/Common/Misc. The results above were obtained using the following parameters :
start_time=tconvert('01/22/2015 15:35') ;
duration = 45*60 ;
The sensor correction matching gains were changed to their new values for the BS ITMX and ITMY in the X dof. The pdf attached are comparing the X platform motion before/after the change, showing improvement (x~2) in the 300mHz 700mHz band.
I confirmed that the factor of two mismatch described in the log above comes from the STS B, looking at the comparison of the three ground seismometers (X dof) of the corner station, see attached screenshot.
It looks like the problem is in the B STS or associated cabling. I took spectra of the raw ADC signal and the B seismometer shows half as much X signal (ADC channel 26 on attached plot) as the other 2 STS's. Arnaud and I went out to check the cabling at the rack, everything is tight there. We are doing measurements with the ITM's right now, so we didn't go out to check the cable on the pod.
I looked though the trends and science channels, and it looks like the auto center on the B STS was pushed at 8:30 am local on Dec 23rd, 2014 and the X channel didn't recover properly. On the first attached trend you can see that before the alleged button push (alleged because there are no logs, I'm just guessing, second trend shows what looks like an auto-centering sequence though) the X channel showed more signal, then after less signal. Y channel looks roughly the same before and after. Attached spectra (3rd png) shows overall spectra is very different, solid red is before, dashed green is after.
