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Entry  Tue Nov 17 20:40:08 2015, yutaro, Summary, ASC, Loss maps of arm cavities 
    Reply  Wed Nov 18 11:22:13 2015, yutaro, Update, ASC, Loss maps of arm cavities ETM_PIT_up.pngETM_PIT_up.pngETM_YAW_up.png
       Reply  Fri Dec 4 10:18:33 2015, yutaro, Update, ASS, Offset in the dither loop of XARM vs beam spot shift on ETMX offset-angleETMXPIT.pngoffset-angleETMXYAW.png
Message ID: 11846     Entry time: Fri Dec 4 10:18:33 2015     In reply to: 11779
Author: yutaro 
Type: Update 
Category: ASS 
Subject: Offset in the dither loop of XARM vs beam spot shift on ETMX 

As I did for YARM (elog 11779), I measured the relation between offsets added just after the demodulation of the dithering loop of XARM and beam spot shift on ETMX. Defferent from YARM, the beam spot on ITMX DOES change because only BS is used as a steering mirror (TT1&2 are used for the dithering of YARM). Instead, the beam spot on BS DOES NOT change.

This time, I measured by oplevs the angles of both ETMX and ITMX for each value of offset, and using these angles I calculated the shift of the beam spot on ETMX so that I got two independent estimations (one from ETMX oplev, and the other from ITMX oplev) as shown below. The calibration of the oplevs reported in elog 11831 is taken into account. 

The difference of two estimations comes from the error of calibration of oplevs and/or imperfect alignment, I think. 

Attachment 1: offset-angleETMXPIT.png  136 kB  | Hide | Hide all
offset-angleETMXPIT.png
Attachment 2: offset-angleETMXYAW.png  129 kB  | Hide | Hide all
offset-angleETMXYAW.png
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