The Cherenkov Telescope Array: the challenges in creating the unique observatory

An image to illustrate the Cherenkov Telescope Array
© briel Pérez Diaz, IAC

Professor Hofmann of the Cherenkov Telescope Array spoke to us about the challenges involved in creating the unique observatory.

Professor Werner Hofmann, Spokesperson for the Cherenkov Telescope Array and Director of the Max Planck Institute for Nuclear Physics in Heidelberg. Hofmann met with SciTech Europa in Bologna, Italy to discuss the progress that has been made so far on the CTA and the challenges involved in creating this unique observatory.

The Cherenkov Telescope Array (CTA) will be the foremost global observatory for very high-energy gamma-ray astronomy over the next decade and beyond. The Cherenkov Telescope Array’s first Science Symposium, which SciTech Europa attended, took place in May in Bologna, Italy, with talks aimed at covering areas such as cosmic particle acceleration, compact objects and relativistic shocks, role of cosmic particles in galaxy evolution and star-forming systems, gamma rays as cosmic probes, fundamental physics, multi-wavelength and multi-messenger observations and, additionally, any topic connected to the scientific possibilities of Cherenkov Telescope Array.

CTA Spokesperson and Director of the Max Planck Institute for Nuclear Physics in Heidelberg, Professor Werner Hofmann, met with SE on the sidelines of the event to discuss the progress that has been made so far and the challenges involved in creating this unique observatory.

Could you begin by outlining the background to the CTA and why it is a necessary development in the field of gamma ray detection/observation?

Gamma-ray astronomy at these energies is a relatively young field, and perhaps the biggest surprise was the fact that there are so many very high energy gamma-ray sources in the Universe which appear to act as cosmic particle accelerators. Before the first discoveries were made, those objects were very exotic and assumed to be very rare; now, over the last decade, we have learned that this type of particle acceleration happens everywhere in the Universe; and we know that it influences the way galaxies evolve, because it feeds a lot of energy into them.

These sources are thus something of a standard phenomenon in cosmic life and, at the same time, they also enable the probing of extreme environments due to the fact that these events occur in supernova explosions, around black holes, and so on, meaning that we are increasingly being able to address a number of additional very interesting questions about these objects. Gamma rays can also be produced by annihilating dark matter particles and may lead to the detection and identification of dark matter.

It has become clear that there is a very rich Universe out there to be explored, and while the current generation of instruments provide a first glimpse, we still don’t fully understand how these cosmic accelerators work. Also, when we search for dark matter, we are just short of achieving the necessary level of sensitivity to really probe theoretical predictions at the required level. Furthermore, if we look at objects in our galaxy, we see those which exist in our galactic neighbourhood, but we don’t see the full galaxy, and that suggests that we are missing some of our galaxy’s key cosmic accelerators.

The inability of the current generation of instruments to provide us with the level of sensitivity needed to probe the full physics is what drove the design of Cherenkov Telescope Array , which will have a factor of 10 more sensitivity and will be able to cover, for example, our entire galaxy, it will be able to probe for dark matter at the expected flux levels and really bring this field forwards.

At the same time, we have the advent of multi messenger astronomy, which is incredibly exciting. We already have instruments that are sensitive enough to look for neutrinos, which is very important because it helps to pin down what is happening in cosmic accelerators such as neutron star mergers (very violent gravitational wave events which send out shockwaves and accelerate particles).

This, then, is an additional incentive to build an instrument like Cherenkov Telescope Array so as to understand these processes in more detail. We know that there is very rich physics out there, and Cherenkov Telescope Array will allow us to explore it.

How important – or perhaps difficult – is it going to be for CTA to work with different areas and different infrastructure projects in order to share best practice or share data when it comes to a better exploration/exploitation of multi messenger astronomy?

