The Rasnik 3-point optical alignment system

Abstract

The Rasnik alignment system was developed initially in 1983 for the monitoring of the alignment of the muon chambers of the L 3 Muon Spectrometer at CERN. Since then, the development has continued as new opto-electronic components become available. Rasnik systems are 3-point optical displacement monitors and their precision ranges from below nanometers to several micrometers, depending on the design and requirements of the systems. A result, expressed in the range/precision ratio of 2 × 10 6 , is presented. According to the calculations of the Cramér-Rao limit, and by means of MonteCarlo simulations, a typical Rasnik image should have enough information to reach deep sub-nanometer precision. This paper is an overview of the technological developments and achievements since Rasnik was applied in high energy physics experiments. K: Detector alignment and calibration methods (lasers, sources, particle-beams); Overall mechanics design (support structures and materials, vibration analysis etc)

Improved performance of the LHCb Outer Tracker in LHC Run 2

“…The image recorded by the CCD camera is then used to reconstruct positions of the chessboard pattern. The resolution of the CCD-RASNIK system is better than 1 µm for a system with a mask-to-CCD distance up to about 8 m [10].…”

The ultimate performance of the Rasnik 3-point alignment system

“…The Red Alignment System Nikhef (Rasnik) was originally developed for the alignment of the inner, middle and outer muon chambers of the Muon Spectrometer of the L3 experiment at CERN (1980CERN ( -2001 [1]. These three chambers, placed at a certain mutual distance, were used to measure the radius-of-curvature of muon tracks, from which the muon momentum can be derived.…”

“…If relevant, the inter-pixel dead area can be taken in account by Image analysis. The SOAP image analysis program for processing the ChessField Rasnik images are set out in detail in [1]. First, the raw image as of Fig 9 is converted into a 2D Fourier image, shown in Fig.…”

“…Secondary (harmonic) peaks can be used to refine these parameters, and the nominal Verification of the SOAP image analysis program. The performance of Rasnik systems has previously been analysed in [1]. From that, the performance of the SOAP analysis program has been certified to be better than 1 nm in resolution and 10 nm in linearity.…”

Geometrical precision alignment of the Micromegas detectors for the ATLAS New Small Wheel upgrade

“…Alignment measurements of two faces of a readout panel are performed with Rasnik masks [5] etched on every PCB and a specific device named Rasfork. The Rasfork is composed of two tubes, each equipped with a prism, lens and CCD, such that when inserted on a panel edge, the Rasnik masks on the two faces are imaged on the CCDs.…”