evan.hall@LIGO.ORG - posted 01:23, Thursday 26 February 2015 - last comment - 14:32, Thursday 26 February 2015(16931)
Closing common ETM alignment loops
Sheila, Alexa, Gabriele, Evan
Summary
In addition to the differential ETM loops, we now have closed the common ETM degrees of freedom using REFLA9I + REFLB9I. These loops are slow, with bandwidths of a few tens of millihertz.
Details
Previously (LHO#16883), we had closed loops around IM4 in order to reduce the amount of reflected light into REFL_A_LF. However, tonight we decided instead to close the common ETM DOF, so that the ETMs are nominally controlled in all four angular degrees of freedom. This (hopefully) leaves us free to pursue more loop-closing with the corner optics.
The common ETM loops are implemented in the CHARD filter modules. These modules are stuffed with the same filters as for their DHARD counterparts.
- For CHARD pitch, we have FM3, FM4, FM6, FM7, FM9, and FM10, with a gain of −14 ct/ct.
- For CHARD yaw, we have FM3, FM4, FM7, FM8, FM9, and FM10, with a gain of −5 ct/ct.
- For DHARD, the gains are 12 ct/ct for pitch and 15 ct/ct for yaw.
This is a screen shot of QPDs durring a well aligned lock tonight.
Since about 10:22 UTC Feb 26th, the IFO has been locked on DC readout with 4 ASC loops closed : DHARD PIT+YAW and CHARD PIT and YAW.
We are leaving this locked with the intent bit undisturbed.
For this lock stretch the ETM ASC loop settings were a bit different from what I said above: