measuring gravitational waves with geo600

Martin Hewitson and the GEO team

Measuring gravitational waves with GEO600

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Introduction

DRMI gives 2 output signals, each containing GW information – P(t) and Q(t)

There is a transfer function from h(t) to P(t) and from h(t) to Q(t)

Tp(f) = P(f) / h(f) and Tq(f) = Q(f)/h(f) Each comprise an optical part and an electronic part Each vary (slowly?) in time

We want to calibrate P(t) and Q(t) to h(t) on-line

Need to estimate Tp(f) and Tq(f) hp(t) and hq(t) Combine hp(t) and hq(t) to get optimal h(t)

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In the steady state….

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Transfer functions h(t)P,Q

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Optical transfer function - equations

For each quadrature, P and Q, Overall gain Pole frequency Pole Q Zero frequency

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Measured optical response - P

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Calibration overview

calibration

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Calibration software tasks

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On-line measurement of Tp(f)

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Optimisation routine

Fit models of the transfer functions to the measured ones

8 parameter fit Gp, Ppf, Ppq, Pzf, Gq, Qpf, Qpq, Qzf

Electronic parameters are fixed Algorithm uses various minimisation

methods to find the best parameter set that describes the data

It also returns a measure of success – 2

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Undoing the effect of the optical response

The parameters from sys id can be used to generate inverse optical response

Poles to zeros, zeros to poles, invert gains

IIR filters are designed for these inverted responses

Overall gains are treated separately

Filters are applied to up-sampled error-point to give better filter response

Inverse P

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Generating loop-gain correction signals

A full set of IIR filters has be constructed to match the response of the feedback electronics in the detection band

One set for fast feedback, one set for slow feedback

Error-point signal is filtered through these electronics filters and then through actuator filters

This produces two ‘displacement’ signals that correct for the loop gain of the MI servo

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Fast path (UG 100 Hz) electronics model

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Slow path (UG 8 Hz) electronics model

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Calibration pipeline – hp(t)

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Parameter estimation results - P

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Parameter estimation results - Q

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2 behaviour

The measure of success from the optimisation routine tells us something about data quality

2 depends on SNR of calibration lines in P

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Quality channel

One 16-bit sample per second

Encodes information from

Lock status Maintenance status 2 threshold crossings

So far, 2 thresholds have been chosen arbitrarily

This will be extended soon – see data quality indicators talk 32 bit

sample per sec

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Measured2 behaviour

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Measured2 behaviour

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Measured2 behaviour

noise estimation (2)

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hp(f) and hq(f) – validation I

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ESD calibration - Validation II

Labbook pages 1587, 1596, 1602

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Combining hp(t) and hq(t)

With ‘correct’ hp(t) and hq(t) we can try to combine them to get some optimal h(t)

Both signals represent (apparent) strain Each contain some differential arm-length

change information (real strain) So, far only tried a couple of simple

examples Simple mean High/low pass filter combination

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Simple mean combination

h(t) = [hp(t) + hq(t)] / 2

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Simple mean combination – phase check

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Filtered combination – highpass+lowpass

h(t) = lowpass{hp(t)} + highpass{hq(t)}

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Filtered combination – results

h(t) = lowpass{hp(t)} + highpass{hq(t)}

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Filtered combination – phase check

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Current and future work

Q quadrature parameters are now successfully estimated and signal is calibrated to hq(t)

Updating of the optical filters needs more extensive studies to look for artefacts

More studies of 2 values for P+Q simulations More studies of 2 values for P+Q ‘real’ data How to combine h(t)_P and h(t)_Q ?

Some simple ideas already exist Other possibilities should be explored The combined h(t) needs studied for artefacts

Include more automation MI loop gains read from LabVIEW Add more data quality checks – extend quality channel bits

Try using recorded feedback signals for loop-gain correction

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Pros and cons

Pros Calibration is updated once per second Accuracy to ~10% from 50Hz to 6kHz Runs on-line with 2 min latency – time-domain Produces calibrated time-series – hp(t), hq(t)

Cons Fast (>1Hz) optical gain fluctuations ignored Outwith valid frequency range, accuracy is

poorer Bottom line is ESD calibration – good to about 5%

Need independent check of ESD Photon pressure calibrator

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Intermission

(Pause)

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Introducing GEO Summary Pages

Track fixed measurements over lock stretches

Same set of measurements is performed on each data segment

Lock stretches can be overnight runs, weekend runs, science runs

A report is produced (web page) for each data segment

Quick-look way of comparing detector status over days

Also gives information about data quality Q: Should this data segment be analysed?

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GEO Summary Pages – calibration quality

GEO Summary pages focus primarily on calibration quality and sensitivity measures so far

Min/Max spectra BLRMS sensitivity Data quality channel – locked?, Maintenance? Recovered parameters 2 values Lock lists and duty cycle

Will be extended to include GEO++ monitor outputs (see Ajith’s talk)

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Where to look

Index of reports appears at

http://www.geo600.uni-hannover.de/georeports/index.html

The list of reports is split into months Each entry is a summary of the full report Links take you to the full report

Let’s have a look…