Hot-Slumped Glass Mirrors for the Cherenkov Telescope Array

Gamma ray astronomy studies very energetic radiation from many different astronomical objects, such as supernova remnants and black holes, as well as seeking to learn more about the fundamentals of the Universe, such as the nature of gravity. Gamma rays do not penetrate the Earth's atmosphere, but nonetheless we can detect them from the ground. It so happens that when a gamma-ray enters the atmosphere, it creates a cascade of highly energetic particles, which in turn produces a flash of light known as Cherenkov radiation. This consists of a faint, brief flash of blue/UV light, which telescopes known as Imaging Atmospheric Cherenkov Telescopes (IACTs) can detect. This technique has been shown to work well, and has revealed to us a sky full of particle accelerators far more powerful than the LHC. Scientists from around the world, including the UK, are now building an observatory for gamma ray astronomy, the Cherenkov Telescope Array, or CTA. This will consist of about 100 telescopes for observing the southern sky, located in Chile, and about 20 telescopes for studying the northern sky, sited in the Canary Islands. Three different sizes of telescope are needed, large, medium and small, for studying the lowest, intermediate and highest energy gamma rays, respectively. The site in Chile will have all three types of telescope, while only large and medium sized telescopes are needed in the Canary Islands, where the number of visible sources of the highest energy gamma rays is small.

There will be about 70 small sized telescopes in Chile. Scientists in the UK are leading the design and prototyping of a two-mirror design for these telescopes, known as the Gamma-ray Cherenkov Telescope (GCT). The mirrors for this telescope are challenging to make. All Cherenkov telescopes mirrors are concave in form. For the single-mirror telescopes that have been used until now, these have a very long focal length and the curvature of the mirrors is therefore small. This makes it possible to pull cold glass down onto a suitable mould to make the reflective surface. However, the GCT mirrors will have much greater curvature, and this makes it impossible to form cold glass to the correct shape. So, we are working with a company in St. Asaph in North Wales on a new technique to form the glass at high temperature. We have made some preliminary studies, which have been encouraging, and this grant application requests funds to take this process further, including creating the correct shape that we will need for the GCT and working with Thin Metal Films in Basingstoke to look at the best way of coating the mirror surfaces. If the project is successful, then UK companies will have a good chance of making mirrors not only for the GCT but also for an American-led dual-mirror telescope design being put forward as a medium size telescope for CTA.