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AdV-COM (AdV commissioning (1st part) )
ruggi - 21:03 Saturday 28 January 2023 (58611) Print this report
Vertical shift of ITMs - chapter 2

Following Fiodor's suggestion, today at lunch time I repeated the experiment performed yesterday, shifting the ITMs in vertical direction, but one at a time. The result was clear: the effect on 12MHz mag and 6MHz QD depends only on NI (fig 1).

This time I tried to avoid any transitory effect on alignment during the shift. This depends on the fact that the optical lever sees the motion of the marionette, even if orthogonal. The pointing loops compensate the drift with a speed too low. Using yesterday's data I evaluated a proper compensation to be added during the shift. I also rescaled a bit NI dither offset: 1.3 mm for 1 mm of shift. The result was satisfying.

Fig 2 shows NI steps, as it seen by three signals sensitive to the axis rotation: NI Y dither; NE TX optical lever; B4 QD 6MHz V. All the signals are expressed in equivalent mm generated by the rotation at the extremity. At the beginning of the ramp a small transient on NI dither is visible, which means that the optical lever drift is not totally avoided. NE TX reacts to keep the error at zero. The quandrant seems still dominated by misalignment at the beginning. Then the misalignment relaxes down: the residual could be in the order of 100 um. At the end the quadrant measures a large variation: 3 mm, or 1 urad, roughly speaking.

Going back to fig 1, it seems that the effect on the sidebands is a bit delayed with respect to the shift, but it could be a consequence of the initial misalignment.

Images attached to this report
Comments to this report:
ruggi - 22:35 Saturday 28 January 2023 (58614) Print this report

Sorry, the legend of the first plot is wrong: NI step is the latest.

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