ALIGNMENT ISSUES FOR RICH 2

1. ALIGNMENT ISSUES FOR RICH 2

2. Alignment in RICH2 • Angular Resolution for RICH 2 (in TR): 0.35 mrad • Emission point: 0.21 mrad • Chromatic: 0.22 mrad • Pixel size: 0.18 mrad • RICH 2 prototype resolutions: obtained with alignments < 0.1 mrad • 2048 pixel HPD: 0.18 mrad • 61 pixel HPD: 0.26 mrad • MAPMT: 0.27 mrad • Experimental aim: alignment to less than 0.1 mrad

3. CURRENT PHILOSOPHY • Mirrors: • Install mirrors on mirror support planes • Survey positions of mirrors with CERN surveyors • Adjust all positions in laboratory • Carry support plane into pit • Install on structure and “hope” mirrors did not move • Survey frames with mirrors w.r.t. structure • Photon Detectors • Detectors surveyed within support structure • Install on structure • Survey photon detectors w.r.t. structure • Data to link detectors, mirrors and frame • Our conclusion: risky strategy

4. Alignment with data • Mirror segmentation of both planes of mirrors complicates problem considerably. • Possible sources of error: tilt angle, tilt orientation and radius of curvature • Between 5-10% of photons have ambiguity in finding mirror segment of origin • Compare difference in reconstructed tilt angles for unambiguous photons (F. Filthaud, 19 March 1999) qcrec- qcexp = A cos (fc- f0) • Frank’s conclusion: if relative alignment ~ 0.5 mrad then alignment accuracy of 0.2 mrad (including mirror quality) can be achieved with data • Study needs to be verified. • Is this accuracy sufficient?

5. Laser Alignment System • A laser alignment system simplifies the problem: • No ambiguous photons • Position of laser beam is known precisely • Allows possibility of performing a first alignment run once mirrors and detectors are installed and modifying mirror tilts (if needed) before closing vessel • It can be used to monitor the alignment during data taking and make corrections if needed (temperature and vibrational stability can modify relative positions) • Complements a final alignment with reconstructed Cherenkov rings.

6. Array of optic fibers • Single mode fibers coupled to silicon laser (690 nm) • Collimating optics: Gaussian beam profile 2-3 mm over 10 m distance • Used for laser alignment ATLAS muon system

7. Optic fibers on mirrors • Alternatively, mount optic fibers with collimators on spherical mirrors • Four points per mirror have to be mapped out on photon detector plane • Possible mechanical problem: ensure optic fibers parallel to surface of mirrors

8. Array of Mirrors • Incident laser light from side of RICH 2 • Array of semi-transparent mirrors to redirect beam • Tube around beam pipe can also be used to reflect alignment beams

9. Mirror with piezoelectric device • A piezoelectric device on a mirror can scan horizontally and vertically the mirror surfaces • Shadows on the photon detector planes identify edges of mirrors

10. Conclusions • Current alignment philosophy for RICH 2 is deemed to be “risky” • Alignment with data would only achieve ~0.2 mrad if survey accurate to 0.5 mrad (to be confirmed) • It would be desirable to have some laser alignment system in place for pre-alignment in pit and during-run monitoring of alignment • Possible options suggested: • Arrays of fibers delivering laser light (in front of mirrors or mounted on mirrors). • Arrays of mirrors with laser injected laterally • Mirror with piezoelectric device to scan beam on mirrors • Other ideas welcome!