With help from Stuart Aston at LLO and RahulK, BetsyW, JeffK, and EdM at LHO, this entry documents our current best understanding of the suspended masses of the ETMs.
Note that the Photon Calibrators generate calibrated forces on the ETMs via radiation pressure. The overall uncertainty of the actuation forces is below 1%, closer to 0.5% for the observing run.
To accurately translate these calibration forces into calibrated displacements, we require accurate masses of the suspended ETMs. A 0.1% error in the suspended mass directly couples into a 1% error in the calibrated displacements.
Here is what we know:
- O3 ETM masses from Galaxy (from Rahul):
LHOx - 39611 grams
LHOy - 39538 grams
LLOx - 39508 grams
LLOy- 39642 grams
- Masses of Ears to be added to each mass (from Rahul):
44 grams (2 x 22 grams)
- Masses of AMDs to be added to each mass (from LIGO-E1700356-v6, first sentence of Section 6, page 10
The total mass of AMD1, 2, 3 and 4 is 0.80gm, 0.54gm and 0.39gm and 0.32gm respectively.
Total: 2.05 g
- Fraction of suspension fibers that contribute to the “suspended” mass
Rahul looking into this issue.
So our current best guesses for the masses are (neglecting the fibers for now):
LHO EX: 39611 + 44 + 2.05 = 39657 g
LHO EY: 39538 + 44 +2.05 = 39584 g
LLO EX: 39508 + 44 +2.05 = 39554 g
LLO EY: 39642 + 44 + 1.78 = 39688 g (Missing one AMD)
- From the suspension Matlab model files in: /ligo/svncommon/SusSVN/sus/trunk/QUAD/Common/MatlabTools/QuadModel_Production/
h1etmx.m
pend.m3 = 39.603+ear_mass_total; % 39.647 = 39.603 + ear_mass_total; Updated on 7 Aug 2015 by K.Izumi, ETM 08 on https://galaxy.ligo.caltech.edu/optics/. +0.044 from Betsy to include ear weight.
h1etmy.m
pend.m3 = 39.597 + ear_mass_total; % 39.641 = 39.597 + ear_mass_total; Updated on 12 feb 2014 by Brett Shapiro, ETM 12 (H1ETMY) on https://galaxy.ligo.caltech.edu/optics/.
l1etmx.m
pend.m3 = 39.508 + ear_mass_total; % 39.552 = 39.508 + ear_mass_total; Updated on 15 Feb 2019 by Stuart Aston, old ETM 07 new ETM 10 (L1ETMX) on https://galaxy.ligo.caltech.edu/optics/.
L1etmy
pend.m3 = 39.642 + ear_mass_total; % 39.686 = 39.642 + ear_mass_total; Updated on 15 Feb 2019 by Stuart Aston, old ETM 09 new ETM 15 (L1ETMY) on https://galaxy.ligo.caltech.edu/optics/.
Thus the masses in the suspension models as of May 22, 2019 seem to be (models may not be including the 2 g from the AMDs):
LHO EX: 39603 + 44 + 2.05 = 39649 g
LHO EY: 39597 + 44 +2.05 = 39643 g
LLO EX: 39508 + 44 +2.05 = 39554 g (was 39552 +46 = 39598 until 2/15/2019)
LLO EY: 39642+ 44 +1.78 = 39688 g (was 39686+46 = 39732 until 2/15/2019)
Summary
H1ETMX
Current model value 39649 g, should be 39657 g, error: model 8 g lighter than actual (0.02%)
H1ETMY
Current model value 39643 g, should be 39584 g, error: model 60 g heavier than actual (0.15%)
L1ETMX
Current model value 39554 g (correct value)
L1ETMY
Current model value 39688 g, was 39732 g until Feb 15, 2019, error: model 44 g lighter than actual (0.11%)
Theoretically, I estimated that each fused silica fibres should have a mass of about 0.494 grams, however if we consider around half of the fibre bending (when pendulum swings) then it should be around 0.247grams. Making a total of 0.988 grams for all four fibres.
If we want more precision, then it is better to take into account the actual bending length (which should around 20 mm from the top of the fibre). This should be around 0.172 grams (0.688 grams for all 4 fibres).
Seems there is a typo in what I wrote earlier.
"A 0.1% error in the suspended mass directly couples into a 1% error in the calibrated displacements." should have been "A 0.1% error in the suspended mass directly couples into a 0.1% error in the calibrated displacements."