jeffrey.kissel@LIGO.ORG - posted 19:18, Tuesday 21 July 2015 - last comment - 09:05, Wednesday 22 July 2015(19814)
IMC WFS Offloaded to MC Optics' Alignment Offsets
J. Kissel, K. Izumi, J. Driggers
This morning, we had a little trouble relocking the IMC because the initial alignment of the IMC SUS causes the light on the IMC WFS trigger PD to be just a hair too low. To solve the problem this morning, Jenne temporary decreased the trigger threshold (H1:IMC-IMC_TRIGGER_THRESH_ON) from 40 to 15 [ct], such that the IMC WFS engaged, which steered the IMC to our known good alignment (and then restored the trigger threshold once the WFS had steered the optics back to a good enough alignment that the normal trigger threshold was easily surpassed). To avoid this in the future, we said "we should offload the DC value of IMC WFS control signal to the MC SUS alignment offsets," but at the time didn't know that was the right script / button / thing to call in order to do so since there have been many in the past which had suffered from bit rot.
Just now, in between lock attempts, I've asked Kiwamu what the right script is, he should me, and I ran it. This changed the alignment offsets as follows:
Before After
H1:SUS-MC1_M1_OPTICALIGN_P_OFFSET 1207.50 1208.3750756649936
H1:SUS-MC1_M1_OPTICALIGN_Y_OFFSET -2025.30 -2024.9455029701996
H1:SUS-MC2_M1_OPTICALIGN_P_OFFSET 527.00 525.7250127471385
H1:SUS-MC2_M1_OPTICALIGN_Y_OFFSET -534.03 -535.7085973743652
H1:SUS-MC3_M1_OPTICALIGN_P_OFFSET -722.14 -721.3522559898023
H1:SUS-MC3_M1_OPTICALIGN_Y_OFFSET -2034.10 -2034.4394052459747
I've spoken with Kiwamu about the seeminly-now-prolific issue of using double-precision python scripts to produce calculated EPICs settings to ridiculous precision, and he agreed to fix it in due time.
The script that does the offloading is launched from the IMC WFS MASTER screen, from the bright blue button marked "! OFFLOAD WFS" just below the output filters in the middle-bottom right. See attached screenshot.
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This alog doesn't really state it, but I assume the PZT is off loaded as well. I also assume each TM is off loaded individually.
- Are you sure that running this script repeatedly will minimize long term drifts?
- What keeps the uncontrolled dof in the IMC from walking away?
- Have we established that the quads are a better long term reference than the suspensions?
Daniel,
- The offload script includes the PZT as well, as you guessed.
- The script does not fix long-term drift. Running the script before you hit a potential risk of loosing the ASC control values is good for speeing up relocking.
- I don't know what introduces drift to the uncontrolled dof. For some reason the uncontrolled dof tends to shift when some computers reboot.
- We don't know if the QPDs are more stable references than the suspensions. In the current configuration, the beam poointing to the interferometer is determined by a combination of a QPD (= MC2 trans) and suspensions (MC1, 2, 3 and IM1,2,3 and 4).