We have made significant progress here. In particular, in Europe many projects are driven by the European Strategy Forum on Research Infrastructures (ESFRI), and the European Commission, of course, has a clear policy towards open data. However, current instruments are experiments, not observatories, and it can be incredibly difficult to analyse the data they generate. With CTA, we are taking a big step forwards; CTA will be an open observatory, meaning that anybody can apply for time, that data will be open, and alerts will immediately be open. Within this general opening up of the framework, there is both a benefit to be had from all sides and a clear willingness for it to happen on the part of all stakeholders.

Gravitational wave triggers are already open, of course, and in the same way CTA triggers will be open. Meanwhile, arrangements will be made for an increased exchange of data; indeed, we already have a Memorandum of Understanding with those working on gravitational waves, for example, despite the fact that CTA will not be operational for some time. This demonstrates that not only is there an appetite for this to happen, but that there has been the realisation that this is a necessity.

How important was it for the CTA to be on the ESFRI Roadmap?

That is an interesting question. The ESFRI Roadmap really triggered CTA, in that we had ideas for a next generation observatory beforehand, but without ESFRI it would have almost certainly taken much longer to turn those ideas into a reality.

It is hard to imagine a project of this size being realised without being on the Roadmap and, what is more, ESFRI helped us to get governments involved which then enabled us to obtain more funding both here and via the European Framework Programmes, which has been crucial to CTA’s development.

Being a part of the ESFRI Roadmap also meant that we were required to focus on things such as the governance and legal frameworks, which we might not otherwise have done. And while these were areas that, as scientists, we might have perhaps skirted around, at least at the early stage, ESFRI’s early requirement that we realise the legal framework is now certainly paying dividends – we are, in fact, aware that one of the things that is currently holding up progress in some areas is the fact that we didn’t move quickly enough on the legal aspects. Without ESFRI, CTA certainly wouldn’t be where it is today.

How does CTA build on technology that exists in other observatories and projects? And what more will it offer when it becomes fully operational?

Technology-wise, CTA is essentially an evolution, rather than a revolution (though it is a revolution in terms of sensitivity); it builds on many of the things we already have and it improves in a lot of them – such as, for example, the photo-sensors which we have developed in conjunction with industry. These have been improved upon for CTA and now offer twice the sensitivity than what was available just a decade ago. This demonstrates how, with an investment of just a few hundred thousand euro for photo sensor development, it is potentially possible to create a telescope which is, effectively, twice as big, at the same cost.

We are also improving the way that we produce mirrors. Many of our current telescopes have glass mirrors which are ground by hand. Now, however, we can utilise large scale fabrication techniques to create the mirrors we need for CTA.

There are two new technologies emerging: the first prototype dual mirror telescopes have started to be operational, and they may or may not come to be used in CTA – they have the advantage of enabling the compensation of optical operations, thereby obtaining better imaging over a large field of view.

Second, our small telescopes will use silicon sensors instead of photomultipliers. Silicon sensors are now at a stage where they are getting cheaper and because they are easier to handle and are more compact, they allow us to build very compact cameras. At some point, we may also come to use them in the big telescopes, too, although that is a discussion for the future. For now, using them in the small telescopes will allow us, for example, to operate the telescopes under full moonlight.

Overall, there are numerous ways in which CTA will build on already existing technologies. CTA will have a relatively large number of telescopes, each of which will have lots of incremental improvements, while we will also employ much better analysis techniques. Combined, CTA provides a factor of 10 gain in sensitivity, as well as a much wider field of view – we will be able to cover three times the area of the sky as compared to current instruments. In terms of speed and how quickly we can survey a certain piece of sky, CTA will be about 400 times faster than current instruments when doing a survey at a given depth.

How will you be handling the logistics of constructing and running/maintaining CTA? Will IAC/ESO be involved here?

The best person to answer this question is Wolfgang Wild, CTAO’s project manager! It will definitely be a team effort. The CTA Observatory (CTAO) is the legal entity in charge of the construction and, later, the operation of both CTA array sites. The CTAO will be in charge of building the sites’ infrastructures, but this will be managed locally by IAC (CTA-North) and ESO (CTA-South). The different CTA telescope teams from around the world will be responsible for installing and commissioning single telescopes, while the integration and commissioning of the overall system of telescopes, software and other devices will be the responsibility of the CTAO.

