Capital funding for CTA-UK programme

The Universe is full of particles with such high energies that they are travelling at very close to the speed of light. These
particles play a significant role in many areas of astrophysics, from the life-cycles of stars to the evolution of galaxies.
The paths of these particles are hard to trace, but they can reveal their presence by producing gamma rays. Like their lower-energy cousins, X-rays, gamma rays do not penetrate the Earth's atmosphere and usually satellite-based telescopes are used to detect
them. However, at very high energies (VHE), gamma rays are extremely rare, and detecting them using spacecraft becomes
impossible - the chance they would hit a reasonably sized satellite is just too small. Luckily, it is possible to observe them from the ground via the flashes of blue light, Cherenkov radiation, they produce when they interact in the atmosphere. The glow of this Cherenkov radiation is 10,000 times fainter than starlight, so telescopes are needed to observe it, and because the flashes last only a few billionths of a second, ultra-fast cameras are needed to record them.

We know from current ground-based gamma-ray telescopes such as HESS that there is a wealth of phenomena to be
studied using gamma rays. VHE gamma ray telescopes have detected the remains of supernova explosions, binary star systems, highly energetic jets of particles produced by black holes in distant galaxies, star formation regions, and many other objects. These
observations can help us to understand not only what is going on inside these objects, but also answer fundamental physics relating to the nature of dark matter and of space-time itself. However, we have now reached the limit of what can be done with current instruments, and so about 1000 scientists from 29 countries around the world have come together to build a new instrument - the Cherenkov Telescope Array (CTA).

CTA will offer a dramatic increase in sensitivity over current instruments, and extend the range of the gamma rays observed to both lower and higher energies. It is predicted that the catalogue of known VHE emitting objects will expand from the 130 known to over 1000, and we can expect many new discoveries in key areas of astrophysics and fundamental physics research. To achieve the wide energy range we require of CTA, it is necessary to build telescopes of three different sizes: 4 m diameter small-sized telescopes (SSTs), 12 m medium-sized telescopes (MSTs) and 23 m large-sized telescopes (LSTs). CTA will consist of two arrays of telescopes, one in the northern hemisphere and one in the southern hemisphere. The northern array will consist of 4 LSTs and 20 MSTs. The southern array will contain similar numbers of large and medium sized telescopes, but add to them an extensive array of 70 SSTs, specifically to investigate the highest energy phenomena which can be observed preferentially from the southern hemisphere. The SSTs will detect the highest energy photons ever seen, with energies approaching a peta-electronvolt, each a thousand billion times more energetic than an X-ray. Production of prototype CTA telescopes in now underway, with full-scale construction of the arrays expected to start in 2016.

There are currently 11 UK universities involved in CTA. The UK groups are concentrating their efforts on the construction of
the SSTs. We have already produced an innovative dual-mirror SST design, which is now being built in sight of the Eiffel Tower in
Paris. In this proposal we request funds to further develop the camera we have designed for this telescope and to support development by UK industry of the mirrors needed for it.

In addition to the applications under investigation from the ongoing CTA-UK programme (high speed amplifier applications, rapidly pulsed LEDs, outreach developments including a UK website and the production of leaflets describing CTA and planetarium resources) the funding requested here will hopefully in result in development of a UK capability to produce mirrors for CTA as well as further camera development. This will provide opportunities for UK industry to be involved in CTA construction. Elements of the camera (for example the enclosure and the required printed circuit boards) will be contracted out to industry. Mirrors must be produced not only for the GCTs but also for the large and medium sized telescopes.