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Entry  Mon Oct 7 14:51:20 2019, aaron, Update, Electronics, WFS head RF measurements WFShead_response.pdf
    Reply  Mon Oct 7 19:50:33 2019, gautam, Update, IOO, IMC locking not working after this work IMCflaky.pngIMG_8015.JPG
       Reply  Tue Oct 8 10:29:19 2019, gautam, Update, IOO, MC Transmission scan MC2_transmission_scatter.pdftransmissionMaps.pdfcorrelStructure.pdf
       Reply  Tue Oct 8 16:54:56 2019, rana, Update, IOO, IMC locking not working after this work 
    Reply  Tue Oct 8 16:00:06 2019, aaron, Update, Electronics, WFS head RF measurements WFS_ACresponse.pdfWFS_DCresponse.pdf
       Reply  Wed Oct 9 12:15:05 2019, rana, Update, Electronics, WFS head RF measurements 
Message ID: 14946     Entry time: Mon Oct 7 19:50:33 2019     In reply to: 14945     Reply to this: 14950   14952
Author: gautam 
Type: Update 
Category: IOO 
Subject: IMC locking not working after this work 

See trend. This is NOT symptomatic of some frozen slow machine - if I disable the WFS servo inputs, the lock holds just fine.

Turns out that the beam was almost completely missing the WFS2 QPD. WTF 😤. I re-aligned the beam using the steering mirror immediately before the WFS2 QPD, and re-set the dark offsets for good measure. Now the IMC remains stably locked. 

Please - after you work on the interferometer, return it to the state it was in. Locking is hard enough without me having to hunt down randomly misaligned/blocked beams or unplugged cables.


I took this opportunity to do some WFS offset updates.

  • First I let the WFS servo settle to some operating point, and then offloaded the DC offsets to the IMC suspensions.
  • Then I disabled the WFS servo.
  • I hand-tweaked MC1 and MC3 PIT/YAW (while leaving MC2 untouched) to minimize IMC REFL (a more sensitive indicator of the optimal cavity alignment than the transmission).
  • Once I felt the IMC REFL was minimized (~1-2% improvement), I set the RF offsets for the WFS while the IMC remained locked. I chose this way of setting the RF offsets as opposed to unlocking the cavity and having the high-power TEM00 mode incident on the WFS QPDs.
  • Overnight, I'm going to run the MC2 spot position scanning code (in a tmux session on pianosa, started ~945pm) to see if we can find a place where the transmission is higher, looking at Kruthi's code now to see it makes sense...
  • The convergence time of the MC2 spot position loop is pretty slow, so the scan is expected to take a while... Should be done by tomorrow morning though, and I expect no work with the IFO tonight.
  • Does this loop have to be so slow? Why can't the gain be higher?
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Attachment 2: IMG_8015.JPG  1.250 MB  Uploaded Mon Oct 7 21:21:42 2019  | Hide | Hide all
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