Höhne, C.

Lebedeva, E.

Mahmoud, T.

GSI Repository

Ronchi test for measurements for the mirror surface of the CBM-RICHdetector∗E. Lebedeva1, T. Mahmoud1, and C. Ho¨hne11JLU Gießen University, Gießen, GermanyThe CBM RICH detector will be operated with CO2 ra-diator gas, MAPMTs (Multi-Anode Photo Multiplier) asphotodetector and spherical glass mirrors as focusing ele-ment. Surface homogeneity is one of the important prop-erties required for the single mirror tiles. The global ho-mogeneity has been tested with the D0 measurement asreported earlier [1]. Local deformations e.g. by the mir-ror holding structure can be investigated with the Ronchitest method from which first results are discussed in thiscontribution.The principle of the Ronchi test is quite simple. Agrating, called Ronchi ruling, consisting of fine, opaque,equally spaced lines ruled onto a transparent substrate isprojected onto the whole mirror surface. The shadows ofthese lines then appear on the face of the mirror under testand will be reflected back onto a camera. The shape andposition of these bands is examined and interpreted to giveinformation about the shape of the mirror’s surface. Con-trary to theD0 measurement the Ronchi test thus allows toget information on local mirror deformations which is ofparticular interest considering e.g. inhomogeneities whichmay be caused by the mirror mounts.The Ronchi test is inherently qualitative and needs de-tailed comparison to a computer model in order to assesspossible distortions more quantitatively. The band shapesobserved in the Ronchi test can be caused by many differ-ent mirror types, i.e. surface profiles. Therefore the pat-tern seen in the Ronchi test has to be compared with a vir-tual, perfect mirror copy of the mirror under test. However,the qualitative picture quickly achieved and presented inthis contribution already reveals a lot of useful informationabout the mirror at a glance. It later may also be used fora fast semi-qualitative test which allows to quickly checkthe local mirror homogeneity in particular after gluing themirror mounts.Figure 1 shows the sketch of the experimental setup forthe D0 measurement, which was used for the Ronchi testas well. The only difference is the usage of the Ronchigrating in front of the CCD camera (c) and the laser pointsource (b). The Source-Camera (SC) unit is located exactlyat the mirror radius R. The active area of SC is located inthe same plane orthogonal to the optical axis of the mir-ror. A mirror prototype with a curvature radius of 3 m fromSLO Olomouc was tested. The mirror is flashed with lightfrom the point source. If the mirror would have an idealspherical shape the fringes seen on the camera would ap-∗Supported by the Hessian LOEWE initiative through the HelmholtzInternational Center for FAIR (HIC for FAIR), and by the BMBF(05P12RGGHM, 05P12RGFCG)Figure 1: Experimental setup.pear straight. Deviations from the spherical surface causedeformations of the fringes, but the measurement is onlysensitive to changes of the curvature radius perpendicularto the grating direction. In order to get a complete pic-ture of a given mirror, several different grating orientationsshould be measured.Figure 2 shows the image of a reflected light on the cam-era chip. The left Ronchigram was obtained at the distanceof 3 m (nominal curvature), the right plot was obtained at3.01 m. A possible interpretation of the deformed fringesin the center can be a depressed center of the mirror. Thethree dark sports correspond to local deformations due tothe mount system [2]. Cutting the squared mirror tiles fromthe produced circular shapes in the manufacturing processcan be the reason for the modification of fringes seen at themirror edges. For further detailed understanding of the mir-ror surface the comparison with a computer model is underway.Figure 2: CCD camera view. Left: for R = 3 m. Right: forR = 3.01 mReferences[1] E. Lebedeva, CBM Progress report 2011, p.37[2] V. Dobyrn et al, CBM Progress report 2011, p.39.GSI SCIENTIFIC REPORT 2013 NQM-CBM-23doi:10.15120/GR-2014-1-NQM-CBM-23 39

Ronchi test for measurements for the mirror surface of the CBM-RICH detector

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Ronchi test for measurements for the mirror surface of the CBM-RICH detector

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