3D DIC tests of mirrors for the single-mirror small-size telescope of CTA

The Cherenkov Telescope Array (CTA) is the next generation very high energy gamma-ray observatory. Three classes of telescopes, of large, medium and small sizes are designed and developed for the observatory. The single-mirror option for the small-size telescopes (SST-1M), of 4 m diameter, dedicated to the observations of the highest energy gamma-rays above several TeV, consists of 18 hexagonal mirror facets of 78 cm flat-to-flat. The goal of the work described in this paper is the investigation of a surface shape quality of the mirror facets of the SST-1M CTA telescope. The mirrors measured are made of composite materials formed using sheet moulding compound (SMC) technology. This solution is being developed as an alternative to glass mirrors, to minimize the production cost of hundreds of mirrors for the network of telescopes, while retaining the optical quality of the telescope. To evaluate the progress of design, production technology and the mirrors’ functionality in operating conditions, the three-dimensional (3D) Digital Image Correlation (DIC) method was selected and implemented for testing selected mirrors. The method and measurement procedure are described. The novel measurement approach based on 3D DIC has been proven to be well suited to the investigation of the mirrors’ behavior with temperature, producing the necessary accuracy.

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