1. Paul Soler,8 June 2000 RAL and Univ. of Glasgow ALIGNMENT ISSUES FOR RICH 2

2. Paul Soler, 8 June 2000 2 Alignment in RICH2 zAngular Resolution for RICH 2 (in TR): 0.35 mrad yEmission point: 0.21 mrad yChromatic: 0.22 mrad yPixel size: 0.18 mrad zRICH 2 prototype resolutions: obtained with alignments < 0.1 mrad y2048 pixel HPD: 0.18 mrad y61 pixel HPD: 0.26 mrad yMAPMT: 0.27 mrad zExperimental aim: alignment to less than 0.1 mrad

3. Paul Soler, 8 June 2000 3 CURRENT PHILOSOPHY zMirrors: yInstall mirrors on mirror support planes ySurvey positions of mirrors with CERN surveyors yAdjust all positions in laboratory yCarry support plane into pit yInstall on structure and hope mirrors did not move ySurvey frames with mirrors w.r.t. structure zPhoton Detectors yDetectors surveyed within support structure yInstall on structure ySurvey photon detectors w.r.t. structure zData to link detectors, mirrors and frame zOur conclusion: risky strategy

4. Paul Soler, 8 June 2000 4 Alignment with data zMirror segmentation of both planes of mirrors complicates problem considerably. zPossible sources of error: tilt angle, tilt orientation and radius of curvature zBetween 5-10% of photons have ambiguity in finding mirror segment of origin zCompare difference in reconstructed tilt angles for unambiguous photons (F. Filthaud, 19 March 1999) c rec - c exp = A cos ( c - 0 ) zFranks conclusion: if relative alignment ~ 0.5 mrad then alignment accuracy of 0.2 mrad (including mirror quality) can be achieved with data zStudy needs to be verified. zIs this accuracy sufficient?

5. Paul Soler, 8 June 2000 5 Laser Alignment System zA laser alignment system simplifies the problem: yNo ambiguous photons yPosition of laser beam is known precisely zAllows possibility of performing a first alignment run once mirrors and detectors are installed and modifying mirror tilts (if needed) before closing vessel zIt can be used to monitor the alignment during data taking and make corrections if needed (temperature and vibrational stability can modify relative positions) zComplements a final alignment with reconstructed Cherenkov rings.

6. Paul Soler, 8 June 2000 6 Array of optic fibers zSingle mode fibers coupled to silicon laser (690 nm) zCollimating optics: Gaussian beam profile 2-3 mm over 10 m distance zUsed for laser alignment ATLAS muon system

7. Paul Soler, 8 June 2000 7 Optic fibers on mirrors zAlternatively, mount optic fibers with collimators on spherical mirrors zFour points per mirror have to be mapped out on photon detector plane zPossible mechanical problem: ensure optic fibers parallel to surface of mirrors

8. Paul Soler, 8 June 2000 8 Array of Mirrors zIncident laser light from side of RICH 2 zArray of semi-transparent mirrors to redirect beam zTube around beam pipe can also be used to reflect alignment beams

9. Paul Soler, 8 June 2000 9 Mirror with piezoelectric device zA piezoelectric device on a mirror can scan horizontally and vertically the mirror surfaces zShadows on the photon detector planes identify edges of mirrors

10. Paul Soler, 8 June 2000 10 Conclusions zCurrent alignment philosophy for RICH 2 is deemed to be risky zAlignment with data would only achieve ~0.2 mrad if survey accurate to 0.5 mrad (to be confirmed) zIt would be desirable to have some laser alignment system in place for pre-alignment in pit and during- run monitoring of alignment zPossible options suggested: yArrays of fibers delivering laser light (in front of mirrors or mounted on mirrors). yArrays of mirrors with laser injected laterally yMirror with piezoelectric device to scan beam on mirrors yOther ideas welcome!