CTAO in Switzerland
Foundations
The Swiss involvement in gamma-ray astronomy instrumentation has been foundational. Nowadays, SiPMs are replacing photo-multiplier tubes in almost every single-photon sensitivity application. Their potential for gamma-ray astronomy was identified very early by the Swiss gamma-ray community that refurbished a HEGRA telescope installed at the observatory of La Palma with a Cherenkov camera based on SiPM sensors. Led by ETHZ, the First G-APD Cherenkov Telescope (FACT) was a path finder of this technology and it is in operation since 2011.
Given the successful achievement of FACT, the UniGe group, in collaboration with a consortium of Polish and Czech institutes, decided to further develop this approach and proposed a Davies-Cotton telescope with a SiPM camera for the SST array, the SST-1M. The camera features custom hexagonal SiPMs developed by DPNC/UniGe in collaboration with Hamamatsu Photonics, and hollow light concentrators.
The latter resulted from a joint development between the UniGe and the Swiss company Thin Film Physics AG to achieve high optical efficiency and adopt a cost-effective injection moulding plastic technique for production.
Always in collaboration with Thin Film Physics AG, the DPNC introduced for the first time an optical filter coated onto the protection window to enhance the response for Cherenkov light and suppress the night sky background. This solution is now used in other Cherenkov telescopes of CTA. SST-1Ms were built and are currently installed in the Czech observatory of Ondřejov where they are performing observations of the brightest and closest known gamma-ray sources The project was scrutinised by a panel of international experts set-up by CTAO for a final selection of the SST array technology. Despite a spotless technical evaluation of the SST-1M solution, being compliant with the observatory requirements and using mature technologies ready for mass production, the dual mirror telescope solution was eventually selected. However, the proven performance and cost-effective design brought the LHAASO collaboration to adopt a similar photosensing plane for the camera of their Wide Field-of-View telescopes. The cost saving of the SST-1M approach allowed to build 16 instead of 12 telescopes for the same budget. It is also worth mentioning that the technologies developed for the light guides and the window with Thin Film Physics AG were adopted by LHAASO which also produced their element in Switzerland.