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Message ID: 12175     Entry time: Tue Jun 14 11:29:25 2016
Author: Johannes 
Type: Summary 
Category: ASC 
Subject: YArm OpLev Calibration 

In preparation for the armloss map I checked the calibration of the Y-Arm ITM and ETM OpLevs with the method originally described in https://nodus.ligo.caltech.edu:8081/40m/1247. I was getting a little confused about the math though, so I attached a document at the end of this post in which I work it out for myself and posteriority. Stepping through an introduced offset in the control filter for the corresponding degree of freedom, I recorded the change in transmitted power and the reading of the OpLev channel with the current calibration. One thing I noticed is that the calibration for ITM PIT is inverted with respect to the others. This can of course be compensated at any point in any readout/feedback chain, but it might be nice to establish some sort of convention where positive feedback to the mirror will increase the OpLev reading.

The calibration factors I get are within ~10% of the currently stored values. The table (still incomplete, need to relate to the current values) summarizes the results:

Mirror DoF Current Relative New
Y-Arm OpLev Calibration
ETM PIT   0.974 ± 0.029  
  YAW   1.077 ± 0.021  
ITM PIT   -0.972 ± 0.020  
  YAW   0.920 ± 0.048  

The individual graphs:

ETM PIT

ETM YAW

ITM PIT

ITM YAW

 

The math:

 

 

Attachment 1: CavityCoupling.pdf  101 kB  | Hide | Hide all
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