Windows for the Small-Sized Telescope (SST) Cameras of the Cherenkov Telescope Array (CTA)

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Gamma-rays are the highest energy photons coming from the Universe. They don't pass through the Earth's atmosphere, but it is possible to detect them from the ground via the flashes of faint, blue light which they create in the upper atmosphere, known as Cherenkov radiation. A team in the UK is involved in building a new system of telescopes, called the Cherenkov Telescope Array (CTA), to detect this radiation and hence pinpoint where the gamma-rays come from and what energy they are. There will be two arrays of telescopes, one in the Northern Hemisphere (on La Palma) and one in the South (in Chile). Both arrays include telescopes of different sizes to detect gamma-rays of different energy. The Southern array will contain many 10s of small-sized telescopes (SSTs) which are designed to detect the most energetic and rarest gamma-rays that CTA will see. The UK team is involved in building the cameras for the SSTs. These cameras need very sophisticated windows to perform 4 functions:i. protect the light detectors (SiPMs) from dust and water;ii. act as filter to remove the night-sky background (in the red region of the spectrum) while providing high transmission in the blue/ultraviolet;iii. act as an anti-reflective coating for high angles of incidence; andiv. stop infrared photons from the SiPMs from being reflected off the rear surface of the window.This challenging problem is one on which we have been working with coating company Thin Metal Films (TMF) in Basingstoke. They have designed a unique hafnium dioxide/silicon dioxide coating which fulfils all the requirements. This grant will fund the production of all 50 windows needed for the cameras in the UK.