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### BEAM ALIGNMENT AND DRIFT OF OPTICS

By: Stephanie Raymond

LSU / Millsaps College

with Gabriela Gonzalez and Ken Watts

12 Nov 2003 LIGO-G030597-00-Z

PURPOSE

- Aid initial alignment by providing a visual configuration for the operator, supplementary to WFS and dithering.
- Compare the image of the beam on the optics during data taking
- Determine good position of beam from a good lock
- Monitor beams on optics and beams on photodiodes
- Determine drift of the beams on the optics

12 Nov 2003 LIGO-G030597-00-Z

Technique

- Image Acquisition
- Access channels (ETMX, ETMY, ITMX, ITMY, AS_PD, AS_Q, BS, and MC_TRANS)
- Capture frames
- Image Processing
- Filtering
- Fourier transform
- Locate center of beam
- Calculate correlation coefficient
- Subtraction of images

single-arm lock

full lock

12 Nov 2003 LIGO-G030597-00-Z

Outline of Software

- Input reference image
- Grab frame
- Access video card
- Select input channel
- Run script
- Process images
- Fourier transforms
- Difference image
- Correlation coefficient
- Display difference image with correlation coefficient
- Clear workspace and repeat

12 Nov 2003 LIGO-G030597-00-Z

Cropping

- Have the option to disclude the OSEMs, but this sometimes clips the image of the beam
- Centering method solves problem of change in luminosity of OSEMs affecting the correlatioin coefficient

12 Nov 2003 LIGO-G030597-00-Z

Filtering MC_REFL

- Commands
- im2bw converts colorscale to grayscale to binary
- medfilt2 eliminates “salt and pepper” noise
- Information is lost to filtering
- The purpose of filtering is to make viewing easier for the Operator

input image

im2bw

medfilt2

12 Nov 2003 LIGO-G030597-00-Z

Difference Image IFO_REFL

- Order of operations is important in accurately displaying which direction the beam needs to move

A

C=fft(A)

D-C

ifft(D)

B

D=fft(B)

12 Nov 2003 LIGO-G030597-00-Z

Fourier Transformvs.Simple Math

Simple Subtraction of Images

- Ignores intensity dependence
- Reduces number of steps in image processing
- Does not expedite script

Fourier Transform

- Prevents having to calibrate the intensity
- Produces sharper image

12 Nov 2003 LIGO-G030597-00-Z

Cont. Example ETMY (single arm)

12 Nov 2003 LIGO-G030597-00-Z

Quantitative and Qualitative Representations of Misalignment

CORRELATION=79.62

CORRELATION=83.46

CORRELATION=85.17

CORRELATION=86.5

12 Nov 2003 LIGO-G030597-00-Z

CONCLUSION

- Fourier transform method

successfully displays

misalignment of beam relative

to reference image during

initial alignment

- Develop beam center method that will be useful in monitoring drift of beam on optics
- Data collected during S3 will determine extent if any to which beams on optics drift
- Study correlations of beam position on AS_PD with data quality

12 Nov 2003 LIGO-G030597-00-Z

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