aLIGO LHO Logbook

H1 ISC

kiwamu.izumi@LIGO.ORG - posted 11:34, Thursday 10 November 2016 - last comment - 17:49, Thursday 10 November 2016(31392)

Checking spots on IMC and PRM

This is a continuity of 31381.

For a bookkeeping purpose, I have extended the spot position thing to PRM and MC mirrors. Here is a summary. They all seem fine as expected.

	p2l or y2l gain	alpha	spot position from the center [mm]	previous spot position record [mm] (18106, Apr 2015)
PRM PIT	+1.23	+0.059	-2.5	N/A
PRM YAW	+ 1.05	-0.050	+2.1	N/A
MC1 PIT	-1.3	-0.020	+0.84	+4
MC1 YAW	+1.6	-0.076	+3.2	+1.9
MC2 PIT	-3.8	-0.18	+7.6	N/A
MC2 YAW	-0.4	+0.019	-0.80	N/A
MC3 PIT	-1.1	-0.053	+2.2	-3.7
MC3 YAW	-2.6	+0.12	-5.0	-2.4

The measurement was done with the interferometer fully locked at 25 W in nominal low noise. The measurement precision should be something like 10% or so.

Comments related to this report

jenne.driggers@LIGO.ORG - 17:49, Thursday 10 November 2016 (31402)

Link

I repeated this for the IMC when the IFO was unlocked, 2W into the vacuum, to see if things are drastically different hot vs. cold. Conclusion: the spots in the IMC are basically the same hot vs. cold, so this probably isn't what is changing our alignment (or something) and giving us the drift down in range as we thermalize after increasing the power into the vacuum.

	P2L or Y2L gain	alpha for UR coil	spot position cold [mm]	spot position hot (from 31392) [mm]	abs(diff) hot vs. cold [mm]
MC1 P	-1.2	-0.057	+2.4	+2.6 (mistyped there as +0.84)	0.2
MC1 Y	+1.5	0.072	+3.0	+3.2	0.2
MC2 P	-3.79	-0.18	+7.6	+7.6	0
MC2 Y	-0.42	-0.02	-0.8	-0.80	0
MC3 P	-1.15	-0.055	+2.3	+2.2	0.1
MC3 Y	-2.6	-0.12	-5.1	-5.0	0.1

Some notes on the method, basically transcribing conversations with Kiwamu:

To calculate alpha as defined in 40m elog 2863, we figure out how an angle actuation signal will get to a single coil. Here As (for angle) can be replaced by Ps or Ys for pitch and yaw.
- (Signal to coil) = [ (A2A gain) * (A to coil EUL2OSEM matrix element) + (A2L gain) * (L to coil EUL2OSEM matrix element) ] * (pitch signal)
- rewriting for easier notation: A2A * A_eul + A2L * L_eul
- rewriting in the form of (1+alpha): A2A * A_eul * [1 + (A2L * L_eul) / (A2A * A_eul) ]
- So, a pitch to pitch force on a single coil will be modified by the amount
  - alpha = (A2L * L_eul) / (A2A * A_eul)
Since all the IMC optics are HSTSs, we use the 42.2mm/alpha number that Kiwamu calculates in alog 14788.
Note that in Kiwamu's measurements (alog 31392), he uses the UL coil as the fiducial coil, while I use the UR so that I don't have to deal with minus signs in the EUL2OSEM matrix elements for yaw. This means that the signs on the yaw alpha values will be opposite for Kiwamu and myself, but the final position comes out the same.
To get the sign of the spot position, we think about how the force on our fiducial coil is modified. For example, for MC1 Pit, alpha is negative so the force on UR is smaller than on LR, so the actuation node is higher than the center of the optic. In the coordinate system used by the SUS team (pictured on all sus screens), up is positive for pitch and closer to the left is positive for yaw. (It is this step that sorts out the minus sign difference between Kiwamu's and my alphas so that the final answer comes out the same.)

Since the ADS system does not actuate on the IMC mirrors, nor does it read in IMC_MCL, this was done by hand (also by hand by Kiwamu yesterday). I put a 100 count dither line at 20.125 Hz into H1:SUS-MC[1, 2, 3]_M3_DITHER_[P, Y]_EXC using awggui and minimized the appearance of that line in H1:IMC-MCL_OUT_DQ.