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CERN’s surveyors conduct diverse measurements to ensure, for example, accurate positioning of magnets, detectors and other equipment. They do so using techniques such as laser tracking and 3D scanning. Two weeks ago, the surveyors were busy measuring the precise position of a new, recently installed layer of Cathode Strip Chambers (CSC), called ME+4/2, mounted on a disk known in CMS as YE+3. To determine the position of the chambers with respect to the local geometry of YE+3, the surveyors used the technique of close-range photogrammetry. Close-range photogrammetry was first used at CERN in 1998 for CMS. Since then photogrammetry has been intensively used for prototyping, geometrical validation, envelope measurements, deformation measurements, preassembly, assembly and positioning of the experiments by the CERN surveyors’ team and many thousands of images have been taken and analysed. The equipment has been taken off-site as well: for example, the full geometrical controls during the CMS barrel and endcap construction were first done by photogrammetry in Germany and Japan at the construction sites, later in the CMS surface hall and finally in the underground cavern. The technique relies on the reconstruction of the object from digital images taken from different positions to ensure a suitable geometry of intersecting rays for the measured points. On 13 November, altogether 212 points on YE+3 were measured: three points per CSC element on the inner ring (ME+4/1), four points per CSC element on the outer ring (ME+4/2) and 14 YE+3 survey reference holes. The reference holes on the CSC and YE+3 are equipped with retro-reflective survey targets. In addition coded targets are temporarily glued on the chambers. These targets have a circular barcode with 516 different codes, and can be recognised by the software to automate the process. For the measurement of YE+3, a Nikon D3x with a 24-megapixel sensor and a NIKKOR lens of 28 mm focal length were used. Images for the measurement of YE+3 were taken from a cherry picker and the cavern access structure. They were then automatically transferred by wifi to the portable PC with the evaluation software AICON DPA. The evaluation software reconstructs the photogrammetric network, and the object coordinates, the camera positions and the calibration parameters of the camera are estimated simultaneously in a process called bundle adjustment. 295 images were taken for this measurement, giving in total 24,402 observations for 2722 unknowns. In order to define the absolute scale for the relative photogrammetric measurement, additional distances were measured using a laser tracker AT401. As a result of the measurement, the position of the CSC will be known with respect to YE+3 local geometry within a precision of +/– 0.3 mm. When YE+3 will be closed at the end of the LHC’s ongoing Long Shutdown 1 and adjusted to its running position with respect to the beam line, the final positions of the CSC chambers with respect to the beam line and the interaction point within CMS can be calculated and be introduced in the CMS parameters database. — Submitted by Antje Behrens and Aurelie Maurisset

Tags / keywords: CMSTimes