Building CTA is a significant challenge, as is gaining the necessary funding to complete the observatory – currently, we have about two-thirds of what we need. With the available funding we can start deploying the telescopes, with the first stage designed to see about two thirds of the arrays operational, which will provide us with a great instrument. Nevertheless, we still want to see the final third come online, and I am confident that the necessary funding will be received in due course. Indeed, many of the countries who are already investing in CTA have indicated that they would be prepared to continue to do so, once stage one is complete.

What challenges have you experienced so far? Has it been a particular challenge to bring the particle physics and astronomy communities together?

Our science consortium, which is made up of people from 31 countries, designed the instrument. The consortium’s members are particle physicists and astronomers and that has presented a challenge, particularly when it comes to reconciling the two very different cultures found in each field. For instance, particle physicists are used to working in big collaborations; they are used to sharing data and publishing large author lists. Whereas in astronomy, it is almost the opposite.

The plans we have now for CTA represent something of a middle ground; there will be publications from the key science projects where everybody publishes together on a paper, but then there will also be a class of publications, where we have short author lists.
Additionally, CTA will also see a big step forward in terms of data analysis. With current instruments, it can take months to get up to speed with data analysis. For CTA, the plan is to create tools which will allow a scientist with some experience in the field to be productive within, say, somewhere between a day and a week. This will involve better data formats so as to allow easier access.

The tools, which are fully documented, will be straightforward and easy to use and are already being positively received by the community. Indeed, existing instruments are now providing their data in this new format and are beginning to use the new tools to analyse their own data because it makes things easier. This means that, at some point, we will be able to make all the data from all the instruments public, in this standard format, thereby allowing us to also look back at archival data. This will have a very positive impact on the field in terms of making analysis simpler.

Alongside this, we are still working to decide on exactly which telescope prototype to implement for the Small- and Medium-Sized Telescopes. We have two MST and three different SST prototypes to choose from, and there is a very intense discussion taking place as to which to actually build.

Finally, a major challenge is to build a system which is simple and easy to maintain; we want something which will work well, but also work every single day. And that has to be weighed up with the necessary technical complexity that the instrument requires. That is also important when you consider that in a decade the people responsible for building the observatory may no longer be available, and so we need to ensure that ‘normal’ engineers and technicians are able to fix any problems as and when they arise.

Alongside choosing which telescopes to use, what will the rest of the pre-production stage entail?

Choosing exactly what telescopes is one. We have the first Large Size Telescope (LST) deployed on La Parma (and its commissioning is ongoing), and we will see whether there any necessary modifications. The next step is then to build the three more LSTs there, as well as one of the Medium-Sized Telescopes.

The other big step is to get started on the work in Chile. This hasn’t been possible so far because it was only in December last year that the hosting agreements with ESO were signed. But now we can really begin to spend money and to do the detailed design of the infrastructure. Of course, we have a general design, but the detailed work now needs to be done. Then, we need to figure out what to deploy in which sequence, and this is in the planning stage.

ESO’s experience and organisation in Chile will be of great help to CTAO, which will make these things easier with regard to the logistics and so on. But nevertheless, it will be important to gain early operational experience with the CTA in this location and to encounter any challenges as early as possible.

It will take five years to deploy this system, but we know from experience that the first telescopes are a challenge, but things get easier as you progress, and so I trust that we’ll be able to smoothly deploy the large-scale observatory, once production gets rolling.

Professor Werner Hofmann
Spokesperson
Cherenkov Telescope Array
Director
Max Planck Institute for
Nuclear Physics
Tweet @CTA_Observatory
www.cta-observatory.org/
www.mpi-hd.mpg.de/mpi/en/start/

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